IMPACT OF WIND TURBINE GENERATORS ON POWER SYSTEM STABILITY

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2 IMPACT OF WIND TURBINE GENERATORS ON POWER SYSTEM STABILITY A THESIS Submitted by JEEVAJOTHI R In partial fulfillment for the award of the degreeof DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING KALASALINGAM UNIVERSITY ANAND NAGAR KRISHNANKOIL AUGUST 2014

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4 iii ABSTRACT Power systems are complex systems that evolve over years in response to economic growth and continuously increasing power demand. Wind power generation has experienced a tremendous growth in the past decade, and has been recognized as an environmental friendly and economically competitive means of electric power generation.in the near future, wind power penetration in electrical power systems will increase and will start to replace the output of conventional synchronous generators (CSGs). As a result, it may also begin to influence overall power system behavior. Hence, the impact of wind power on the dynamics of power systems should be studied thoroughly in order to identify potential problems and to develop measures to mitigate those problems. The dynamic behavior of a power system is determined mainly by the generators. Wind turbine generators (WTGs) affect the dynamic behavior of the power system in a way that might be different from CSGs. The major issues to be considered are voltage stability and transient stability. A power system is said to be voltage stable if it maintains voltage within operational limits.maintaining the transient stability of the system is another major issue in the operation of power system. It is the ability to maintain synchronous operation of the machines when subjected to a large disturbance.

5 iv The dynamic behavior of a power system is determined mainly by the generators. Wind turbine generators (WTGs) affect the dynamic behavior of the power system in a way that might be different from CSGs. The major issues to be considered are voltage stability and transient stability. A power system is said to be voltage stable if it maintains voltage within operational limits.maintaining the transient stability of the system is another major issue in the operation of power system. It is the ability to maintain synchronous operation of the machines when subjected to a large disturbance. Dominant WTGs in use at present are fixed-speed squirrel cage induction generators (SCIGs), variable speed geared drive doubly fed induction generators (DFIGs),and variable speed direct drive electrically excited synchronous generators (EESGs) and permanent magnetsynchronous generators (PMSGs).SCIGs with capacitor bank for self-excitation and can reach peak efficiency at a particular wind speed. In DFIGs, the rotor winding is fed through back-to-back variable frequency, voltage source converters. In EESGs, additional converter is required for exciting its rotor. In PMSGs, the requirement of a larger pole number can be met with permanent magnets which allow small pole pitch. Also absence of field windings results in higher efficiency. Different generator converter combinations with different control strategies are available. In this work, the significance of DC-DC boost converter, step and search control method of tracking the maximum power, d-

6 v q current controland adaptive hysteresis band current control areanalyzed. Step and search control tracks the maximum power by sensing the V DC aloneand controls the same. The d-qtransformation current control enables the separate control of real and reactive component of ac output power. Adaptive hysteresis band current control adjusts the hysteresis bandwidth as a function of the reference compensator current variation, to optimize the switching frequency and THD of supply current. Switching frequency varies with respect to the adaptive band size. This thesis attempts to model variable speed direct drive WTGs with modified controllers and investigates the impact of fixed-speed SCIGs with capacitor banks, variable speed DFIGs with standard control and direct drive variable speed EESGs and PMSGs with modified controllers on power system voltage stability and transient stability. The direct drive EESG has been modeled with d-q current controlled converter and a direct drive PMSG has been modeled with maximum power point tracking (MPPT) controlled dc-dc boost converter, adaptive hysteresis band current controlled voltage source converter(vsc) which maintained constant DC link voltage at different wind speeds and different load conditions respectively. Since a large proportion of existing wind farms are based on fixedspeed wind turbines (FSWTs) which are equipped with simple induction generators (IGs), the voltage stability issue is a key problem. IGs consume

7 vi reactive power during system contingency, which deteriorates the local grid voltage stability. DFIGs make use of power electronic converters and are thus able to regulate their own reactive power to operate at a given power factor as well as able to control grid voltage. Because of the limited capacity of the pulse-width modulation (PWM) converter, when the voltage control requirement is beyond the capability of the DFIG, the voltage stability of the grid is also affected. DFIG offers a vast reduction in converter size; however they are susceptible to grid disturbances since their stator windings are directly connected to the grid. The synchronous generators are direct drive systems which supplies more reactive power and thus provides better performances in contrast to DFIG to recover post-fault voltage. EESG and PMSG are coming under this direct drive category. EESGs are salient pole machines and are excited from power grid. For low speed operation, high pole count synchronous generators are recommended. With EESGs, over excitation is easily possible. So operation at unity power factor is utilized to reduce machine side inverter to the real power value. High pole count increase the field ampere turns which expresses the need to utilize permanent magnets. Also increased exciting ampere turns yield an increase of excitation losses. Simulation has been performed on IEEE 14-bus test system to study the loading margin, voltage collapse, voltage magnitude and reactive power

8 vii delivered by the various WTGs. Further simulation was carried out on IEEE 9- bus test system to study the rotor angle swing, rotor speed deviation and oscillation, critical clearing time (CCT), voltage magnitude, active power support and reactive power support by the different WTGs. Simulation results demonstrate the superior performance of EESGs and PMSGs with modified controllers in improving the voltage stability and transient stability of power system.

9 viii ACKNOWLEDGEMENT First and foremost I thank God for providing His grace and strength to achieve this work. I express my sincere gratitude to my Supervisor Dr.D.Devaraj, Senior Professor and Head, Dept. of Electrical & Electronics Engineering, Kalasalingam University, for his technical guidance, his intellectual support and encouragement of my research work. I am extremely grateful for having the privilege to work under him and learn from his expertise. I express my sincere thanks to Mr. R.Jeyasingh, Manager, R.S.Wind Tech. Pvt. Ltd. for the valuable discussions and technical support towards my research work. I would like to thank the management and officials of Kalasalingam University for providing support for doing my research work. I want to thank my late parents for getting me where I am now. I want to dedicate the effort done in this project to them who always believed in me and stood by my decisions. I am forever indebted to my family for their understanding, patience and encouragement when it was most required. Lastly, I offer my regards and blessings to all of those who supported me in any respect for the successful completion of my research work. R.JEEVAJOTHI

10 ix CHAPTER NO. TABLE OF CONTENTS TITLE ABSTRACT LIST OF TABLES LIST OF FIGURES LIST OF SYMBOLS & ABBREVIATIONS PAGE NO. iii vii viii xx 1 INTRODUCTION INTRODUCTION ENERGY CONVERSION FROM WIND CONTROL OF WIND TURBINE Stall Control Pitch control OPERATING CHARACTERISTICS OF WIND 6 TURBINE 1.5 TYPES OF WIND TURBINE GENERATORS Fixed Speed Wind Turbine Generators Variable Speed Wind Turbine Generators Geared drive Doubly fed 9 Induction Generator Direct drive Synchronous Generator POWER ELECTRONIC CONVERTERS IN 12

11 x WIND ENERGY CONVERSION SYSTEMS 1.7 IMPACT OF WIND TURBINE GENERATORS 13 ON POWER SYSTEM PERFORMANCE 1.8 OBJECTIVES OF THE RESEARCH WORK ORGANISATION OF THE THESIS 15 2 LITERATURE SURVEY INTRODUCTION MODELING OF WIND TURBINE 18 GENERATORS 2.3 CONVERTER TOPOLOGIES FOR WIND 23 TURBINE GENERATORS 2.4 CONVERTER CONTROL STRATEGIES MPPT Control Control of Voltage Source Converter IMPACT OF VARIABLE SPEED WIND 28 TURBINE GENERATORS ON POWER SYSTEM STABILITY 2.6 ISSUES IDENTIFIED CONCLUSION 34 3 MODELING AND SIMULATION OF WIND 36 TURBINE GENERATORS 3.1 INTRODUCTION MODELING OF INDUCTION GENERATORS VARIABLE SPEED WIND TURBINE WITH 38 DIRECT DRIVE SYNCHRONOUS GENERATORS Direct drive EESG Direct drive PMSG 40

12 xi 3.4 MODELING OFDIRECT DRIVE 41 SYNCHRONOUS GENERATORS Modeling of EESG Modeling of PMSG MODELING AND CONTROL OF POWER 43 ELECTRONIC CONVERTERS 3.5.1Full-wave Diode Bridge Rectifier DC-DC Boost Converter Control of DC-DC Boost Converter Modeling of Voltage Source Converter Control of Voltage Source Converter d-q Current Control Adaptive Hysteresis Band Current 54 Control 3.6 SIMULATION RESULTS Direct Drive EESG Direct Drive PMSG Effect of Pitch control Results of constant DC link 71 voltage control with MPPT at wind speeds of 12 m/ sec. and at 14 m/sec Results of constant DC link 74 voltage control with adaptive hysteresis band current controller at load currents of 50A and 130A 3.7 SUMMARY 78 4 IMPACT OF WIND TURBINE GENERATORS ON POWER SYSTEM VOLTAGE STABILITY INTRODUCTION 80

13 xii 4.2 VOLTAGE STABILITY ANALYSIS PV curve Loading margin POWER SYSTEM VOLTAGE STABILITY IN 83 THE PRESENCE OF WIND TURBINE GENERATORS Fixed Speed Wind Turbine with Squirrel 84 cage Induction Generator Variable Speed Wind Turbine with Geared 84 drive Doubly fed Induction Generators 4.3.3Variable Speed Wind Turbine with Direct 85 drive Synchronous Generator Electrically Excited Synchronous 86 Generator Permanent Magnet Synchronous Generator VOLTAGE CONTROLLERS IN WIND 88 TURBINE GENERATORS 4.5 SIMULATION RESULTS Voltage Stability with Conventional Synchronous Generators Computation of Loading 90 margin Voltage Vs time curve after 91 the contingency Voltage profile Voltage Stability with Wind Turbine Generators Computation of Loading 96

14 xiii margin Voltage Vs. Time curve after the Contingency Voltage profile SUMMARY POWER SYSTEM TRANSIENT STABILITY IN THE PRESENCE OF WIND TURBINE GENERATORS INTRODUCTION TRANSIENT STABILITY ANALYSIS TRANSIENT STABILITY ASSESSMENT Critical Clearing Time Rotor Angle Deviation Rotor Speed Oscillation TRANSIENT STABILITY IN THE PRESENCE OF WIND TURBINE GENERATORS Fixed Speed Wind Turbine Generators Variable Speed Wind Turbine Generators Geared drive Wind Turbine Generators Direct drive Wind Turbine Generators SIMULATION RESULTS Transient stability with CSGs alone Impact of WTGs on transient stability Impact of SCIGs on transient stability Impact of DFIGs on transient 120

15 xiv stability Impact of EESGs on transient 123 stability Impact of PMSGs on transient 127 stability 5.6 SUMMARY SUMMARY OF FINDINGS AND CONCLUSION INTRODUCTION SUMMARY OF THE RESEARCH FINDINGS SIGNIFICANT RESEARCH CONTRIBUTION CONCLUSION OF THE THESIS SUGGESTIONS FOR FUTURE WORK 137 APPENDIX APPENDIX APPENDIX REFERENCES 147 LIST OF PUBLICATIONS 154 CURRICULUM VITAE 156

16 xv TABLE NO. LIST OF TABLES TITLE PAGE NO. 3.1 Parameters of Wind Turbine Parameters of Electrically Excited Synchronous 60 Generator 3.3 Parameters of Wind Turbine Parameters of Permanent Magnet Synchronous 66 Generator 3.5 Converter parameters Parameters of Conventional Synchronous Generator Loading Margin under Base case and Contingency 90 states in p.u. 4.3 Voltage magnitude and Reactive power flows with 92 Conventional Synchronous Generators 4.4 Parameters of Fixed Speed SCIG Parameters of variable speed DFIG Values of Loading Margin under Base case and 96 Contingency states in p.u. 4.7 Values of voltage magnitude and reactive power in 99 p.u. 5.1 Transient stability assessment with CSGs and fixed and 131 variable speed WTGs A3.1 Transmission line parameters of IEEE 14 bus test system 146

17 xvi FIGURE NO. LIST OF FIGURES TITLE PAGE NO. 1.1 C versus characteristic Performance characteristics of wind turbine under pitch control 1.3 Three dimensional view of group of C versus λ 5 6 characteristic with pitch angle 1.4 Typical wind turbine power output with steady wind 7 speed 1.5 Schematic representation of the fixed speed wind turbine 8 with squirrel cage induction generator 1.6 Schematic representation of the variable speed wind 10 turbine with doubly fed induction generator 1.7 Schematic representation of the variable speed wind 11 turbine with direct drive electrically excited synchronous generator 1.8 Schematic representation of the variable speed wind 12 turbine with direct drive permanent magnetsynchronous generator 1.9 Power electronic converters in wind energy conversion 12 systems 3.1 Equivalent circuit of Fixed Speed Squirrel Cage Induction Generator Equivalent circuit of Variable Speed Doubly Fed 37 Induction Generator 3.3 Direct Drive Synchronous Generator Block diagram representation of the Variable Speed 39

18 xvii Direct Drive EESG 3.5 Block diagram representation of the Variable Speed 40 Direct Drive PMSG 3.6 Electrical model of the EESG Three-phase Diode bridge Rectifier DC-DC Boost Converter Circuit (a) Equivalent circuit of the DC-DC converter in the first 46 operating phase 3.9(b) Equivalent circuit of the DC-DC converter in the second 47 operating phase 3.10 Step and search control strategy to track maximum 48 power 3.11 Circuit diagram of a IGBT based DC- AC Single phase 49 Full bridge Converter 3.12 PWM signal for a Voltage Source Converter Current control scheme of a Voltage Source Converter Adaptive hysteresis current controller concept Simulation diagram of direct drive EESG Simulation diagram of Wind Turbine Simulation diagram of d-q current control scheme of 59 VSC 3.18 Wind speed Tip speed ratio Power coefficient Mechanical speed of VSWT with direct drive EESG Real power output of VSWT with direct drive EESG Reactive power generated by VSWT with direct drive 63 EESG 3.24 Generated phase voltage in p.u. of VSWT with direct 63

19 xviii drive EESG 3.25 Vdc link of VSWT with direct drive EESG Phase voltage in p.u in grid side of VSWT with direct 64 drive EESG 3.27 Injected real power in grid side of VSWT with direct 64 drive EESG 3.28 Injected reactive power in grid side of VSWT with 64 EESG 3.29 Simulation diagram of direct drive PMSG Simulation diagram of MPPT control of DC-DC boost 67 converter 3.31 Simulation diagram of reference current generator of 67 Adaptive hysteresis band current controlled VSC 3.32 Simulation diagram ofadaptive hysteresis bandwidth 67 calculation 3.33 Simulation diagram of switching pulses of VSC Wind speed profile Coefficient of Performance Tip speed ratio (a) Generator phase Voltage at 12m/sec (b) Generator phase Current at 12m/sec (a) Generator phase Voltage at 14m/sec (b) Generator phase Current at 14m/sec (a) DC link Voltage at 12m/sec (b) DC link Voltage at 12m/sec. (with zooming) (c) DC link Voltage at 14 m/sec (d) DC link Voltage at 14 m/sec. (with zooming) Grid Voltage (a) Grid Current 75

