CHAPTER 5 ROTOR RESISTANCE CONTROL OF WIND TURBINE GENERATORS

Transcription

1 88 CHAPTER 5 ROTOR RESISTANCE CONTROL OF WIND TURBINE GENERATORS 5.1 INTRODUCTION The advances in power electronics technology have enabled the use of variable speed induction generators for wind energy systems. The volts / hertz-controlled generator are one of the earliest variable speed systems used for efficient wind power capture. Although, it enabled fast output power control and full range speed control, it was very expensive, as the power electronics converters had to be rated for the maximum output power of the system.however, the use of rotor-side control decreased the cost of the converters by reducing their power ratings. The limited speed control range of rotor-controlled systems was sufficient for wind turbine generator systems, as the required speed variation is only about 10 %. In this chapter, an alternative method of connecting fixed- speed wind turbines, with active pitch mechanisms or variable-slip generators, is proposed and evaluated. Instead of using the conventional soft starter, external resistors are used. 5.2 WIND MACHINES The U.S Federal government is in the process of constructing and demonstrating a number wind energy conversion systems (WECS) for farm, home and other dispersed applications, including the generation of electricity, the pumping of water for household use, stock-watering, irrigation purposes,

2 89 the heating of working fluids to provide hot water, space heating and cooling. Other potentially important applications are the use of wind power for desalination of water using reverse osmosis, and the production of both low temperature and high temperature process-heat for industry and agriculture. Most of the WECS units that are being used under this program are the two bladed and three bladed horizontal axis designs, although multi bladed horizontal-axis rotors and various types of vertical axis wind machines are also being used as shown in Figure Figure 5.1 Dimension of wind energy conversions system test bed One of the earliest rotor control schemes was the rotor resistance control as shown in Figure 5.2. The speed of the induction machine is controlled by changing the external resistance in the rotor circuit. This method enables fast response (Filho et al 1997), since power electronics converters are used to vary external resistance. The drawback of this type of control is

3 90 the substantial losses in the external resistor at high speed. This reduces the efficiency of the overall system. The use of a slip-power recovery induction generator is also called the static Kramer drive, as shown in Figure 5.2 removed the drawback of the rotor resistance controller, as the rotor power is fed back into the grid. The grid-side converter can be used to control the speed of the generator and the converter rating is typically 50 % of the rated power. However, the power flow can only occur from the rotor to the grid, due to the use of the diode bridge rectifier on the rotor side (Woei-Luen et al 2006). Figure 5.2 Rotor resistance control of a wound rotor induction machine This unidirectional power flow limits the generation action of the machine to super-synchronous speeds. This limitation, which is also a drawback of the rotor resistance controller, does not enable wind power capture at low wind speeds. One of the simplest rotor-side control schemes is the rotor resistance control. The principle involved is to control the speed of the generator by varying the external resistance introduced in the rotor circuit.

4 91 The resistance is controlled by power electronics circuitry to provide fast response. This chapter describes the design of the rotor resistance controller and discusses the performance of the system compared to the pitch control scheme. 5.3 PRINCIPLE OF CONTROL Wound rotor induction machine speed variation can be obtained by inserting external impedance into the rotor circuit. The external impedance necessitates a higher slip for a desired electromagnetic torque (Pena et al 1996). The initial current that is flowing through the external resistor is mainly resistive since the external resistive dominates the electric circuit of the wind turbine. The reactive current needed by the turbine is largely, supplied by the capacitor during the connection procedure. This leads to a smaller impact on an inductive grid compared to a resistive one Induction Machine Torque-Speed Characteristics The steady state, single-phase equivalent circuit for the induction machine is shown in Figure 5.3 (Fitzgerald et al 1990). Figure 5.3 Single-phase equivalent circuit for the induction machine

5 92 The parameters shown in the Figure 5.3 are R 1,R 2 - Stator and rotor resistances. X 1,X 2 - Stator and rotor leakage inductances. X m - Magnetizing inductance. R c - Core loss resistance. V phase - Single phase stator voltage and Slip = ω s ω r / ω s. The variable resistance shown on the rotor side represents the mechanical power produced by the machine. The torque expression can be derived from the single-phase Thevenin circuit as, T e = 2 qv 1 ( R2 s) ( R + R s) + ( X + X ) 1 phase 2 2 s ω (5.1) where q 1 is a constant. Figure 5.4. A typical speed-torque curve for an induction machine is plotted in Figure 5.4 Torque-speed curve for an induction machine

6 Output Power Control The power input to the induction generator (P m ) is P m = T e * ω r (5.2) A fraction of this power is dissipated in the rotor resistance. The stator and rotor powers can be expressed as P stator = T e *ω s (5.3) P rotor = T e * (ω r ω s ) (5.4) If the electromagnetic torque is considered positive for the generating mode, then both P stator and P rotor are positive. The stator and rotor powers can be regarded as areas in the torque-speed characteristics of the generator as shown in Figure 5.5. Figure 5.5 Stator and rotor power in an induction machine (generating mode) The control of the stator power of the induction machine by changing the rotor resistance is illustrated in Figure 5.6.