20 xix 3.41 (b) Inverteroutput phase Current (c) Adaptive hysteresis band at 50 A (d)(i) DC link voltage at 50 A with Adaptive hysteresis band 75 current controller DC link voltage at 50 A with Adaptive hysteresis band 76 d)(ii) currentcontroller (with zooming) 3.42 (a) Grid Current (b) Inverter output phase Current (c) Adaptive hysteresis band at 130 A (d)(i) DC link voltage at 130 A (d)(ii) DC link voltage at 130 A (with zooming) Typical PV curve One line diagram of IEEE 14- bus system Loading margin with CSGs Bus-6 voltage variation after the disconnection of line (a) Voltage profile of bus-2 under Base case and 92 Contingency states in p.u. 4.5(b) Voltage profile of bus-5 under Base case and 93 Contingency states in p.u. 4.6(a) Reactive power flow from bus-1 to bus-2 under Base 93 case and Contingency states in p.u. 4.6(b) Reactive power flow from bus-1 to bus-5 under base 93 case and contingency states in p.u. 4.7 Profile of loading margin (a) BUS-6 voltage with SCIGs after the disconnection of 97 line (b) BUS-6 voltage with DFIGs after the disconnection of 98

21 xx line (c) BUS-6 voltage with EESGs after the disconnection of 98 line (d) BUS-6 voltage with PMSGs after the disconnection of 98 line (a) Voltage profile of bus-2 under Base case and 100 Contingency states in p.u. 4.9(b) Voltage profile of bus-5 under Base case and 100 Contingency states in p.u. 4.10(a) Reactive power flow from bus-1 to bus (b) Reactive power from bus-1 to bus Typical allowable maximum rotor speed deviation and 108 oscillation duration 5.2 One-line diagram of IEEE 9- bus system (a) Voltage at generator buses with CCTwith only CSGs (b) Rotor angle deviation at generator bus-2 and bus-3 with 113 only CSGs 5.3(c)(i) Rotor speed oscillationat generator bus-2 and bus-3 with 114 only CSGs 5.3(c) (ii) Rotor speed oscillationat generator bus-2 and bus-3 with 114 only CSGs 5.3(d) (i) Real power at generator buses with only CSGs (d) (ii) Real power at generator buses with only CSGs (e) Reactive power at generator buses with only CSGs (a) Voltage at generator buses with CCTwith SCIGs (b) Rotor angle deviation at generator bus-2 and bus-3 with 117 SCIGs 5.4(c)(i) Rotor speed oscillationat generator bus-2 and bus-3 with 117 SCIGs

22 xxi 5.4(c) (ii) Rotor speed oscillationat generator bus-2 and bus-3 with 118 SCIGs 5.4(d) (i) Real power at generator buses with SCIGs (d) (ii) Real power at generator buses with SCIGs (e) Reactive power at generator buses with SCIGs (a) Voltage at generator buses with CCT with DFIGs (b) Rotor angle deviation at generator bus-2 and bus-3 with 119 DFIGs 5.5(c) (i) Rotor speed oscillationat generator bus-2 and bus-3 with 120 DFIGs 5.5(c) (ii) Rotor speed oscillationat generator bus-2 and bus-3 with 121 DFIGs 5.5(d) (i) Real power at generator buses with DFIGs (d) (ii) Real power at generator buses with DFIGs (e) Reactive power at generator buses with DFIGs (a) Voltage at generator buses with CCT with EESGs (b) Rotor angle deviation at generator bus-2 and bus-3 with 124 EESGs 5.6(c)(i) Rotor speed oscillationat generator bus-2 and bus-3 with 124 EESGs 5.6(c)(ii) Rotor speed oscillationat generator bus-2 and bus-3 with 125 EESGs 5.6(d) (i) Real power at generator buses with EESGs (d) (ii) Real power at generator buses with EESGs (e) Reactive power at generator buses with EESGs (a) Voltage at generator buses with CCT with PMSGs (b) Rotor angle deviation at generator bus-2 and bus-3 with PMSGs 128

23 xxii 5.7(c)(i) Rotor speed oscillationat generator bus-2 and bus-3 with 128 PMSGs 5.7(c) (ii) Rotor speed oscillationat generator bus-2 and bus-3 with 129 PMSGs 5.7(d) (i) Real power at generator buses with PMSGs (d) (ii) Real power at generator buses with PMSGs (e) Reactive power at generator buses with PMSGs 131 A1.1 Illustration of prediction-correction steps 138 A1.2 Flow chart for Continuation power flow 142

24 xxiii LIST OF SYMBOLS AND ABBREVATIONS SYMBOLS C - Coefficient of performance λ - Tip speed ratio R - Radius of the wind turbine rotor in m ω - Angular velocity of the rotor in rad/sec. V ω - Wind speed in m/sec. A - Swept area of wind turbine rotor in m 2 ρ - Air density in kg/m 3 P t - Power obtained from wind turbine T - Torque developed by the wind turbine V - Stator voltage V - Rotor voltage R - Stator resistance R - Rotor resistance I - Stator current I - Rotor current R - Magnetizing resistance L - Magnetizing inductance I - Magnetizing resistance current L - Stator leakage inductance L - Rotor leakage inductance ω - Stator frequency ω - Slip frequency ω - Rotor speed s - Slip V - d-axis voltage

25 xxiv V - q-axis voltage i - d-axis current i - q-axis current Ψ - d-axis flux linkage Ψ - q-axis flux linkage - Angular frequency of rotor - Amplitude of the flux induced by the permanent magnets of the rotor in the stator phases p -- Number of pole pairs T - Electromagnetic Torque V, V, V - Phase voltages V - Amplitude of the phase voltage V - Root-mean-square (RMS) value of the phase Voltage V - DC component of the output voltage I - DC component of the output current I - RMS value of input current P - Output power of the rectifier P - Inputpower of the rectifier V - Input voltage of the boostconverter V - Output voltage of the boostconverter D - Duty cycle L - Minimum value for inductance C - Minimum value for capacitance R - Output resistance V - Ripple voltage f - Switching frequency L - Inductance C - Capacitance

26 xxv i - Current through the inductor P - Electric power generated by synchronous generator V - Generator phase voltage I - Generator phase current E - Induced voltage in the armature I f - Field current ω e - Electrical angular speed V - DC link voltage V - Desired voltage magnitude P - Desired real power Q - Desired reactive power P - Actual real power Q - Actual reactive power P - Maximum power C - Maximum coefficient of performance λ - Optimal tip speed ratio PF - Power factor - Electrical efficiency of generator and inverter T θ - Rotational d qtransformation matrix Vo Vd Vq - Variables on the o-d-q frame Va Vb Vc - Variables on the a-b-c frame θ - Phase angle of Vain radian V - d-axis voltage at VSWT terminal V - q-axis voltage at VSWT terminal I - d-axis current at VSWT terminal I - q-axis current at VSWT terminal P - Instantaneous active power output Q - Instantaneous reactive power output

27 xxvi V - Instantaneous VSWT voltage magnitude I _ - d- axis reference current I _ - q- axis reference current I _ - a- axis reference current I _ - b-axis reference current I _ - c- axis reference current θ - Reference phase angle T θ - Rotational inverse d q transformation matrix I _ - Desired current vector of the VSWT I - Actual current vector of the VSWT I - Error signal vector I _ - Upper limit of the d-axis reference current I _ - Upper limit of the q-axis reference current I _ - Lower limit of the d-axis reference current I _ - Lower limit of the q-axis reference current Q - Reactive power capability limits of the inverter S - Apparent power of inverter P - Real power of inverter E - Error signal of grid connected inverter I - Measured line current of the grid connected inverter I - Reference line current of the grid connected inverter - Measured line current of phase A V - Grid voltage per phase HB - Hysteresis bandwidth T,T - Switching intervals m - Slope of command current wave f Modulation frequency. M - Moment of inertia

28 xxvii P - Acceleration power δ - Rotor angle P - Input mechanical power P - Output electrical power

29 xxviii ABBREVATIONS ABH - Adaptive Band Hysteresis AVR - Automatic voltage regulator CCT - Critical clearing time CCT - Critical clearing time CPF - Continuation power flow CSC - Current source converter CSG - Conventional synchronous generator DDSG - Direct drive synchronous generators DFIG - Doubly fed induction generator EESG - Electrically excited synchronous generator FRC - Fully rated converter FSWT - Fixed-speed wind turbine GSC - Grid side converter GUI - Graphical user interface HAWT - Horizontal axis wind turbine HCS - Hill-climb search IG - Induction Generator LVRT - Low Voltage Ride Through Function MPPT - Maximum power point tracking OEL - over excitation limiter OLTC - On load tap changer PLL - Phase locked loop PMSG - Permanent magnet synchronous generator POI - Point of interconnection PSAT - Power system analysis toolbox PSF - Power signal feedback PWM - Pulse-width modulation

30 xxix RSC - Rotor side converter SCIG - Squirrel cage induction generator STATCOM - Static synchronous compensator SVC - Static VAR compensator TSA - Transient stability Assessment TSR - Tip speed ratio UPF - Unity power factor VOC - Voltage oriented control VSC - Voltage source converter VSWT - Variable speed wind turbine WECS - Wind energy conversion system WPP - Wind power plant WTG - Wind turbine generator

31 1 CHAPTER 1 INTRODUCTION 1.1 INTRODUCTION As a result of increasing environmental concern, more and more electricity is generated from renewable sources. Renewable energy can contribute to securing energy supplies and smoothen the transition to a fossilfree economy. Renewable energy replaces conventional fuels in electricity generation. Renewable energy provides 19% of electricity generation worldwide. Wind power is one of the most competitive renewable technologies and, in developed countries with good wind resources, onshore wind is often competitive with fossil fuel-fired generation. Wind power generation has experienced a tremendous growth in the past decade, and has been recognized as an environmental friendly and economically competitive means of electric power generation. The size of wind turbines and wind farms are increasing quickly; a large amount of wind power is integrated into the power system. A huge penetration of wind energy in a power system may cause important problems due to the random nature of the wind and the characteristics of the wind generators. In large wind farms connected to the transmission network (110 kv 220 kv) the main technical constraint to take into account is the power system transient stability that could be lost when, for example, a voltage dip causes the switch off of a large number of wind turbine generators (WTGs). Another major technical issue to be considered is the voltage

32 2 instability and voltage collapse problem. The aim of this research is to evaluate the impact of strategically placed WTGs on power system stability with respect to the variation in load and occurrence of contingencies. This chapter explains the structure of WTGs, operating characteristics of WTGs, types of WTGs, interconnection of WTGs with electric power systems and the impact of WTGs on performance of power system. The objectives of this research work are explained in detail. Further it also includes an outline of the dissertation. 1.2 ENERGY CONVERSION FROM WIND In a wind turbine, the aerodynamic rotor converts the wind power into mechanical power which in turn is converted into electricity through the generators. The wind turbines can be classified into horizontal axis and vertical axis wind turbines. A horizontal axis wind turbine has its blades rotating on an axis parallel to the ground. A vertical axis wind turbine has its blades rotating on an axis perpendicular to the ground. The performance of wind turbine is characterized by the non-dimensional curve of coefficient of performance C, as a function of tip-speed ratio λ. C as a function of λ is expressed by equation (1.1) and it is shown in Figure 1.1. Figure 1.1 versus characteristic

33 3 C λ λ 0.146λ λ λ λ The tip-speed ratio is given by the expression, (1.1) λ ω ω (1.2) where R is the radius of the wind turbine rotor in m, ω is the angular velocity of the rotor in rad/sec. and V ω is the velocity of the wind in m/sec. The output power of the wind turbine P is given by, P 0.5 C λ A V ω (1.3) where A is the swept area of wind turbine rotor. It can be observed from Figure 1.1 that C is maximum when λ is equal to 7.5. In general, P T ω (1.4) Combining equations (1.2), (1.3) and (1.4), the expression for torque T developed by the wind turbine is written as T 0.5 A R λ λ V ω (1.5) The power extracted from the wind is maximum when the power coefficient C is at its maximum. This occurs at a defined value of the tip speed ratio (TSR). Hence, for each wind speed; there is an optimum rotor speed where maximum power is extracted from the wind. Therefore, if the wind speed is assumed to be constant, the value of C depends on the wind turbine rotor speed. Thus, by controlling the rotor speed, the power output of the turbine is controlled.

34 4 1.3 CONTROL OF WIND TURBINE The rotor power is limited by generator rating. At high wind speeds, the power is regulated by any one of the following controls: Stall Control Stall control works by increasing the angle at which the relative wind strikes the blades called angle of attack, and it reduces the induced drag associated with lift. Stall control is simple because it can be made to happen passively (it increases automatically when the winds speed up), but it increases the cross-section of the blade face-on to the wind, and thus the ordinary drag. A fully stalled turbine blade, when stopped, has the flat side of the blade facing directly into the wind. A fixed speed wind turbine inherently increases its angle of attack at higher wind speed as the blades speed up. A natural strategy, then, is to allow the blade to stall when the wind speed increases. This technique was successfully used on many early horizontal axis wind turbines (HAWTs). However, on some of these blade sets, it was observed that the degree of blade pitch tended to increase audible noise levels Pitch Control Pitch angle control is the most common means for adjusting the aerodynamic torque of the wind turbine when wind speed is above rated speed. Pitch control is not to optimize the generation, i.e. to work at the most efficient operating level or maximum power output level. This allows a good level of control over the angle of attack, thus control over the torque. The purpose of the control is to extend the range of operation of the wind turbine beyond the rated wind speed upto the cut-off speed. But for the control, the

35 5 machine should be stopped as soon as the wind speed reaches the rated wind speed. If the wind turbine is operated beyond the rated wind speed without stall or pitch control, the turbine will absorb more power from the wind than its capability to withstand. So, the control limits the power absorbed by the turbine from the wind to its capacity, even though much higher amount of power is available in the wind. Since the absorbed power is much less than the available power, naturally the efficiency will be less, which means that the C will be less or TSR is either more or less than the optimum. Figure 1.2 shows the performance characteristics of wind turbine under pitch control (Sachin Khajuria et al 2012). Figure 1.2 Performance characteristics of wind turbine under pitch control Beyond rated wind speed, optimum power generation or maximum C cannot be expected because the intention of the controller is only to increase the grid-connected duration in a day, i.e. overall energy per day and not power at each moment. Figure 1.3 shows the three dimensional view of

36 6 group of C versus λ characteristic with pitch angle. (Jianzhong Zhang et al 2008) clearly depicts this concept. Figure 1.3 Three dimensional view of group of versus characteristic with pitch angle The pitch angle controller used employs a PI controller. As long as the wind turbine output power is lower than the rated power of the wind turbine, the error signal is negative and pitch angle is kept at its optimum value. But once the wind turbine output power exceeds the rated power P the error signal are positive and the pitch angle changes to a new value, at a finite rate, thereby reducing the effective area of the blade resulting in the reduced power output. 1.4 OPERATING CHARACTERISTICS OF WIND TURBINE All wind machines share certain operating characteristics, such as start-up wind speed cut-in, rated and cut-out wind speeds. Figure 1.4 shows a typical wind turbine power output with steady wind speed.

37 7 Figure 1.4 Typical wind turbine power output with steady wind speed Some of the common terminologies associated with the operation of WTG are defined below. Start-up wind speed is the wind speed that will turn an unloaded rotor. Cut-in speed is the minimum wind speed at which the blades will turn and generate usable power. The cut-in wind speed of most turbines is around 12 Km/h. The rated speed is the minimum wind speed at which the wind turbine will generate its designated rated power. Cut-out speed is the maximum wind speed that the wind turbines cannot operate normally. The theoretical maximum amount of energy in the wind that can be collected by a wind turbine rotor is approximately 59%. This value is known as the Betz limit. 1.5 TYPES OF WIND TURBINE GENERATORS WTGs are generally of two types: fixed and variable speed.

38 Fixed Speed Wind Turbine Generators Fixed speed WTGs are squirrel cage induction generators (SCIGs) with capacitor bank for self-excitation. Figure 1.5 shows the schematic representation of the fixed speed SCIG with capacitor bank. In fixed speed WTGs, owing to the different operating speeds of the wind turbine rotor and the generator, a gearbox is necessary to match these speeds. The generator slip slightly varies with the amount of generated power and is therefore not entirely constant. However, because these speed variations are in the order of 1 %, this wind turbine type is normally referred to as constant-speed or fixedspeed. In fixed speed WTGs, (Camm E H et al 2009), the turbine speed is fixed (or nearly fixed) to the electrical grid s frequency, and generates real power (P) when the turbine shaft rotates faster than the electrical grid frequency creating a negative slip (positive slip and power is motoring convention). The fixed speed WTGs only reach peak efficiency at a particular wind speed. Fixed speed WTGs cannot have an optimal TSR and hence the efficiency will not be maximized. Power can only be controlled through pitch angle variations. Figure 1.5 Schematic representation of the fixed speed wind turbine with squirrel cage induction generator

39 Variable Speed Wind Turbine Generators Variable speed operation continuously adapt the rotational speed to the present wind speed, so that, ideally the maximum obtainable power is produced by the wind energy conversion system (WECS). Variable speed operation yields 20 to 30 percent more energy than the fixed speed operation, providing benefits in reducing power fluctuations and improving VAR supply. Variable speed wind turbines are connected to the grid through power electronic converters and maximize effective turbine speed control. Variable speed generators are classified according to drive trains as direct drive and geared drive systems Geared drive doubly fed induction generator The wind turbines, which use gear ratios bigger than 1, is categorized as geared drive systems. Wind turbines with doubly fed induction generator (DFIG) comes under this category. A gearbox, located between the rotor shaft and the generator shaft, is used for increasing the rotational speed of the generator input shaft while decreasing the torque. By the help of the increased rotational speed of the generator input shaft, the small number of poles is enough to obtain the desired frequency as the generator output. Smaller and cheaper generators can be used in these systems. On the other hand, because of the gearbox, the complexity of the system is higher than direct drive systems and these systems are less reliable. Besides, due to the failure in the gearboxes, the operation and maintenance cost of these systems are higher. In DFIG, the stator winding of the generator is coupled to the grid, and the rotor winding to a power electronic converter. Usually a back-to-back VSC with current control loops is used. In this way, the electrical and

40 10 mechanical rotor frequencies are decoupled, because the power electronic converter compensates the difference between mechanical and electrical frequency by injecting a rotor current with variable frequency. Figure 1.6 shows the schematic representation of the variable speed wind turbine with DFIG. Figure 1.6 Schematic representation of the variable speed wind turbine with doubly fed induction generator Direct drive synchronous generator In direct drive systems, the gear ratio is equal to one, which means the rotor of the wind turbine is directly coupled with the generator. A low speed multi pole synchronous generator with the same rotational speed as the wind turbine rotor converts the mechanical energy into electricity. The generator can have a wound rotor or a rotor with permanent magnets. The stator is not coupled directly to the grid but to a power electronic converter. This may consist of a back-to-back voltage source converter or a diode rectifier with a single VSC. The power electronic converter makes it possible to operate the wind turbine at variable speed.

41 11 Figure 1.7 shows the schematic representation of the variable speed wind turbine with direct drive electrically excited synchronous generator (EESG). Figure 1.7 Schematic representation of the variable speed wind turbine with direct drive electrically excited synchronous generator EESGs are salient pole machines and are excited from power grid. For low speed operation, high pole count synchronous generators are recommended. With EESGs, over excitation is easily possible. So, operation at unity power factor is utilized to reduce machine side inverter to the real power value. High pole count increase the field ampere turns which leads to increase of excitation losses. PMSGs are excited by the converters, not from power grid. PMSGs eliminate the excitation losses which lead to an increase in efficiency and reduce thermal problems on the rotor side. No brushes and slip rings are necessary, thus reduces the maintenance costs (Andreas Binder et al 2005). Figure 1.8 shows the schematic representation of the variable speed wind turbine with direct drive permanent magnet synchronous generator.

42 12. Figure 1.8 Schematic representation of the variable speed wind turbine with direct drive permanent magnet synchronous generator 1.6 POWER ELECTRONIC CONVERTERS IN WIND ENERGY CONVERSION SYSTEMS In variable speed drives, the changes in the rotational speed of the rotor directly affects the rotational speed of the generator input shaft during variable speed operation. This situation gives rise to the variable frequency problem. The frequency of the grid is stable with a close range of variation at 50 Hz. The frequency of a generator is determined by the number of poles of the generator and the rotational speed of the generator input shaft. The variable frequency problem can be solved by employing power electronic converters between the generator and the grid. These power electronic converters are simply a rectifier that converts the alternative current (AC), which has unstable frequency, to direct current (DC) and an inverter, which converts DC to AC with stable frequency. Figure 1.9 shows the power electronic converters used in wind energy conversion systems. Figure 1.9 Power electronic converters in wind energy conversion systems

43 13 Fixed speed WTGs generally have a capacitor bank for selfexcitation. Power converters such as Static Kramer drive and SCR converter, Back-to-back pulse width modulation (PWM) converter and matrix converters are generally used in DFIGs(Jamal A Baroudi et al 2007). An AC/DC/AC IGBT-based PWM converter is used in this work. It has standard rotor speed control and voltage control. The grid side converter controls the transfer of real and reactive power between the grid and the DC link. A constant DC link is maintained. Power converters such as thyristor supply-side inverter, hardswitching supply-side inverter, intermediate DC/DC converter and back-toback PWM converters are generally used in direct drive synchronous generators (DDSGs). For direct drive EESG, rectifier and voltage source converter (VSC) circuit with LC harmonic filter combination is used. This diode rectifier converts ac power generated by the wind generator into dc power in an uncontrollable way. Current-controlled VSCs can generate an ac current which follows a desired reference waveform and so can transfer the captured real power along with controllable reactive power. VSC circuit are used. For direct drive PMSG, diode rectifier, DC-DC boost converter and 1.7 IMPACT OF WIND TURBINE GENERATORS ON POWER SYSTEM PERFORMANCE An important issue when integrating large scale wind farms is the impact on the system stability and transient behavior. System stability is largely associated with power system faults in a network such as tripping of

44 14 transmission lines, loss of production capacity (generator unit failure) and short circuits. These failures disrupt the balance of power (active and reactive) and change the power flow. Though the capacity of the operating generators may be adequate, large voltage drops may occur suddenly. The unbalance and re-distribution of real and reactive power in the network may force the voltage to vary beyond the boundary of stability. A period of low voltage (brownout) may occur and possibly be followed by a complete loss of power (blackout). Many of power system faults are cleared by the relay protection of the transmission system either by disconnection or by disconnection and fast reclosures. In all the situations, the result is a short period with low or no voltage followed by a period when the voltage returns. A wind farm nearby will see this event. In early days of the development of wind energy, only a few wind turbines were connected to the grid. In this situation, when a fault somewhere in the lines caused the voltage at the wind turbine to drop, the wind turbine was simply disconnected from the grid and was reconnected when the fault was cleared and the voltage returned to normal. Because the penetration of wind power in the early days was low, the sudden disconnection of a wind turbine or even a wind farm from the grid did not cause a significant impact on the stability of the power system. With the increasing penetration of wind energy, the contribution of power generated by a wind farm can be significant. If the entire wind farm is suddenly disconnected at full generation, the system will lose further production capability. Unless the remaining operating power plants have enough spinning reserve, to replace the loss within very short time, a large frequency and voltage drop will occur and possibly followed by complete loss of power. Therefore, the new generation of wind turbines is required to be able to ride through during disturbances and faults to avoid total disconnection

45 15 from the grid. In order to keep system stability, it is necessary to ensure that the wind turbine restores normal operation in an appropriate way and within appropriate time. This could have different focuses in different types of wind turbine technologies, and may include supporting the system voltage with reactive power compensation devices, such as interface power electronics, SVC, STATCOM and keeping the generator at appropriate speed by regulating the power etc. 1.8 OBJECTIVES OF THE RESEARCH WORK 1. To model and simulate a variable speed direct drive EESG model with diode rectifier and d-q current controlled VSC and a variable speed direct drive PMSG model with pitch angle control, diode rectifier, MPPT controlled DC-DC boost converter and adaptive hysteresis current controlled VSC for constant DC link voltage. 2. Investigating the performance of fixed speed SCIG, variable speed DFIG with standard control, modeled variable speed direct drive EESG and PMSG with modified controllers on voltage stability by analyzing the loading margin, voltage collapse, voltage magnitude and reactive power delivered and compare with CSGs. 3. Investigating the performance of fixed speed SCIG, variable speed DFIG with standard control, modeled variable speed direct drive EESG and PMSG with modified controllers on transient stability by analyzing CCT, rotor angle swing, rotor speed deviation, active and reactive power support and maintaining voltage magnitude and compare with CSGs. 1.9 ORGANISATION OF THE THESIS This thesis is organized into six chapters namely; the introduction, literature review, modeling and simulation of variable speed direct drive synchronous generators, impact of WTGs on voltage stability of power

46 16 system, power system transient stability in the presence of WTGs and summary of findings and conclusion. The summary of each chapter is given below: Chapter 1 explains about the energy conversion from wind, control of wind turbine with stall and pitch control, operating characteristics of wind turbine, fixed and variable speed WTGs, power electronic converters in WECSs and impact of WTGs on power system performance. It also presents the objectives of this research work in detail. Chapter 2 discusses the summary of literature review undertaken in modeling of fixed and variable speed WTGs, various converter topologies for WTGs, control strategies and impact of variable speed WTGs on power system stability. Chapter 3 describes the modeling of fixed speed SCIG and variable speed DFIG, special features of variable speed direct drive synchronous generators, modeling of direct drive EESG, modeling of direct drive PMSG, modeling of full-wave diode bridge rectifier, modeling and control of DC-DC boost converter, modeling of VSC, d-q current control of VSC, adaptive hysteresis band current control of VSC. This chapter also discusses the simulation results of modeled variable speed direct drive EESG and PMSG with modified controllers and presents a summary of results. Chapter 4 describes the voltage stability analysis using PV curve and loading margin, voltage stability analysis including fixed speed SCIGs, variable speed DFIGs, variable speed direct drive EESGs and PMSGs. This chapter also presents the simulation results of voltage stability by computing loading margin, voltage vs. time curve after the contingency and voltage

47 17 profile with CSGs alone first and then with WTGs and presents a brief summary of results. Chapter 5 describes the transient stability analysis, how the transient stability computed with CCT, rotor angle deviation, rotor speed oscillation, active power support and reactive power support, transient stability in the presence of WTGs. Also presents the simulation results with CSGs alone first and then with WTGs and presents a brief summary of results. Chapter 6 summarises the research findings and significant research contributions. Also identifies some area for future research work and concludes the thesis. are given. At the end of the thesis, a list of relevant references and appendices

48 18 CHAPTER 2 LITERATURE SURVEY 2.1 INTRODUCTION Wind power is one of the fastest growing electricity generation sources with a 20% annual growth rate for the past 10 years. The vast majority of wind turbines that are currently installed use one of the three main types of electromechanical conversion system: Squirrel cage induction generator, Doubly fed induction generator and Direct drive synchronous generator. Often, they are directly connected to the transmission grid and will, sooner or later, replace conventional power plants. Such wind farms will be expected to meet very high technical requirements, such as to perform frequency and voltage control, to regulate active and reactive power and to provide quick responses during transient and dynamic situations in the power system. This chapter reviews the recent publications in dynamic models of WTGs, power electronic converters for WTGs and the impact of WTGs on the performance of power system. 2.2 MODELING OF WIND TURBINE GENERATORS A wide variety of wind turbine technologies are in use today. A typical wind turbine employs a blade and hub rotor assembly to extract power from the wind, a gear train to step up the shaft speed at the slowly spinning rotor to the higher speeds needed to drive the generator, and an induction generator as an electromechanical energy conversion device. Induction machines are popular as generating units due to their asynchronous nature, since maintaining a constant synchronous speed in order to use a synchronous generator is difficult due to variable nature of wind speed. Power electronic

49 19 converters are used to regulate the real and reactive power output of the turbine. A wind farm typically consists of a large number of individual WTGs connected by an internal electrical network. In the near future, wind turbines may start to influence the behavior of electric power systems by interacting with conventional synchronous generators (CSGs) and loads. To study the impact of wind farms on the dynamics of the power system, an important requirement is to develop appropriate wind farm models to represent the dynamics of many individual WTGs. Wind turbine models that can be integrated into power system simulation software need to be developed.(slootweg J G et al 2003) presented a general model of wind speed, rotor and rotor speed controller, generator/converter, pitch angle controller, voltage controller and protection system used to represent all types of variable speed wind turbines in power system dynamics simulations. Also, it has been shown experimentally that in variable speed wind turbines, the shaft properties are hardly reflected at the grid connection due to the decoupling effect of the power electronic converters. Dynamic models of wind farms with fixed speed WTGs are presented in various literatures. A typical fixed speed SCIG employs a capacitor bank arrangement. Some literatures have modeled fixed speed SCIGs with control strategies also. (Mihet-Popa L et al 2004) presented a fixed speed SCIG model which uses an alternative control strategy, where the rotational speed is the controlled variable and it is tested during normal operation and transient grid fault events. (Wei Qiao et al 2007) explained a detailed model and three reduced order equivalent model of fixed speed SCIG and explained about how to choose an appropriate wind farm model for power system dynamic and transient studies.

50 20 Variable speed DFIGs are generally more complex and expensive than fixed speed SCIGs. DFIGs have independent active real and reactive power control. DFIGs have some advantages over full converter machines as well. DFIGs are to be rated for 30% output power of the generator, thus decreasing the cost relative to DDSGs. DFIGs have started to influence the behavior of electrical power systems. Detailed DFIG models to study the impact of wind turbines on electrical power system behavior are needed. (Slootweg J G et al 2001) simulated a DFIG model equipped with rotor speed, pitch angle and terminal voltage controllers. Grid codes demand complete models and simulation studies to avoid the detrimental impact on the network while connecting WTGs. (Ekanayake J B et al 2003) developed a dynamic model with reduced order double cage representation for the DFIG and its associated control and protection circuits which is suitable for inclusion in large power system transient stability programs. (a) A high proportional gain in the rotor converter limited the rotor current during the fault to a level below the trip setting of the crowbar circuit and (b) fast-acting reactive power control (applied through either converter) improved the stability of the generator. Voltage control using the rotor side converter is likely to be preferred to using the network side converter for this task. This is mainly because of the reduction in the converter rating requirement as reactive power injection through the rotor circuit is effectively amplified by a factor of 1/slip. A DFIG model which has the form of traditional generator model and hence is easy to integrate into the power system is developed by (Yazhou Lei et al 2006). In this, the power electronic converter is simulated as a

51 21 controlled voltage source, regulating the rotor current to meet the requirement of real and reactive power production. The dynamic behavior of WTGs is quite different from that of CSGs. It is to be expected, therefore, that the dynamic performance of power systems may change as traditional generation is displaced by ever increasing number of WTGs. Dynamic interactions of the models of converter control, pitch control and the wind turbine are analyzed by (Ian A Hiskens 2012) and is governed by interactions between the continuous dynamics of state variables, and discrete events associated with limits. Switching hysteresis is proposed for eliminating deadlock situations created by interactions. The dynamic characteristics of this WTG that are important from the grid perspective are dominated by the response of controllers that regulate active power, pitch angle and terminal voltage. To analyse the dynamic performance and grid impact analysis capability of VSWT system with an EESG, (Seul-Ki Kim et al 2007) proposed a model of EESG with fixed-pitch stall regulated wind turbine, diode rectifier and a six-igbt VSC with controllable power inverter strategy intended for capturing the maximum energy from varying wind speeds and maintaining reactive power generation at a pre-determined level for constant power factor or voltage regulation. Direct drive PMSG plays an important role in the modern wind generating systems. The PMSG model described in (Ming Yin et al 2007) includes pitch angle control and a drive train. PMSG model was established in the d-q synchronous rotating reference frame. The pitch angle control in wind turbine model used wind speeds and electric power output as the input signals to ensure normal operation in high wind speed. The speed control is realized through field orientation where the d-axis current is set to zero and the q-axis

52 22 current is used to control the rotational speed of the generator according to the variation of wind speed. An aggregate model reduces the simulation time without significantly compromising the accuracy of the results in comparison to the detailed model during transient interaction between a large wind farm and a power system. (Conroy J et al 2009) modeled an aggregate PMSG wind farm which employs a braking resistor in the DC circuit to satisfy the latest grid code requirements. This system is relevant for transient stability studies of large-scale systems. (Rolan A et al 2009) modeled the wind speed, wind turbine and drive train of a variable speed direct drive PMSG. The maximum power point tracking (MPPT) concept utilized here adjusts the generator rotor speed according to instantaneous wind speed. (Cultura A B et al 2011) developed a PMSG model with diode rectifier, boost dc to dc converter and inverter. A reliable and speedy simulation of the PMSG is significant which is achieved by (Junfei Chen et al 2012). They replaced the PMSG s power electronic device with math equivalence and developed models of windmill, PMSG and its control, VSC, dc-line, filter and grid. Control includes MPPT, independent active and reactive power control and variable speed constant frequency operation. The control scheme in (Ziping Wu et al 2012) comprised of MPPT and double PWM active/reactive power independent control strategy. A DC-link over-voltage protection scheme is also designed. This model possessed desirable capabilities of operation at the maximum power point as well as enhanced low voltage ride through (LVRT) function.