7 94 Figure 5.6 Effect of changing the rotor resistance on stator and rotor powers The various areas indicated are as shown in Figure 5.6. When the input power increases, the addition of the external rotor resistance causes the speed of the generator to increase, maintaining constant torque. The stator power remains fixed, whereas the excess power is dissipated in the rotor as shown in Figure 5.7 (Torbjorn Thiringer 2002). Figure 5.7 Power dissipated in the stator and rotor

8 Design of the Rotor Resistance Controller The standard form of the rotor resistance controller is shown in Figure 5.8 (Nayar and Bundell 1987). The error between the desired power and the actual power output is fed into an integrator and the actuator acts on the output of the P-I controller to determine the amount of external resistance added. The gearbox ratio chosen in this case is 115 %, as this value would limit the speed variation of the generator from 100 % to 115 % of the rated speed over the required wind velocity range of 14 ms -1 to 20 ms -1. Figure 5.8 Block diagram of the rotor resistance controller The actuator model used is based on the implementation of the rotor resistance control scheme shown in Figure 5.9. Since the response of the power electronics converters is much faster than the machine response, the converters are modeled as ideal blocks. The values of the resistors chosen for the simulation are R 1 =0.01p.u and R 2 =0.01p.u. The P-I controller was designed using the Zeigler-Nichols techniques and the values are K p ~ 5 and K i ~ 50

9 96 Wound rotor machine Diode bridge rectifier R 1 R d Figure 5.9 Implementation of the rotor resistance control scheme The SIMULINK implementation (SIMULINK Manual 1997) of the entire rotor resistance controller is shown in Appendix A ELECTRICAL SYSTEM OF WIND TURBINES An important part of the turbine is the asynchronous generator. It converts the mechanical energy from the gearbox to electrical energy. The generator is connected to the grid, which transfers the electrical energy to the consumers. The public supply system is 50 Hz. The generator s rotational speed depends on the frequency.

10 97 The energy conversion for most modern wind turbines (WT s) can be divided into three main configurations, fixed speed machines with one or two speeds and variable speed machines (Dongdong et al 2004). The preferred configuration is the two-speed machine, as shown in Figure A tendency towards variable speed is noticeable in the development of the megawatt wind turbines. Figure 5.10 Fixed and variable speed graph Fixed Speed Wind Turbines In fixed speed machines, the generator is directly connected to the main supply grid. The frequency of the grid determines the rotational speed of the turbine rotor, which in turn is translated into the generator rotational speed (through gearbox). The generator speed depends on the number of pole pairs and the frequency of the grid. This feature in the controller requires additional design effort and manual of control in the region may lead to an optimization that considers life of the gearbox and other wind turbine elements.

11 98 The greatest advantage of WT s with asynchronous generators is their simple and robust construction. In addition, there is no need for synchronization device. With the exception of bearings, no parts are subjected to wear and tear. The wind turbine with two generator performs better mainly due to the fact that the small generator has more poles than the large generators, that is the rotor speed of smaller generator becomes closer to the rotor speed that would capture more energy and also the price tends to be slightly lower Variable Speed Wind Turbines In variable speed machines, the generator is connected to the grid by an electronic inverter system or the generator excitation windings are fed by an external frequency from an inverter (Eduard et al 2007). The basic idea of the variable speed turbine is the decoupling of the rotational speed of the generator and thus of the turbine rotor from the frequency of the grid (Suel Kim et al 2007). The rotor can operate with variable speed adjusted to the actual wind speed situation. The wind turbine will go in to a higher variable speed control mode, the motor RPM will be reduced in a controlled manner to enable the airfoils to operate less efficiently (Dionisio et al 2007). Regenerating braking will be used wherever possible to reduce the rotor speed during such a controlled mode. The lower motor RPM at a constant power level will lead to slightly higher drive train torques. There is also less mechanical stress and rapid power fluctuations are scarce because rotor acts as a flywheel. The variable speed turbine allows grid voltage control, as reactive power generation can be varied. The variable speed design with doubly fed induction generator adopts cheaper power electronics