53 CONVERTER TOPOLOGIES FOR WIND TURBINE GENERATORS The WTG system requires a power conditioning circuit called power converter that is capable of adjusting the generator frequency and voltage to the grid. Several types of converter topologies have been developed in the last decades; each of them have some advantages and disadvantages. Most of the proposed converters require line filters and transformers to improve the power quality and step-up the voltage level, respectively. These heavy and bulky components significantly increase the tower construction, and turbine installation and maintenance costs. Recent advances in power semiconductors and magnetic materials have led to the development of new topologies of converters, which would be a possible solution to reduce the size, weight, and cost of power converters. Several types of converter topologies have been developed in the last decades. They are: diode rectifier based converter, back to back converter, matrix converter, Z-source converter, improved Z-source converter, cycloconverter, and multilevel converter. (Jamal A Baroudi et al 2007) provided a comprehensive review of past and present converter topologies applicable to PMSGs, IGs, EESGs and DFIGs. The different generator converter combinations are compared on the basis of topology, cost, efficiency, power consumption and control complexity. (Kawale Y M et al 2009) carried out an analysis to test the behavior of PMSG with different converter topologies. (Jamil M et al 2012) presented a review of recent and past converter topologies on PMSGs.

54 24 (Md Rabiul Islam et al 2013) conducted a comprehensive study of power converter technologies, current research and development. Mainly two converter topologies are currently used in the commercial WTG systems. They are: diode rectifier based converter and back to back converter. In diode rectifier based converter, variable frequency and variable magnitude AC power from the WTG is converted to a DC power by a diode rectifier circuit and then converted back to an AC power at different frequency and voltage level by a controlled inverter. The diode rectifier based converter system transfers power in a single direction e.g. from generator to the grid. This type of power converter is normally used in an EESG or a PMSG based WTG system instead of an induction generator (IG). In EESG based system, to achieve variable speed operation, the systems use an extra excitation circuit, which feeds the excitation winding of EESG. The PMSG based WTG systems are equipped with a step-up chopper circuit. The step-up chopper adapts the rectifier voltage to the DC-link voltage of the inverter. Controlling the inductor current in the step-up chopper can control the generator torque and speed. In this converter system, the grid side converter (GSC) controls the active and reactive power delivered to the grid. The back to back converter consisting of controlled rectifier and controlled inverter based converter. The controlled rectifier gives the bidirectional power flow capability, which is not possible in the diode rectifier based power conditioning system. Moreover, the controlled rectifier strongly reduces the input current harmonics and harmonic losses. The grid side converter enables to control the active and reactive power flow to the grid and keeps the DC-link voltage constant. The generator side converter works as a driver, controlling the magnetization demand and the desired rotor speed of the generator. The decoupling capacitor between GSC and rotor side converter (RSC) provides independent control capability of the two converters. The back

55 25 to back converter can be used for PMSG and SCIG based wind power generation systems. 2.4 CONVERTER CONTROL STRATEGIES Wind energy, even though abundant, varies continually as wind speed changes throughout the day. The amount of power output from a WECS depends upon the accuracy with which the peak power points are tracked by the MPPT controller of the WECS irrespective of the type of generator used MPPT Control A concise review of MPPT control methods proposed in various literatures for controlling WECS with various generators have been presented in (Jogendra Singh Thongam et al 2011). The maximum power extraction algorithms researched so far can be classified into three main control methods, namely TSR control, power signal feedback (PSF) control and hill-climb search (HCS) control. The TSR control method regulates the rotational speed of the generator in order to maintain the TSR to an optimum value at which power extracted is maximum. This method requires both the wind speed and the turbine speed to be measured or estimated in addition to requiring the knowledge of optimum TSR of the turbine in order for the system to be able to extract maximum possible power. In PSF control, it is required to have the knowledge of the wind turbine s maximum power curve, and track this curve through its control mechanisms. The maximum power curvesneed to be obtained via simulations or off-line experiment on individual wind turbines. In this method, reference power is generated either using a recorded maximum power curve or using the

56 26 mechanical power equation of the wind turbine where wind speed or the rotor speed is used as the input. The HCS control algorithm continuously searches for the peak power of the wind turbine. It can overcome some of the common problems normally associated with the other two methods. The tracking algorithm, depending upon the location of the operating point and relation between the changes in power and speed, computes the desired optimum signal in order to drive the system to the point of maximum power. (Kesraoui M et al 2010) proposed a variable speed PMSG with gear box, a diode bridge rectifier, a MPPT controlled dc-to-dc boost converter and a current controlled VSC. The MPPT extracts maximum power from the wind turbine from cut-in to rated wind velocity by sensing only dc link power. This MPPT is an advanced HCS control called as step and search control which senses V DC alone and controls the same is utilized in this thesis. The effectiveness of the WECS can be greatly improved, under grid fault, by using an appropriate control. (Errami Y et al 2013) proposed a control strategy which combines MPPT and a pitch control scheme to maximize the generated power. This effective control strategy not only captures the maximum wind energy, but also maintained the frequency and amplitude of the output voltage Control of Voltage Source Converter The utilization of VSC for the interconnection of WECS to the grid requires application of control systems capable of regulating the active and reactive output current, ensuring high power quality levels and achieving high immunity to grid perturbations.

57 27 The VSC control ensures that the strict power quality standards (frequency, power factor, harmonics, flicker, etc) are met. In the case of a grid fault, the WECS should remain connected; thus they should cope with sudden and important loads, and even assist the grid in voltage or frequency control. The increasing requirements for WECS to remain connected and to provide active grid support have added stringent control objectives for the power converters. The hysteresis band current control technique has proven to be most suitable for all the applications of current controlled VSCs (Murat Kale et al 2005). The hysteresis band current control is characterized by unconditioned stability, very fast response, and good accuracy. On the other hand, the basic hysteresis technique exhibits also several undesirable features; such as uneven switching frequency that causes acoustic noise and difficulty in designing input filters. The current control with a fixed hysteresis band has the disadvantage that the switching frequency varies within a band because peak-to-peak current ripple is required to be controlled at all points of the fundamental frequency wave. Adaptive Band Hysteresis (ABH) control with phase locked loop (PLL) is based on indirect PQ power control. It is similar with the voltage oriented control (VOC)-PI control strategy. Hysteresis control is known to exhibit high dynamic response as the concept is to minimize the error in one sample. As the typically sampling frequencies are in the range of khz, this means a very high bandwidth. A constant band for the hysteresis comparator leads to variable switching frequency. An on-line adaptation of the band can be done in order to keep the switching frequency quasi-constant. The PLL is used in order to orientate the output of the P and Q controller with grid angle and for grid monitoring.

58 28 Even if this strategy is using a PLL for orientation of the reference currents, it will exhibit improved performances under grid voltage variations due to the higher bandwidth of the current controller that help in order to keep the currents under the trip limits. An adaptive hysteresis band current controller proposed in (Murat Kale et al 2005) for active power filter changes the hysteresis bandwidth according to modulation frequency, supply voltage, dc capacitor voltage and slope of the reference compensator current wave. Hysteresis band can be modulated as a function of V and m so that the modulation frequency remains nearly constant. (Giraldo E et al 2014) proposed an adaptive control strategy for a WECS based on PWM- current source converter (CSC) and PMSG. Reactive power is generated according to the capacity of the converter, the wind velocity and the load profile. It uses an adaptive PI which is self-tuned based on a linear approximation of the power system calculated at each sample time. 2.5 IMPACT OF WIND TURBINE GENERATORS ON POWER SYSTEM STABILITY With the scenario of wind power constituting up to 20% of the electric grid capacity in the future, the need for systematic studies of the impact of WTGs on both voltage stability and transient stability of the grid has increased. (Milano F 2002) used a power system analysis (PSAT)/Matlab toolbox for electric power system analysis and control. PSAT includes power flow, continuation power flow, optimal power flow, small signal stability analysis and time domain simulation. All operations can be assessed by means of

59 29 graphical user interfaces (GUIs) and a simulink-based library provides a user friendly tool for network design. The impact of WTGs on the operation of power system stability is discussed in many literatures. In (Muljadi E et al 2008), voltage stability is analysed by using (P-V) curves of the system at the point of interconnection (POI) for the base case as well as for contingencies.also they analysed the transient stability with and without the wind power plant (WPP). Under the condition of sudden short circuit disturbance, (Hosaka N et al 2008) investigated grid voltage characteristics with IG, PMSG and found that PMSG gives a better voltage support performance both during and after the fault. In (Devaraj D et al 2011), loading margin, voltage collapse and voltage magnitude are examined for investigating the long term voltage stability. They examined the system with SCIG, DFIG and DDSG and found that DDSG has the potential to improve the long term voltage stability of the grid by injecting reactive power. Some authors have introduced modified controllers in WTGs and analysed its performance. (Nayeem Rahmat Ullah et al 2007), (Shu J et al 2009) and (Rahimi M et al 2010) have introduced modified controllers with DFIGs. (Nayeem Rahmat Ullah et al 2007) introduced variable power factor operation in DFIG with P/Q control in VSC and investigated possible improvements in grid voltage and transient stability by comparing it with fixed-speed SCIG, variable speed full power converter WTG with standard control. (Shu J et al 2009) introduced a controller in a DFIG WECS which consists of two control loops. In steady state, the main control loop ensures

60 30 that the wind power generators without wind speed measurement can perform active power control tasks below the nominal wind speed while the auxiliary stability control loop restraining the wind power system oscillations by eliminating the system unbalancing energy during system disturbances. The control strategy adjusted the power of the DFIG WECS accurately under wind speed fluctuation and when a disturbance occurs in the system, the strategy is effective in improving the power system transient stability in different operating conditions without deteriorating the system voltage stability. (Rahimi M et al 2010) proposed a nonlinear control scheme applied to the GSC of DFIGs. It stabilized the internal dynamics and limits the dc-link voltage fluctuations during the fault. It also introduced a coordinated control of RSC and GSC to improve the LVRT capability. The proposed ride-through approaches limit the peak values of rotor current and dc-link voltage at the instants of occurring and clearing the fault. They also limited the oscillations of electromagnetic torque, and consequently, improved the DFIG voltage dip behavior. This system also has a stator damping resistor which is used to limit the rotor inrush current and to reduce the oscillations and settling time of DFIG transient response during the voltage dip. Also, the GSC is controlled to limit the dc-link overvoltage during the voltage drop. It is found that the dynamics of the GSC and dc-link voltage exhibit non minimum phase behavior, and thus there is an inherent limitation on the achievable dynamic response during the fault. (De Rijcke S et al 2012) proposed voltage control and reactive power support with DDSGs and revealed that preferred mode for voltage support during a voltage dip depends on the grid characteristics, short-circuit power and X-R ratio. Also, it is found that the angle stability of induction motor loads and nearby CSGs could be improved by adding reactive power support by these DDSGs.

61 31 (Londero R et al 2014) presented two control strategies for fully rated converter (FRC) and DFIG. One control strategy is with GSC at unity power factor (UPF), which is usually adopted, and the second control strategy is GSC controlling reactive power. They have considered wind turbine capability curves and its variable limits, since they are subjected to several limitations that changes with the operating point and wind speed. They also considered the dynamic models of over excitation limiter (OEL) and on-load tap changers (OLTC) combined with static and dynamic loads using time domain simulations. Different penetration levels of wind generation are analyzed. It is found that long-term voltage stability could be improved when GSC of DFIG is controlling reactive power. It is concluded that capability curve plays an important role in this analysis since reactive power is a key requirement to maintain voltage stability. (Revel G et al 2014) exploited the ability of PMSG WECS to rapidly modify its active and reactive power and provide additional support to the power grid and enhance the overall stability of the system. Three control loops are incorporated to achieve supporting tasks such as short-term frequency regulation, oscillation damping and voltage regulation. Recent grid codes require the wind farms not only to ride through the fault disturbances but also support the stability of nearby grid during severe network disturbances. (Mokui H T et al 2012) proposed various operational strategies i.e. without reactive power support, considering reactive power support complying with the Danish grid codes (with and without considering overloading of the converter currents). Control strategies enabled the DDSGs to inject the required reactive power in order to help stabilize the nearby fixed speed SCIGs during faults.

62 32 The impact of WTG on transient stability is investigated by a number of authors: (Muyeen S M et al 2007), (Djemai Naimi et al 2008), (Folly K A et al 2009) and (Nanou S et al 2011). Transient stability analysis using six-mass, three-mass and twomass drive train models have not been reported sufficiently in the literature. The (Muyeen S M et al 2007) have examined the effects of inertia constant, spring constant and damping constant on stability using the above mentioned drive train models and concluded that two-mass shaft model is sufficient for the transient stability analysis of WTGs. (Djemai Naimi et al 2008) investigated the angular stability by using critical clearing time (CCT) by replacing the power generated by fixed speed SCIG by variable speed DFIG, increasing gradually a rate of wind power penetration and changing the location of wind resources. (Folly K A et al 2009) analysed the transient stability by connecting fixed speed SCIG, variable speed DFIG and DDSG to a power system network. They analysed the system under two scenarios: CSG without an automatic voltage regulator (AVR), CSG equipped with an AVR. In the first case, fixed speed SCIG performed poorly and contributed negatively to the transient stability of the power system's network as compared to both the variable speed DFIG and DDSG. In the second case, level of penetration of the SCIG was increased without losing its stability. (Nanou S et al 2011) examined DDSG WTG equipped with GSC control strategy and found that transient stability can be further improved if the power electronic converters can withstand an additional amount of reactive current during low voltage conditions. They also found that, if the wind farm active and reactive power injection is reconfigured in order to satisfy typical

63 33 present-day grid codes, transient stability margins can be significantly improved. 2.6 ISSUES IDENTIFIED Identified that the positive point behind using IGs in WECS is that it has no synchronization problem with grid. While integrating with grid, it works as an induction motor with positive slip and draws electrical energy from grid and after it captures the wind speed and starts rotating more than the rotating magnetic field of stator ie under negative slip, instead of consuming electrical energy, it starts delivering the electrical energy to grid. Therefore the synchronization difficulty of interconnecting direct drive synchronous generators with grid is completely avoided with IGs. Identified that the DFIG equipped wind turbine is currently the most popular one due to its capability of controlling reactive power, high energy efficiency, and the fact that the converter rating of appropriately 20% - 30% of the total machine power is needed. Direct drive wind turbine is the second most common WT in the MW range in the market while the most common is the one based on the geared drive DFIG WT. However, the direct drive WT will replace the DFIG in the near future. Recently, larger systems use EESGs. In comparison to IG, the use of SG is advantageous since they are self-excited machines and the pole pitch of the machine can be smaller. From the literature review, it is identified that the modeling of EESGs has not been reported sufficiently in the literature. In this work, variable speed direct drive EESG and PMSG with modified controllers are modeled. Various converter topologies and different control schemes have been analysed in detail in the literature. At the generator side,

64 34 diode rectifier is a widely used and accepted option. Both direct drive EESG and PMSG can utilized this diode rectifier. The use of MPPT techniques would cost more than a simple lookup table method. However, higher order control and converter designs increase efficiency of the overall system. The inclusion of a DC-boost stage helps reduce the control complexity of the grid inverter with a small increase in cost. In order to maximize the benefits of the WECS, a compromise between efficiency and cost must be made. Thus direct drive PMSG model with MPPT and DC-DC boost converter can be developed. Adaptive hysteresis band current control is a known technique. But, majority of the literatures have demonstrated the suitability of adaptive hysteresis band current control of VSC in active power filters. The adaptive hysteresis band current control of VSC can be utilized for WECS also to maintain a constant DC link voltage and hence to improve its capability in injecting reactive power to enhance the stability of the system. It is also identified that only a few literatures have compared the performance of impact of WTGs with CSGs. This work attempts to compare the performance of different WTG categories such as fixed speed SCIG, variable speed geared drive DFIG with standard control and variable speed direct drive EESG and PMSG with the CSGs. 2.7 CONCLUSION This chapter has reviewed the various models proposed in the literature for fixed speed, variable speed geared drive and direct drive WTGs suitable for dynamic analysis. The various converter topologies proposed in the literature for WTGs are also discussed. Further various MPPT control algorithms, current control algorithms for VSC are also reviewed. The papers

65 35 which discuss impact of WTGs on power system voltage stability and transient stability are also analysed. Finally, various technical issues to be addressed with respect to modeling and impact analysis are also reported in this chapter.

66 36 CHAPTER 3 MODELING AND SIMULATION OFWIND TURBINE GENERATORS 3.1 INTRODUCTION In many Countries, there is a tendency towards increasing the amount of electricity generated from wind turbines. Hence, wind turbines will start to replace the output of conventional generators. As a result, it may begin to affect the overall behavior of the power system. Hence the impact of wind power on the dynamics of power system should be investigated. This chapter develops mathematical models suitable for analyzing the impact of variable speed wind turbine (VSWT) on the dynamics of power systems. The variable speed WTGs commonly used are geared drive DFIG, direct drive EESG and PMSGs. MATLAB/Simulink is used to simulate the developed models and the simulation results obtained under different operating conditions are presented. 3.2 MODELING OF INDUCTION GENERATORS This section presents the equivalent circuit offixed speed SCIGand variable speed DFIGsuitable for analyzing the impact of WTG on stability studies (Andreas Petersson 2005). Figure 3.1 shows the equivalent circuit of fixed speed SCIG and Figure 3.2 shows the equivalent circuit ofvariable speed DFIG.

67 37 Figure 3.1 Equivalent circuit of fixed speed squirrel cage induction generator Figure 3.2 Equivalent circuit of variable speed generator doublyfed induction V R I j ω L λ I j ω L I I I V s R s I j ω L λ I j ω L I I I 0 R I j ω L I I I ω ω ω ω ω where,v is thestator voltage, V is therotor voltage,r is thestator resistance, R is therotor resistance, I is thestator current,i is therotor current,r is themagnetizing resistance, L is themagnetizing inductance,i is themagnetizing resistance current, L λ is thestator leakage inductance, L λ is therotor leakage inductance, ω is thestator frequency,ω is the slip frequency,ω is the rotor speed and sis theslip.

68 VARIABLE SPEED WIND TURBINE WITH DIRECT DRIVE SYNCHRONOUS GENERATORS In direct drive synchronous generators, rotor and generator shafts are mounted to the same shaft without gear-box. The synchronous generator is designed with more number of poles for low speed operation. Synchronous generators are suitable for high capacities. Higher speed operation create no problems other than difficulties in manufacturing of synchronous generators with large capacities. It can be utilized independently. The output voltage of the synchronous generator terminals can be regulated. The power factor of the front and rear phases and the reactive power can be controlled. No impact is generated during paralleling connection to the network. It can either be an EESGor a PMSG. For a large DDSG, with a diameter of several meters, the air-gap should not exceed few millimeters, to avoid excessive magnetization requirements. Direct drive generator has a large diameter to produce higher torque because the torque is proportional to the square of air gap diameter. Absence of gearbox consequently decrease the operation and maintenance cost. Moreover, direct drive systems are more reliable and operate for a relatively longer time with fewer problems due to the reduced complexity. In order to produce electricity at the desired frequency in low rotational speeds; the pole number of the generators in direct drive systems is high. Hence, the generators used in these systems are bigger, heavier and more expensive. For allowing variable speed operation, the synchronous generator must be connected to the grid through a frequency converter. Figure 3.3 shows the principal arrangement of a direct drive synchronous generator.

69 39 Figure 3.3Direct drive synchronous generator The major components in frequency converters are diode rectifier, dc link and pulse-width modulated inverter. The generator is connected to an intermediate DC-circuit by a diode rectifier. The grid-side connection is realized by a self commutated PWM converter that imposes a pulse-width modulated voltage to the AC-terminal. The PWM converter is connected to the network through a filter, symbolized by the L-C circuit. The level of harmonics in the voltage at the connection point is extremely low Direct drive EESG The block diagram representation of the variable speed direct drive EESGis shown in Figure 3.4.The system includes a fixed-pitch, stall regulated wind turbine, an EESG and a controllable power electronics system, which consists of a six-diode rectifier and three phase VSCwith d-q current control. Figure 3.4 Block diagram representation of the Variable Speed Direct Drive EESG

70 40 High pole count in EESGs increases the field ampere turns. So exciting ampere turns yield an increase of excitation losses. With increased pole count inpmsgs, due to lower flux per pole, the danger of demagnetization decreases; hence smaller magnets and hence reduced costs are possible for high pole count machines Direct Drive PMSG The block diagram representation of the variable speed direct drive PMSGis shown in Figure 3.5.The system consists of a pitchable wind turbine, a PMSG, a passive rectifier, a MPPT controlled dc-to-dc boost converter and an adaptive hysteresis band current controlled VSC. Figure 3.5 Block diagram representation of the Variable Speed Direct Drive PMSG PMSGs have several advantages overeesgs. They are: Higher power to weight ratio Improvement in efficiency High energy and light weight No additional power supply for the field excitation Higher reliability without slip rings.

71 41 PMSGs are still considerably more expensive and require more advanced rectifiers because they won t allow for reactive power or voltage control. PMSGs with light weight and low cost are most suitable for applications inwecs. 3.4 MODELING OFDIRECT DRIVESYNCHRONOUS GENERATORS This section presents the mathematical modeling ofdirect drive synchronous generatorssuitable for analyzing the impact of WTG on stability studies Modeling of EESG The EESGmodel takes into account the dynamics of the stator, field, and damper windings. The equivalent circuit of the model is represented in the rotor reference frame (q-d frame). All rotor parameters and electrical quantities are viewed from the stator. They are identified by primed variables. The subscripts used in model are defined as follows: d,q: d and q axis quantity R,s: Rotor and stator quantity l,m: Leakage and magnetizing inductance f,k: Field and damper winding quantity The electrical model of the EESG is shown in Figure 3.6and the related equations are given below:

72 42 Figure 3.6 Electrical model of the EESG V R i Ψ Ψ (3.1) V R i Ψ Ψ (3.2) where,v and V are the d and q axis voltages, R is the stator resistance, i and i are the d and q axis currents,ψ and Ψ are the d and q axis flux linkages and is the angular frequency of rotor.this model assumes currents flowing into the stator windings Modeling of PMSG The mode of operation ofpmsg is dictated by the sign of the mechanical torque (positive for motor mode, negative for generator mode). The electrical and mechanical parts of the machine are represented by aseparate second-order state-space model. The sinusoidal model assumes that the flux established by the permanent magnets in the stator is sinusoidal, which implies that the electromotive forces are sinusoidal. The block implements the following equations.

73 43 These equations are expressed in the rotor reference frame (d-qframe). All quantities in the rotor reference frame are referred to the stator. i v i pω i (3.13) i v i pω i (3.14) T 1.5p i L L i i (3.15) where, L is the d-axis inductance,l is the q-axis inductance,λ is the amplitude of the flux induced by the permanent magnets of the rotor in the stator phases, p is the number of pole pairs and T is the electromagnetic torque. 3.5 MODELING AND CONTROL OF POWER ELECTRONIC CONVERTERS Full-wave Diode Bridge Rectifier The output voltage of synchronous generator is rectified using a three-phase passive bridge rectifier. This rectifier consists of a three-phase diode bridge, comprising diodes D1 tod6 which converts ac power generated by the wind generator into dc power in an uncontrollable way. Figure 3.7 shows the three-phase diode bridge rectifier(pejovicp 2007). Figure 3.7Three-phase Diode bridge rectifier

74 44 In the analysis, it is assumed that the impedances of the supply lines arelow enough to be neglected, and that the load current I is constant intime.v,v,v are the phase voltages. The amplitude of the phase voltage V is V V 2 (3.16) where,v is the root-mean-square (RMS) value of the phase voltage.assuming that I is strictly greater than zero during the whole period,in each time point two diodes of the diode bridge conduct. The firstconducting diode is from the group of odd-indexed diodes D, D, D, andit is connected by its anode to the highest of the phase voltages at the timepoint considered. The second conducting diode is from the group ofevenindexed diodes D, D, D, and it is connected by its cathode to thelowest of the phase voltages. The DCcomponent of the output voltage is given by V π V 1.65 V 2.34 V (3.17) The input currents have the same RMS value, given by I I (3.18) The output power of the rectifier is P V I π V I P (3.19) and it is the same as the input power P, since losses in the rectifier diodesare neglected in the analysis DC-DC Boost Converter DC-DC boost converter is a most efficient topology which ensures good efficiency along with low cost. A DC-DC boost converter is connected next to full-wave diode bridge rectifier to raise the voltage of the diode rectifier. A capacitorc1 is connected across rectifier to lessen the variation in

75 45 the rectified AC output voltage waveform from the bridge. Figure 3.8shows the arrangement of the DC-DC boost convertercircuit. Figure 3.8 DC-DC boostconverter circuit The model of the boostconverter is needed to simulate and analyzethe behavior. The input and output voltage of the boostconverter under an ideal condition can be related as V V 1 D (3.20) V isthe input voltage, V is the output voltage and Dis duty cycle.given the value of D, it is possible to find the minimum values for inductance and capacitance using the equations given below. L (3.21) C (3.22) where,v is the ripple voltage, R is theoutput resistance and f is the switching frequency. An important consideration in DC-DC converters is the use of synchronous switching which replaces the flywheel diode with a power IGBT with low "On" resistance, thereby reducing switching losses. This is achieved by using a PWM switched mode control design orpwm. The PWM performs the control and regulation of the total output voltage.

76 46 If the semiconductor device is in the off-state, its current is zero, and hence, its power dissipation is zero. If the device is in the on-state, the voltage drop across it will be close to zero, and hence, the dissipated power will be very small. The most common strategy for controlling the power transmitted to the load is the PWM. A control voltage is compared to a triangular voltage. The triangular voltage determines the switching frequency. The switch T is controlled according to the difference of the voltage. The variation of the voltage across the inductance L and the current through the capacity depend on the operating phase. The two operating phases are: a. T state-on and D state-off During this time period, the equivalent diagram of the circuit is presented in Figure 3.9(a). In this time period the inductance L stores energy. Figure 3.9(a) Equivalent circuit of the DC-DC converter in the first operating phase During this period, the output voltage u and the current through the inductor i satisfies the following equations: ; (3.23) b. T state-off and D state-on In the moment when the transistor switch in OFF state, the voltage across the inductor will change the polarity and diode will switch in ON state. The equivalent diagram of converter during this period is shown in Figure 3.9(b).

77 47 Figure 3.9(b) Equivalent circuit of the DC-DC converter in the second operating phase During this period, the output voltage u and the current through the inductor i satisfy the following equations: ; Control of DC-DC Boost Converter i (3.24) The electric power generated by synchronous generatorp is given by(kesraoui M et al 2010) P V I (3.25) where,vis the generator phase voltage and I is the generator phase current. For an ideal system, equation (1.4) and (3.1) can be equated. T ω V I (3.26) I (3.27) where, E is the induced voltage in the armature and R is the stator resistance. Using equations (1.1) to (1.5) and (3.1) to (3.3), P is expressed as P ω V ki ω (3.28) where,i is the field current,ω is the electrical angular speed and V is the generator phase voltage. Maximum power occurs when 0 (3.29)

78 48 The maximum power can be tracked by searching the rectified dc power, rather than environmental conditions, such as wind speed and direction. The step and search control strategy for tracking maximum power is explained below: The step and search control strategy makes use of the fact that the generated voltage and V DC depend upon the speed of the turbine. Therefore, instead of sensing the turbine speed, it senses the V DC and tries to control the same. The set point for this voltage is not constant. That is because the wind speed is varying every now and then which causes the optimum turbine speed to vary frequently. The set point is floating and has to be decided by trial and error method. The method is called Peak seeking. Figure 3.10 shows the step and search control strategy to track maximum power. Figure 3.10Step and search control strategy to track maximum power The strategy is to start with any arbitrary set point (A) i.e. reference dc voltage and check the output dc power. Then give a small increment to the set point. Again check the output at point B. If the output has increased, give an additional increment and check the output once again. Incrementing the set point by small steps should be continued till the stage (H) when the increment does not yield favorable result. At this stage, a small decrement to the set point should be given. The set point will be moving back and forth around the optimum value. Thus the power output could be maximized. In this method, after giving increment to the set point, both the power output as well as the

79 49 voltage level has to be checked. Four possibilities arise. Power increased voltage increased Power increased voltage decreased Power decreased voltage increased Power decreased voltage decreased Only when power output and the voltage are increased (case 1) the set point has to be incremented. If the wind speed changes from one value to another, the turbine is not being operated at the maximum power point at the new value. The MPPT controller has to search for the new maximum power point for the new wind speed. Thus depending upon the MPPT controller output, dc-dc boost converter switch operates and maintains a constant V DC link across the capacitor Co. MPPT control the output power as well as adjust theelectrical torque, the speed of the generator is indirectly controlled and then it obtains theoptimum speed for driving the power to the maximum point Modeling of Voltage Source Converter Figure 3.11 shows the circuit diagram of aigbt based dc- ac single phase full bridge converter.the model of VSCis needed to simulate the circuit and to analyze its behavior. Figure 3.11 Circuit diagram of aigbt based DC- AC Single phasefull bridge Converter

80 50 Figure 3.12PWM signal for a voltage source converter The DC voltage which is obtained from converter output is given to the inverter, for converting it to a smooth sinusoidal waveform. An inductor current flowing through filter and load voltage are considered as state variables. The state variables with S 1 ands 3 ON (d interval) and S 1 ands 3 OFF (1-d interval) are expressed as: v i v v (3.30) v i v v (3.31) The basic operation of the dc ac full-bridge switching converter is that each pair of switches, S1 S3 and S2 S4, are operated alternately for each switching period with their duty cycle (d). The duty cycle (d) is the ratio of the ON time (t on ) to the switching period (T), d= t on /T S = t on f s, as shown in Figure Control of Voltage Source Converter d-qcurrent control Current-controlled VSCs can generate an ac current which follows a desired reference (Chen Z et al2001) waveform and can transfer the captured real power along with controllable reactive power. d-q currentcontrol method is employed to control the VSC.

81 51 Maincontrol targets are the desired real and reactive power, P andq to be followed by actual real and reactive power, P andq.p andq can be calculated by using the following equations (3.32), (3.33) &(3.34). P ηp (3.32) P πρr ω (3.33) λ Q P (3.34) where, is the electrical efficiency of generator and inverter. The desired values are specified according to power control strategy of the VSWT. The strategy is to capture the maximum energy from varying wind speed while maintaining reactive power generation(seul-ki Kim et al 2007). Below the rated wind speed, the real power of the VSWT isregulated to capture the maximum wind energy from varyingwind speed. The maximum power available can be described by(3.33). This simply means that the maximum power is obtained by varying the turbine speed with wind speed such that it is on the track of the maximum power curve P (Patel M R 1999) and (Muljadi E et al 2001)at all times. Specify the desired real power P ofthe inverter using equation (3.32). Above the rated wind speed, the maximum power control is overridden by stall regulation for constant power. In this thesis,the wind blade is assumed to be ideally stall regulated at ratedpower so that rotor speed keeps constant at rated speed underhigh wind speeds.

82 52 Specify the desired reactive power Q using equation (3.34). The voltage magnitude of thevswt terminal is to be kept constant at a specified level. Therefore,the target for reactive control is the desired voltage magnitudev. By using equations (3.32), (3.33) and (3.34), once the target values are determined,d qtransformation control is applied to enable real and reactive component of ac output power to be separately controlled.the basic concept of d qcontrol is as follows: variables in the a b ccoordinate may be transformed into those in the d qcoordinate rotating at synchronous speed by the rotational d qtransformation matrix T θ (Machowski J 1997). where, 1/2 1/2 1/2 T θ cos θ cos θ 2π/3 cos θ 2π/3 sin θ sin θ 2π/3 sin θ 2π/3 (3.35) Vo Vd Vq T θ Va Vb Vc Vo, Vd, Vq= variables on the o-d-q frame Va Vb Vc= variables on the a-b-c frame θ= phase angle of Vain radian In the three-phase balanced system, the instantaneous active and reactive power outputs, P and Q, of the wind turbine are described by equation (3.36). P V I V I, Q V I V I (3.36) where,v, V ared- and q-axis voltage at VSWT terminal [V]

83 53 Here, V is identical to the magnitude of the instantaneous voltage at the wind generation system and V is zero in the rotating d-q coordinates, so equation (3.36) may be simplified intoequations(3.37) and (3.38). P V I (3.37) Q V I (3.38) where, V is the instantaneous VSWT voltage magnitude.since the voltage magnitude remains almost constant as grid ac voltage, the real and reactive power can be controlled by regulating the q- and d-axis current, I and I,respectively. Figure 3.13Current control scheme of a voltage sourceconverter Through appropriate proportional-integral (PI) control gains,errors between P and P and between Q and Q (orbetween V and V depending on reactive power controlmode) in Figure 3.13are processed into the q- and d-axis referencecurrent I _ and I _, respectively, which are transformedinto the a-, b- and c- axis reference current I _, I _, I _ by the d-q to abctransformation block. The PLL block generates a signal synchronized in phase tothe inverter output voltage Vato provide the reference

84 54 phaseangle θ for the rotational inverse d qtransformation T θ.when the desired currents on the a-b-cframe are set, a pulsewidth modulation (PWM) technique is applied. In the PWM generatorblock, the desired current vector I _ and the actual currentvector I of the VSWT are compared. The error signal vector I is compared with a triangular waveform vector to create switching signals for the six IGBTs of the VSC. The upper and lower limits of the q-axis reference current I _ and I _ are usually set at 1.1 to 1.5 times the VSC s rated current to protect the system from excessive heating. The d-axis reference current limits I _ and I _ may be specified based on (3.22) and (3.23) and reactive power capability limits of the inverter, (3.24). Q S P (3.24) in EESG. The d-q current control strategy is utilized to control the VSC used Adaptive hysteresis band current control band current control. Another commonly used control strategy for VSC is hysteresis Hysteresis control is known to exhibit high dynamic response as it minimizes the error in one sample.the fixed hysteresis band method has the drawbacks of variable switching frequency,heavy interference,harmonic content around the switching side band and irregularity of the modulation pulse position. These drawbacks result in high current ripples and acoustic noise. To overcome these drawbacks,adaptive hysteresis band current control technique is used which adjusts the hysteresis bandwidth as a function of the reference compensator current variation, to optimize the switching frequency

85 55 and THD of supply current. Switching frequency varies with respect to the band size, the inverter and the grid parameters. The VSC can act both as an inverter and as a rectifier. The VSC requires a minimum dc link voltage in order to operate, and here a DC-DC boost converter is introduced to increase the voltage level for the VSC. Variable voltage and frequency supply is invariably obtained from the threephase VSC. Adaptive hysteresis type modulation is used to obtain variable voltage and constant frequency supply. Adaptive hysteresis current control in VSC forces the IGBT s to switch only when it is necessary to keep on tracking the reference of the current. Figure 3.14Adaptive hysteresis band current controller concept Figure 3.14 illustrates the concept of adaptive hysteresis band current control. The adaptive hysteresis band current control of three phase grid connected VSC and its working as explained in (Murat Kaleet al 2005) is considered here. The adaptive hysteresis band current controller adjusts the hysteresis band width, according to the measured line current of the grid connected inverter. Let I be the reference line current and I be the measured line current of the grid connected inverter. The error signal E can be written as:

86 56 E I I (3.25) When the measured line current I a of phase A tends to cross the lower hysteresis band at point 1, then switch S 1 isswitched ON. When this touches the upper band at point P, switch S4 is switched ON. The expression for adaptive hysteresis bandwidth is derived as below. d 0.5 V V (3.26) d 0.5 V V (3.27) where, L is the line inductance, V is the grid voltage per phase and V be the DC link voltage. From Figure 3.8 we obtain, T T 2HB (3.28) T T 2HB (3.29) T T T (3.30) where,t and T are the respective switching intervals and f is the modulation frequency. Simplifying the above equations the hysteresis bandwidth (HB) is obtained as: HB. 1 m (3.31) where,m is the slope of command current wave. The profile of HB and HB are same as HB but have phase difference. According to and V voltage, the hysteresis bandwidth is changed to minimize the influence of current distortion on modulated waveform. Thus the switching signals for the VSC are generated by the adaptive hysteresis band current controller. The VSC used in PMSG is controlled by the adaptive hysteresis band current control technique.

87 SIMULATION RESULTS Direct Drive EESG This section presents the details of the simulation carried out to demonstrate the effectiveness of the modeling of variable speed direct drive EESG.Table 3.1and Table 3.2 provides the parameters of wind turbine and parameters of the EESG respectively.matlab/simulink is used to simulate the modeled systems. The performance of the WTG modeling has been examined under two different wind speeds. The comprehensive simulation results are presented below.simulation diagramsimplemented in MATLAB/SIMULINK are given below.figure3.15represents the simulation diagram of direct drive EESG,Figure 3.16represents the simulation diagram of wind turbineand Figure 3.17represents the simulation diagram of d-q current control scheme ofvsc.

88 58 [Pmeas] Pmeas C ontinuous powergui Signal 1 Signal Builder1 [wr] 0 [wind] Generator speed (pu) Pitch angle (deg) Tm (pu) Wind speed (m/s) WIND TURBINE MODEL [Tm] [Vabc] Vabc_B1 [Pmeas] signal magnitude Pmes [Qmeas] Qmeas [Iabc] [Iabc] Iabc pu Iabc_B1 Fourier Qmes -T- [Vabc] Vabc pu In Mean [wr] 0 Init wr [wr] wind gen speed Mean Value Cp [cp] [pitch] pitch angel [Tm] Tm [cp] cp [Vabcs] [pitch] TSR [TSR] Pitch_angle (deg) [Vdc] Vdc [Tm] Tm [Pinv] [Vabcp] Vabc (pu) [Vf] Vf SCOPES [Vabcp] V I PQ <Stator v oltage vq (pu)> [Qinv] 1 qref Pref [Iabcp] Active & Reactive Power2 [Qinv] <Stator v oltage vd (pu)> [Pinv] Pmeas pulses ga ga' [wr] <Rotor speed wm (pu)> [Iabcp] 0 q q Iabc (pu) Demux gb gb' wt gc series converter controller gc' [Tm] + [wind] 1 - a 1.0 v ref v d Vf v q v stab Excitation System Pm Vf _ [Vf] m A B C aa bb cc A a b B c C n2 Three-phase Transformer1 66ohms A + B C - Universal Bridge + - v [Vdc] b c Subsystem4 Aa Bb Cc A B C a b c n2 Aa Bb C c grid A B C a b c A B C A B C + - v Three-phase Transformer load + - v + - v Figure3.15Simulation diagram of direct drive EESG

89 59 9 m/sec 3 1/9 Wind speed 1/wind_base (m/s) wind_speed_pu Avoid division by zero u(1)^3 Pwind_pu wind_speed^3 Iabc (pu)2 Product Pm_pu -Kpu->pu Pwind*Pnormal/pelctrical 1 1/1 Generator speed (pu) pu->pu 45 lambda lambda_pu cp Product beta 1/normal speed 2 Pitch angle (deg) cp(lambda,beta) 1/2 1/cp_nom Iabc (pu)1 [cp] Avoid division by zero -K- 1 Tm (pu) Figure 3.16Simulation diagram of wind turbine Figure 3.17 Simulation diagram of d-q current control scheme of VSC 1 Vabc (pu) 7 wt Freq Sin_Cos wt Discrete Virtual PLL 50 Hz Product abc dq0 sin_cos abc_to_dq0 Transformation qref 5 q 2 Selector Vd Vq emu PI Vr.1 V0 emu hypot modulation index Terminator2 Vd Vq inverter dq0 sin_cos abc dq0_to_abc Transformation 6 Iabc (pu) emu emu Uref Pulses Discrete PWM Generator 1 pulses 3 Pref PI 4 Pmeas

90 60 Table 3.1 Parameters of wind turbine Rating 1.5MW Blade radius 38m No. of Blades 3 Air density 0.55kg/m 3 Rated wind speed 12 m/sec. Rated speed rad/sec. Cut-in speed 4m/sec. Cut-out speed 25 m/sec. Blade pitch angle 0 0 Stator resistance Ω Stator inductances 0.02µH Table 3.2 Parameters of electrically excited synchronous generator Rating 1.75MVA Rated RMS line to neutral voltage 1.269kV Rated RMS line current 0.433kA Number of poles 84 Base angular frequency rad/sec. Inertia constant of generator sec. The rating of the inverter is 1.2 MVA and its PWM switching frequency is 20 khz. The capacitor value of grid interface rectifier is 6900 µf and dc link voltage is 3.35 kv. The transformer rating of grid connected side is 2.2 kv/132 kv. The p.u. voltage magnitude of primary of the transformer is 0.99 p.u. The grid voltage is 132 kv.

91 61 For the variable speed operation of the WECS, a step change in wind speed is applied in MATLAB, with a step size of 0.5, a wind speed of 12 m/sec. and 11.5 m/sec. is considered in this system is shown in Figure wind speed. Figure 3.18 Wind speed A glitch occurred in Figures3.21 to3.26 is due to this change in Figure 3.19 Tip speed ratio The power coefficient in Figure 3.20is maintained at the maximum of 0.44, which indicates that the turbine speed iswell controlled and maintained an optimum TSR(Figure 3.19) to capture the maximum energy.

92 62 Figure 3.20Power coefficient Figure 3.21 Mechanical speed of VSWT with direct drive EESG Figure 3.21shows the turbine angular speed variation in response to the varying wind speed. The rotor speed has varied smoothly in response to changes in wind speed, owing to the inertia of the turbine and generator. Figure 3.22 Real power output of VSWT with direct drive EESG

93 63 Figure 3.23 Reactive power generated by VSWT with direct drive EESG Figure 3.22 &3.23 present the real and reactive power of the VSWT. The real and reactive power has varied smoothly. This is possible due to the inertia smoothing effect and VSC interface control. Figure 3.24 Generated phase voltage in p.u. of VSWT with direct driveeesg The VSWT voltage variation is given in Figure 3.24 and the voltage magnitude fluctuated with wind speed. Figure 3.25 link of VSWT with direct driveeesg

94 64 Figure 3.25 shows the dc link voltage and it was maintained at a level 3.35 kvsufficient to meet the ac conversion requirement. The grid voltage is 132kV. Figure 3.26 Phase voltage in p.u in grid side of VSWT with direct drive EESG To see the performance of the system, additional load was added. The VSWT with DDSG system has the capability to supply reactive demand to the power grid and maintained the load voltage at a constant specified level, as shown in Figure Figure 3.27 Injected real power in grid side of VSWT with direct driveeesg Figure 3.28 Injected reactive power in grid side of VSWT with EESG

95 65 Figure 3.27 shows the simulation waveform of injected real power 1.5 MW and Figure 3.28 shows the injected reactive power 0.25 MVAR in grid side of VSWT Direct Drive PMSG This section presents the details of the simulation carried out to demonstrate the effectiveness of the modeling of proposed adaptive hysteresis controlled VSWT driven PMSG with MPPT.The system consists of a dc dc converter which is controlled by MPPTwith step and search control strategy and VSC with adaptive hysteresis band current control technique Simulation results are taken for two wind speeds 12 and 14 m/sec. and different load conditions. Table 3.3 shows the parameters of the wind turbine model. Table 3.4 shows the basic parameters used for the direct-drive generator model. Table 3.5 shows data used for the dc-dc converter of the VSWT. The comprehensive simulation results are presented below. Table 3.3 Parameters of Wind Turbine Rating 1.5MW Blade radius 38m No. of Blades 3 Air density 0.55kg/m 3 Rated wind speed 12.4 m/sec. Rated speed 3.07rad/sec. Cut-in speed 4m/sec. Cut-out speed 25m/sec. Blade pitch angle 0 0 at 12m/sec. and 4/0.7 degree/sec. at 14m/sec.

96 66 Table 3.4 Parameters of Permanent Magnet Synchronous Generator Rating 1.75MVA Rated RMS line to neutral voltage 1.269kV Rated RMS line current 0.445kA Number of poles 64 Base angular frequency rad/sec. Inertia constant of generator sec. Stator resistance Ω Stator inductances 0.02µH Table 3.5 Converter parameters Low voltage side capacitor C µf High voltage side capacitors 8000 µf Inductor L 5mH Switching frequency 20kHz Figures show the simulation diagram of direct drive PMSG,MPPT control of DC-DC boost converter, reference current generator of adaptive hysteresis band current controlled VSC, adaptive hysteresis bandwidth calculation and switching pulses of VSC respectively.

97 67 Gain PI Controller Continuous -1 PI powergui [pitch] Rate Limiter [wr] [pitchangle] Wind Turbine1 Generator speed (pu) Tm (pu) Pitch angle (deg) [Tm] [Vabc] [Iabc] Vabc_B1 Iabc_B1 Pmes [Pmeas] c p ref [wind] Wind speed (m/s) Tip speed ratio [wr] wr Signal 1 Signal Builder1 [Vs] <Stator v oltage v q (pu)> [Vs] <Stator v oltage v d (pu)> <Rotor speed wm (pu)> [wr] <Stator current is_q (A)> [Iq] <Stator current is_d (A)> [Id] m PMSG Tm A B C A B C Vabc Iabc a b c Three-Phase V-I Measurement Scope8 [Tm] c [wind] Vabc A B C Iabc1 + Universal Bridge v Va2 [Tm] [pitch] [A2] V1 Vdc2 Tm Qmes Pitch_angle (deg) [A] Idc I2 i + - Ia1 L1 R C [Qmeas] Out1 MPPT Diode2 + g - A dc dc converter Pulse + v - 2 ica 3 Va 4 Vdc1 ica Va HB1 Vdc hys band cal HB Iabc* pulses Iabc 6 Iabc inverter switching pulses Universal Bridge2 + - [Vdc] g A B C 5 Iabc*1 A B C a b c n2 Three-phase Transformer 1.75 MVA 2.2 kv / 130 kv Aa Bb Cc To Workspace3 invvol Out1 I abc Iabc* 1 Vdc Out3 Vdc ref cur gen VIM1 Vabc A Iabc a B b C c load 1 Iabc* A B C Figure 3.29Simulation diagram of direct drive PMSG

98 68 Figure 3.30 Simulation diagram of MPPT control of DC-DC boost converter Figure 3.31 Simulation diagram of reference current generator of Adaptive hysteresis band current controlled VSC Figure 3.32 Simulation diagram ofadaptive hysteresis bandwidth calculation

99 69 Figure 3.33Simulation diagram of switching pulses of VSC Figure 3.34 Wind speed profile Effect of pitch control A wind turbine of 1.5 MW rating has been connected to the 1.75MVA, 2.2kVPMSG. The rating of the inverter is 1.2 MVA. Figure 3.34shows the wind speed profile in which at t = 10 sec., wind speed is changed from 12 to 14 m/sec. in step. Since 12.4 m/sec. is the rated wind speed, at 12m/sec., pitch angle need not be activated. During this period, Cp is obtained as At t=10 sec., as the wind speed is 14m/sec., which is above rated wind speed of 12.4 m/sec., pitch control is activated. As the wind speed increases, the power generated by the wind turbine also increases. Once the maximum rating of the power converter is reached, the pitch angle is increased (directed to feather) to shed the aerodynamic power.

100 70 Here the pitch rate is chosen to be 4/0.7 degrees. That is, the pitch angle can be ramped up at 4 degrees per second and it can be ramped down at 0.7 degrees per second. Small changes in pitch angle can have a dramatic effect on the power output. Cp has changed to 0.39 at 14m/sec. as shown in Figure Figure 3.35 Coefficient of Performance Figure 3.36 shows the variation of tip speed ratio with time. From figure 3.36, it is observed that the turbine speed is well controlled to maintain an optimum tip speed ratio of 7 from 0 to 10 sec. at wind speed of 12m/sec. When wind speed is increased to 14m/sec., the optimum TSR is normally higher than the value at 12m/sec., but due to pitch control, it is kept at 7 itself. In general, three bladed wind turbines operate at a TSR of between 6 and 8, with 7 being the most widely reported value(muljadi E et al 2001). Figure 3.36 Tip speed ratio

101 71 This indicates that the turbine speed is well controlled to maintain an optimum tip speed ratio to capture maximum energy. It shows that the MPPT controller is able to track maximum power and keepc of the wind turbine very close to maximum Betz's coefficient of It is the maximum fraction of the power in a wind stream that can be extracted Results of constant DC link voltage control with MPPT at wind speeds of 12 m/ sec. and at 14 m/sec. Simulation results of generator phase voltage and generator phase current at 12 m/sec. with zooming effect between 0.2 to 0.4 sec. are shown in Figure 3.37 (a) & Figure 3.37 (b). Figure 3.37 (a) Generator phase Voltage at 12m/sec. Figure 3.37 (b) Generator phase current at 12m/sec. The Figure 3.38 (a)& Figure 3.38 (b) show the generator phase voltage and generator phase current at 14 m/sec.

102 72 Figure 3.38 (a) Generator phase Voltage at 14m/sec. Figure 3.38 (b) Generator phase Current at 14m/sec. At 12m/sec., the generator rms phase voltage is1.03kv and generator rms phase current is A. At 14m/sec., the generator rms phase voltage is1.27kv and generator rms phase current is A. The power output at 14m/sec. is higher than at 12m/sec. So with increase in wind speed, power output of wind generator also increases. With MPPT control under both wind speed conditions, the switching signals to boost converter are controlled in such a way that DC link voltage across Co is maintained constant which is shown in Figures3.39(a-d). Figure 3.39(a) and Figure 3.39(c) show the DC link voltage from t= 0 to 1 sec. at 12m/sec. and 14m/sec. respectively. Simulation result of DC link voltage with zooming effect between 0.2 to 0.4 sec. is shown in Figure. 3.39(b) and Figure3.39(d). In the WECS with MPPT control proposed in this

103 73 work, it is possible to maintain a DC link voltage of kv under the wind speeds of 12m/sec. and 14m/sec. Figure 3.39 (a) DC link Voltage at 12m/sec. Figure 3.39 (b) DC link Voltage at 12m/sec. (with zooming) Figure 3.39 (c) DC link Voltage at 14 m/sec.

104 74 Figure 3.39(d) DC link Voltage at 14 m/sec. (with zooming) Results of constant DC link voltage control with adaptive hysteresis band current controller at load currents of 50A and 130A To analyse the dynamic response of adaptive hysteresis current controller, the grid current is increased from 50A to 130A by applying load.the adaptive hysteresis current controller acts under this condition and made the load current to track the reference current command at a faster rate and avoided the grid waveforms getting distorted. Figure 3.38 shows the grid voltage at the point of common coupling. Figure 3.40 Grid Voltage Figures 3.41(a-d) show the grid rms current of 50A and inverter output rms phase current, corresponding hysteresis band and DC link voltage at 50A of grid current.

105 75 Figure 3.41 (a) Grid Current Figure 3.41 (b) Inverter output phase Current Figure 3.41(c) Adaptive hysteresis band at 50 A Figure 3.41 (d)(i) DC link voltage at 50 A with Adaptive hysteresis band current controller

106 76 Figure 3. 41(d) (ii)dc link voltage at 50 A with Adaptive hysteresis band current controller (with zooming) With adaptive hysteresis current controller, it is possible to maintain a DC link voltage of 4.369kV which is the value maintained with MPPT algorithm in dc-dc converter under variable wind speeds. Figures 3.42(a-d) show the grid current of 130A and inverter output rms phase current, corresponding hysteresis band and DC link voltage at 130A of grid current. Figure 3.42(a) Grid Current Figure 3.42 (b) Inverter output phase Current

107 77 Figure 3.42(c) Adaptive hysteresis band at 130A As indicated in Figure 3.41 (c) and Figure 3.42 (c), the adaptive hysteresis band is varied according to the variation in load in order to maintain the constant switching frequency of operation. Figure 3.42 (d)(i) DC link Voltage at 130A Figure3.42(d) ii. DC link Voltage at 130A (with zooming)

108 SUMMARY In electric utilities perspective, grid interface of intermittent generation sources such as wind turbine has been a challenge because such interface may lower power quality of power systems. Therefore, comprehensive impact studies are necessary before adding wind turbines to real networks. In addition, users or system designers who intend to install or design wind turbines in networks must ensure that their systems have well performed while meeting the requirements for grid interface. This chapter presents models of two direct drive WTGs. First is the VSWT with EESGand d-qcurrent controlled VSC.Second is the VSWT with PMSG, MPPT controlled DC-DC converter and adaptive hysteresis band current controlledvsc.theyhave been simulated in MATLAB/ Simulink. By using function and control blocks provided in the MATLAB software, VSWT is built. Dynamic responses were simulated and analyzed based on the modeled system. With d-qcurrent controlled VSC, desired real power and reactive power are maintained in the EESG.It supplied the necessary reactive demand during additional load and maintained the terminal voltage magnitude at a specified level. The VSWT with PMSGhas been simulated with MPPT algorithm. Simulation results have shown that the proposed set up is effective in tracking the maximum power. Adaptive hysteresis band current control in VSC is tested under transient grid currents.fast dynamic response and constant switching frequency characteristics of the adaptive hysteresis band current control maintained the DC link voltage constant.

109 79 The work illustratedin this chaptermay provide a reliable tool for evaluatingthe performance of a direct drive variable speedwtgs and to analyseits impacts on powernetworks in terms of dynamic behaviors.

110 80 CHAPTER 4 IMPACT OF WIND TURBINE GENERATORS ON POWER SYSTEM VOLTAGE STABILITY 4.1 INTRODUCTION Recently, wind power generation has been experiencing a rapid development in many Countries. The size of wind turbines and wind farms are increasing quickly; a large amount of wind power is integrated into the power system. As the wind power penetration into the grid increases quickly, the influence of wind turbines on the power quality and voltage stability is becoming more and more important. Voltage stability is one of the important aspects in maintaining the security of the power system. This chapter explains the voltage instability phenomenon in power system and the tools available for assessing the voltage stability level of the power system. Further, the impact of wind power on voltage control and voltage stability of power system is also investigated. Simulation results based on IEEE 14-bus system are also presented. 4.2 VOLTAGE STABILITY ANALYSIS Voltage stability is the ability of a power system to maintain steady acceptable voltages at all buses in the system under normal operating conditions and after being subjected to a disturbance. Instability occurs in the form of a progressive fall in voltage in some buses. A possible result of voltage instability is a loss of load in an area. The factors contributing to voltage stability are the generator reactive power limits, load characteristics, the characteristics of the reactive power compensation devices and the action of voltage control devices. Contingencies such as unexpected line outages in

111 81 stressed system may often result in voltage instability which may lead to voltage collapse. Unavailability of sufficient reactive power sources to maintain normal voltage profiles at heavily loaded buses are the prime reasons for the voltage instability. Although the voltage instability is a localized problem, its impact on the system can be wide spread as it depends on the relationship between transmitted active power, injected reactive power and receiving end voltage PV curve The relationship between transferred power (P) and the voltage (V) can be illustrated by the PV curve. Figure 4.1 Typical PV curve Figure 4.1 shows a typical PV curve. PV curves are used to analyze the voltage stability. The curve shows how the voltage falls as the demand increases. At the knee of the PV curve, the voltage drops rapidly with an increase in load demand. Load flow solutions do not converge beyond this point, which indicates that the system has become unstable. This point is called the critical point.the critical point of the PV curve defines the maximum

112 82 demand that can be served (the Power Limit ) for a particular power factor and the associated critical voltage. The upper part of the PV curve is considered to be stable whilst the lower part is considered to be unstable. Consequently normal operation is restricted to the upper part of the curve alone Loading Margin The loading margin P is the difference between the present operating point of the system and knee (critical loading) point of the PV curve. It represents the additional power that may be transferred to the load that is located in a node or in a load zone so that power system initially found in a stable zone moves to the final state that corresponds to the voltage stability limit (Jan Machowski et al 2008). Loading margin is usually calculated starting from the current operating point and assuming small load increments, for which a new load flow calculation is performed until the nose of the P-V curve is reached. For load flow calculations, it is necessary to represent the input of active and reactive power, P and Q. Wind turbines with induction generators must be represented with an induction machine, with the active power P set to the momentary value. Based on the machine impedance, the load flow program will then calculate the reactive power consumption Q corresponding to the terminal voltage. Variable speed wind turbines however have reactive power controllability. Therefore, the most suitable representation is a fixed PQ representation or if the wind turbine is set to voltage control, a PV representation. At the loadability limit, or tip of the nose curve, the system Jacobian of the power flow equations will become singular as the slope of the nose curve become infinite. Thus the traditional Newton- Raphson (NR) method of obtaining the load flow solution will break down. In this case, a

113 83 modification of the Newton- Raphson method known as the continuation power flow method is employed. The continuation power flow method employs an additional equation and unknown into the basic power flow equations. The additional equation is chosen specifically to ensure that the augmented Jacobian is no longer singular at the loadability limit. The additional unknown is often called the continuation parameter. The continuation power flow analysis uses iterative predictor and corrective steps. The details of continuation power flow method are given in Appendix POWER SYSTEM VOLTAGE STABILITY IN THE PRESENCE OF WIND TURBINE GENERATORS When the penetration of wind generation is high, it is important to keep WTGs on line as much as possible during grid disturbances as per grid code requirements. Therefore, there is a significant interest in investigating the dynamic performance and characteristics of the system under high penetration of wind generation. This section investigates the effects of wind turbines on power system voltage stability. In transmission networks and distribution grids, node voltage and reactive power are correlated and therefore node voltages can be controlled by adjusting the reactive power generation or consumption of generators. Fixed speed wind turbines have SCIG that always consume reactive power and consequently, it is present practice to provide capacitor banks at each wind turbine. A variable-speed wind turbine with a voltage controller effectively performs voltage control task. Unfortunately, this does not come for free. The variable-speed wind turbines require power electronic converters with a rating that is higher than the rating for operation at unity power factor.

114 Fixed Speed Wind Turbine with Squirrel cage Induction Generator SCIG always consume reactive power. The amount of reactive power consumption is governed by rotor speed, active power generation and terminal voltage. SCIG cannot be used for voltage control because the reactive power exchange with the grid cannot be controlled but is governed by the above factors. In the case of large wind turbines or wind farms and/or weak grids, the reactive power consumption may cause severe node voltage drops. Therefore, the reactive power consumption of the generators is in most cases compensated by capacitors. By adding compensating capacitors, the impact of the wind turbine on the node voltage is reduced, but the voltage control capabilities as such are not enhanced. To enhance the voltage control capability and hence to improve the voltage stability of the system, power electronic based reactive power compensation devices like static VAR compensator (SVC) and static synchronous compensator (STATCOM) have to be installed at the terminals of the WTG Variable Speed Wind Turbine with Geared drive Doubly fed Induction Generator The reactive power generation of a DFIG can be controlled by the rotor current. Here, there is no unique relationship between reactive power and other quantities, such as rotor speed and active power generation. Instead, at a particular rotor speed and the corresponding active power generation, a widely varying amount of reactive power can be generated or consumed. The amount of reactive power is, to a certain extent, affected by rotor speed and active power generation, as in the case of SCIG even though it does not depend on these quantities. The reason is that both generator torque and reactive power generation depend directly on the current that the power electronic converter feeds into the rotor. The part of the current that generates torque depends on

115 85 the torque set point that the rotor speed controller derives from the actual rotor speed. The current that is needed to generate the desired torque determines, in turn, the converter capacity that is left to circulate current to generate or consume reactive power. Since the DFIG is able to generate or consume reactive power, it can well contribute to voltage stability enhancement. But, this requires power electronic converters with a rating that is higher than the rating for operation at unity power factor. The converter current rating for reactive power consumption can be lower than for reactive power generation. The reason is that the generator in this wind turbine type is grid coupled. The magnetizing current can be drawn from the grid instead of being provided by the converter. In this mode of operation, reactive power is consumed and fewer converters current is needed than for the generation of the same amount of reactive power. Full voltage control capability requires that reactive power can be both generated and consumed. Therefore, reactive power generation is the determining factor in sizing the converter when equipping a DFIG based wind turbine with a voltage controller. The relative increase of converter size is largest in the case of the DFIG based wind turbine Variable Speed Wind Turbine with Direct drive Synchronous Generator In WTGs with DDSGs, the reactive power exchange with the grid is not determined by the properties of the generator but by the characteristics of the GSC. The generator is fully decoupled from the grid. Therefore, the reactive power exchange between the generator and the generator side of the converter as well as between the grid side of the converter and the grid are decoupled. This means that the power factor of the generator and the power

116 86 factor of the grid side of the converter can be controlled independently. As the generator and the grid are decoupled, the rotor speed hardly affects the grid interaction. The reactive power is changed by controlling the GSC Electrically excited synchronous generator The voltage stability depends on the balance of reactive power demand and generation in the system. Like a conventional power plant, the direct drive EESG supplies reactive power to the grid when it is needed, regulating system voltage and stabilizing weak grids. Each wind turbine maintains precise torque and pitch regulation, controlling power and speed during changing wind and grid conditions. With a control system, direct drive EESG will be able to operate more like a conventional power plant. Electrically excited synchronous generator has wound rotor with electrical excitation, and over excitation is easily possible. The operation of the generator at unity power factor is utilized to reduce machine side inverter power rating to the real power value. With the ability to supply and regulate reactive and active power to the grid when it is needed, direct drive EESG is becoming a standard feature in large wind farms. It provides smooth fast voltage regulation by delivering controlled reactive power through all operating conditions. WECS with controlling capability ensures that the reactive power performance of a wind power plant can meet and often exceed the performance of a conventional (non-wind) power plant. Even when wind turbines are not generating rated active power, the reactive power control feature can provide reactive power. The provision of continued voltage support and regulation provides grid benefits not possible with conventional generation, while mitigating adverse voltage impacts of wind turbines being off-line due to wind conditions. This feature can eliminate the need for grid reinforcements

117 87 specifically designed for no-wind conditions, and may allow for more economic commitment of other generating resources that will enhance grid security by reducing the risk of voltage collapse. With increased pole count, field ampere turns increase. The number of turns of exciting field winding yields an increase in excitation losses Permanent magnet synchronous generator The excitation losses which occur in direct drive electrically excited synchronous generator are eliminated here due to the usage of permanent magnets. This leads to increase in efficiency and reduces the thermal problems on the rotor side. Thus they are more efficient than CSGs.The design of the permanent magnet circuit has to take into account the demagnetization limit, which may be reached by high stator current loading (over load condition ) which causes opposing stator field on rotor trailing magnet edge. With increased pole count due to lower flux per pole, the danger of demagnetization decreases, hence smaller magnets and thus results in reduced cost for high pole count machines. Further no brushes and slip rings are necessary, which reduces maintenance costs. The main advantages of direct drive PMSG are the full decoupling between the RSC and GSC. In fact, in case of grid disturbances, the GSC is controlled so that it can support the voltage recovery by supplying reactive power and at the same time it ensures the grid stability. No significant mechanical stress (torque or speed) occurs due to their high dynamic compared to electrical dynamics (O B K Hasnaoui et al 2006). The direct drive PMSG is able to ride through the balanced voltage grid fault by reducing active power and supplying the maximum possible reactive power to maintain the current constant until clearance of the voltage fault. Due to decoupling between RSC and GSC, the dynamic behavior of the

118 88 generator is slightly affected in presence of grid fault, this disturbance creates a slight increase in speed of the generator. 4.4 VOLTAGE CONTROLLERS IN WIND TURBINE GENERATORS Wind turbines equipped with a voltage controller compensate the grid voltage drop and keep the voltage at its reference value. The variablespeed wind turbines with a voltage controller have the capability of controlling terminal voltage independent of the grid voltage, unless their operating limits are exceeded. Thus, the voltage magnitude is maintained above 0.95 p.u. The terminal voltage variation is smooth in the case of variable-speed wind turbines with voltage control. It is more expensive to equip a DFIG based wind turbine with a voltage controller than a direct-drive wind turbine. However, this conclusion is not necessarily correct. The converter in a direct-drive wind turbine is larger and thus more expensive than in a DFIG based wind turbine. This means that, although the relative increase in converter cost will be smaller in the case of the direct-drive wind turbine, the absolute cost increase may be substantially higher. This voltage control capability could improve the voltage stability margin at distribution and transmission levels. 4.5 SIMULATION RESULTS This section presents the details of the simulation study carried out on IEEE 14- bus system for analysis of voltage stability using the WTGs. Figure 4.2 shows the one line diagram of IEEE 14- bus system.

119 89 Figure 4.2 One line diagram of IEEE 14- bus system IEEE 14- bus system consists of 5 generator buses, 11 load buses and 20 transmission lines. The transmission line parameters of IEEE 14- bus system are given in Appendix 3. For this test system, based on the contingency analysis conducted at different loading conditions, 3 single line outages 2-3, 5-6, 7-9 were identified as most severe cases Voltage Stability with Conventional Synchronous Generators within limit. CSG has inbuilt AVR which could maintain the voltage level PSAT, a MATLAB based open source package is utilized for system analysis. CPF is used to compute the loading margin. The CPF algorithm consists of a predictor step which computes a normalized tangent

120 90 vector and a corrector step that can be obtained either by means of a local parameterization or a perpendicular intersection. is the loading parameter, which is used to vary base case generator and load powers. Table 4.1 shows the parameter values of conventional synchronous generator. Table 4.1 Parameters of Conventional Synchronous Generator Parameters Values Power, voltage and frequency ratings 1.75 MVA, 2.2kV, 50Hz Stator resistance Rs and Leakage Reactance Xl 0.01 p.u., p.u. d- axis reactances Xd, X d, X d 1.45 p.u., 0.28 p.u.,0.11 p.u. d- axis open circuit time constants T d0 and T d0 3.4 s,0.06 s q- axis reactances Xq, X q, X q 1.42 p.u., 1.07 p.u., 0.11 p.u. q- axis open circuit time constants T q0 and T q0 2.7 s, s Inertia constants H, damping s,2 p.u. Speed and active power additional signals Kw, Kp 0.0 p.u., 0.0p.u. Percentage of active and reactive power at bus 1p.u., 1p.u. d- axis additional circuit leakage time constant Taa 0.0 s Computation of loading margin Table 4.2 shows the values of loading margin under base case and contingency states in p.u. and Figure 4.3 shows the profile of loading margin under base case and contingency states with only CSGs. Table 4.2 Loading Margin under Base case and Contingency states in p.u. Condition Loading margin with CSGs Base case Line outage Line outage Line outage

121 91 Loading margin with CSGs Loading margin in p.u Base case Line outage 2-3 Line outage 5-6 Line outage 7-9 Loading margin with CSGs Figure 4.3 Loading margin with CSGs From figure 4.3, it is found that, under the contingency states, the loading margin has reduced much from the base case value Voltage Vs. Time curve after the contingency One highh voltage line 6-11 is disconnected to analyze the variation of voltage with respect to time. The voltage drop in bus-6 is measured and plotted. Figure 4.4 shows the variation at bus-6 voltage after the disconnection of line Figure 4.4 Bus-6 voltage variation after the disconnection of line 6-11 After the line disconnection, the bus-6 voltage drops due to the increasing reactive losses in the line, and due to the reduced line charging. But, the voltage magnitude at bus 6 is within the acceptable limit (Vbus 6 < 0.95 p.u.) ).

122 Voltage profile The values of voltage magnitude at bus-2 and bus-5 and the reactive power flow from bus-1 to bus-2 and reactive power flow from bus-1 to bus-5 under base case and contingency states with CSGs are given in Table 4.3. Table 4.3 Voltage magnitude and Reactive power flows with Conventional Synchronous Generators Condition Base case Line outage 2-3 Line outage 5-6 Line outage 7-9 Bus-2 Voltage Voltage Q from bus-1 to magnitude magnitude with bus-2 with CSGs CSGs Bus-5 Q from bus-1 to bus Figure 4.5 (a) shows the voltage profile of bus-2 under base case and contingency states in p.u. Figure 4.5(b) shows the voltage profile of bus-5 under base case and contingency states in p.u. Voltage magnitude of bus-2 with CSGs Voltage magnitude in p.u Base case Line outage 2-3 Line outage 5-6 Line outage 7-9 V magnitude with CSGs Figure 4.5(a) Voltage profile of bus-2 under Base case and Contingency states in p.u.

123 93 Figure 4.5(b) Voltage profile of bus-5 under Base case and Contingency states in p.u. Figure 4.6(a) shows the reactive power flow from bus-1 to bus-2 and Figure 4.6(b) shows the reactive power from bus-1 to bus-5. Figure 4.6(a) Reactive power flow from bus-1 to bus-2 under Base case and Contingency states in p.u. Figure 4.6(b) Reactive power flow from bus-1 to bus-5under base case and contingency states in p.u.

124 94 Under base case and under severe contingencies considered, voltage magnitude is 1 p.u. and above at both bus-2 and bus-5. Negative value of reactive power from bus-1 to bus-2 shows that bus-1 receives reactive power from bus-2. Similarly positive value of reactive power from bus-1 to bus-5 shows that bus-1 delivers reactive power to bus Voltage Stability with Wind Turbine Generators The impact of WTGs on voltage stability is analysed by replacing the CSGs with fixed speed SCIG, variable speed DFIG, EESG and PMSG. Fixed speed SCIG and variable speed DFIG in MATLAB /PSAT library are connected at bus-1 of IEEE 14-bus system. Fixed speed SCIG has capacitor banks. Reactive power absorbed by the SCIGs is compensated by capacitor banks connected with it. Variable speed DFIG with standard control inbuilt in MATLAB /PSAT consists of a wound rotor induction generator and an AC/DC/AC IGBT-based PWM converter. The stator winding is connected directly to the 50 Hz grid while the rotor is fed at variable frequency through the AC/DC/AC converter. The DFIG technology allows extracting maximum energy from the wind for low wind speeds by optimizing the turbine speed, while minimizing mechanical stresses on the turbine during gusts of wind. It has standard rotor speed control and voltage control inbuilt in it. The modeled controllable power inverter strategy of VSWT with direct drive EESG and VSWT with direct drive PMSG along with the power converters implemented in MATLAB/SIMULINK and presented in chapter 3 are converted to MATLAB codings and interconnected with the PSAT. The 360 numbers of VSWT with direct drive EESGs and PMSGs of 1.5 MW are connected at bus-1. The parameter values of SCIG and DFIG used in the analysis are given in Table 4.4 and Table 4.5 respectively. The parameters of variable speed EESG and PMSG presented in Table ( ) of chapter 3 are considered here also.

125 95 Table 4.4 Parameters of Fixed Speed SCIG Parameters Values Power, voltage and frequency ratings 1.75 MVA, 2.2kV, 50Hz Stator resistance Rs and Reactance Xs 0.01 p.u., p.u. Rotor resistance Rr and Reactance Xr 0.01 p.u., 0.08 p.u. Magnetization Reactance Xm 3 p.u. Inertia constants Hwr Hm and Ks 2.5KWs/KVA, KWs/KVA, 0.3 p.u. Number of poles and gearbox ratio 4, 1/89 Blade length and number 75m, 3 Table 4.5 Parameters of Variable speed DFIG Parameters Values Power, voltage and frequency ratings 1.75 MVA, 2.2kV, 50Hz Stator resistance Rs and Reactance Xs 0.01 p.u., 0.101p.u. Rotor resistance Rr and Reactance Xr 0.01 p.u., 0.08p.u. Magnetization Reactance Xm 3 p.u. Inertia constants Hm KWs/KVA Pitch control gain and time constant Kp, Tp 10p.u, 3 sec Voltage control gain Kv 10p.u Power control time constant Te 0.01 sec. Number of poles and gearbox ratio 4, 1/89 Blade length and number 75 m, 3 Pmax and Pmin 1p.u., 0p.u. Qmax. And Qmin +0.7 p.u., -0.7 p.u.

126 Computation of loading margin The values of loading margin under base case and contingency states with CSGs and with SCIGs, DFIGs, EESGs and PMSGs connected at bus-1 are given in Table 4.6. The profile of loading margin under base case and contingency states is given in Figure 4.7. Table 4.6 Values of Loading Margin under Base case and Contingency states in p.u. With SCIG With DFIG With EESG With PMSG Condition at bus-1 at bus-1 at bus-1 at bus-1 Base case Line outage Line outage Line outage Figure 4.7 Profile of loading margin By using the reactive power injection facility of variable speed WTGs, maximum deliverable power has been increased. In other words, the voltage stability margin has been increased by reactive power injection from variable speed WTGs.

127 97 With fixed speed SCIG which is less capable of injecting reactive power integrated into the system, loading margin is much lesser. VSWTs namely DFIGs, direct drive EESGs and PMSGs have much influence in improving the loading margin. Direct drive EESGs and PMSGs which are implemented with modified control strategies in VSC have more influence than DFIGs. Moreover it is found that voltage stability improvement is larger when the control strategy is modified in a variable speed WTG, rather than when standard variable speed WTGs are used Voltage Vs time curve after the contingency To analyse the performance of the system under contingency, line 6-11 is disconnected and the system is analysed. After the line disconnection, voltage drops due to increasing reactive losses in the line and the reduction in reactive power produced by the inherent shunt capacitance in a line. The voltage drop in bus-6 is measured. Figures 4.8 (a-d) represent the BUS-6 voltage after the disconnection of line 6-11 and the response of system with SCIGs, DFIG, EESGs, and PMSGs. Figure 4.8 (a) BUS-6 voltage with SCIGs after the disconnection of line 6-11

128 98 Figure 4.8 (b) BUS-6 voltage with DFIGs after the disconnection of line 6-11 Figure 4.8 (c) BUS-6 voltage with EESGs after the disconnection of line 6-11 Figure 4.8 (d) BUS-6 voltage with PMSGs after the disconnection of line 6-11 With SCIGs integrated into the power system, the transmission level voltage (Bus-6) drops further. The voltage at bus 6 is now below the acceptable limit (Vbus 6 < 0.95 p.u.). The tap-changing action restores the voltage to some extent but unfortunately has a negative impact on the grid-side voltage, and initiated a voltage collapse event with SCIGs and DFIGs.

129 99 A possible voltage collapse event is avoided with variable speed WTGs equipped with modified power-electronics control. In this case, the wind turbine system utilizes its reactive-power injection capability to maintain voltage on the transmission level within the allowable limit (±5% deviation) after the grid disturbance. Voltage level is restored by the wind farm action, and part of the load-side voltage is restored, in this case by a few transformer tap movements. Transmission level voltage reduction, due to this tap movement is counteracted by subsequent reactive power injection by the wind farm Voltage profile Table 4.7 Values of voltage magnitude and reactive power in p.u. Condition Bus-2 Bus-5 SCIG DFIG EESG PMSG SCIG DFIG EESG PMSG Base case Line outage 2-3 Line outage 5-6 Line outage 7-9 Base case Line outage 2-3 Line outage 5-6 Line outage 7-9 Voltage magnitude Reactive power Voltage magnitude Reactive power The values of voltage magnitude at bus-2 and bus-5, reactive power flow from bus-1 to bus-2 and, reactive power flow from bus-1 to bus- 5 under base case and contingency states with SCIGs, DFIGs, EESGs and PMSGs at bus-1 are given in Table 4.9. Figure 4.9(a) shows the voltage profile of bus-2 under base case and contingency states in p.u., Figure 4.9(b) shows the voltage profile of bus-5 under base case and contingency states in

130 100 p.u., Figure 4.10(a) shows the reactive power flow from bus-1 to bus-2 and Figure 4.10(b) shows the reactive power flow from bus-1 to bus-5. Figure 4.9(a) Voltage profile of bus-2 under Base case and Contingency states in p.u. Figure 4.9(b) Voltage profile of bus-5 under Base case and Contingency states in p.u.