12 99 converter and cost of semi-conductor components used in AC-DC-AC converters is less (Nayeem Rahmat Ullah et al 2008). The disadvantages are complex construction activity and generation of harmonics, which may result in failure of electronic components. Both basic wind turbine concepts have proved their reliability and efficiency in energy production in wind farms and single site applications in the recent past ( In machines with such inflexible speeds of rotation, turbine torque fluctuations place heavy loads on the drive train because of the almost rigid coupling between the grid and the generator as shown in Figure By raising design slip levels, such systems may be made more flexible and thus offer greater protection to drive the train. However, this comes at the expense of greater volume, mass and losses. Figure 5.11 Cut way drawing of configuration for two speed wind machines

13 100 A greater flexibility is desired since the rotating blades of the turbine are subject to considerable airstrip turbulence, especially near to the tower. These result in fluctuations in turbine performance. The effects of tower shadow fluctuations cannot be ironed out entirely. At low slip values, small variations in the speed of rotation therefore result in relatively large fluctuations in power. Fluctuations in electrical output power can, as already mentioned, are considerably reduced by the use of asynchronous machines with higher nominal slip. Figure 5.12 (a) and (b) show comparative values for a 20 KW machine running under partial load, i.e. without application of blade pitch regulation. Considerable differences in output behavior may be observed. While fluctuations of more than 0.2 P N can be seen for generators with 2 % nominal slip as shown in Figure 5.12(a), due to tower-shadow effects, at 8 % nominal slip as shown in Figure 5.12(b), these are reduced to some 5 % of nominal output (0.05 P N ). The dominant variations in power shown here are measurable in seconds and are clearly due to variations in wind speed. ' ' In accordance with equations S = R σ X and k 2 2 S k = R R + X X R X ' ' σ 1 high slip values can be achieved by designing the generator with High rotor resistance, Low total leakage factor and Lower rotor inductance. Constructional details such as form and resistance of conductor materials and conductor cross-sections in the rotor are very important.

14 101 Application of power smoothing, can also considerably reduce demands on turbine components. Figure 5.12(a) 2 % nominal slip Figure 5.12(b) 8 % nominal slip In addition to setting higher fixed slip values by using extra resistors in the rotor circuit, we can control slip to adapt to output power variations as shown in Figure The slip value and thus the proportional loss are thereby kept low and system efficiency is increased by varying the resistance of the rotor. It is also possible to limit the torque on the drive train to values around the normal value and to smooth the power output. This principle is employed in the opti-slip control in the Vestas turbines models V44 and V60/63/66 and the USA export V80.

15 102 Figure 5.13 Dynamic slip control in slip-ring rotor synchronous machines (Simulation results) 5.5 PERFORMANCE CHARACTERISTICS FROM MEASUREMENTS We will show the results of metrological investigation on five different synchronous machines of the same size with output-class-dependent slip value and increased slip values due to Constructional changes in the case of cage rotor machines or Additional resistances in the rotor circuit of slip-ring rotor machines.

16 103 Stator voltages between 280 V and 500 V or between 320 V and 480 V. Electrical output ranges of Generator no load operation or Motor no-load operation respectively and Up to a maximum of 1.3 times nominal output. To record the measured values in a stable temperature, steadystate condition, a warm running phase of around two hours was necessary for each measurement. The rating of the machine was selected such that the characteristics attributes of large unit class would be accurately represented. To demonstrate the transient behavior of individual machines and in order to specify guidelines for the design of generators specifically for use in wind turbines, there follows a small selection from the comprehensive program of investigations based on some performance characteristics from a selection of machine types (generator or motor design, slip ring rotor). The relevant performance characteristics for Active power consumption, Apparent power, Reactive power, Stator current, Efficiency, Power factor and Generator slip. are represented in the following to Electrical output power and Stator voltage

17 104 The grid voltage was adapted to the selected rotor voltage with aid of a regulating transformer. Because the power values were measured at various distances, they had to be interpolated for spatial representation in the form of computer graphics before they could be represented in selected power grid (based on the 1 KW increments). Values that decrease with increasing voltage, such as the power factor and slip, remain difficult to recognize due to being partially obscured. The increases in voltage are caused by the effects of saturation in the magnetic circuit and the associated increase in current with rising voltage, particularly at no-load and low outputs. The minimum current runs from the lowest voltage values (280 V) at no-load to maximum output at approximately 460 V. High electrical output is significantly reduced. Measurements on machines of differing designs have shown that, assuming that driving torque fluctuations are similar, generators with slip values four times higher than nominal slip transfer around a quarter of the output fluctuations to the grid compared to a nominal design as shown in Figure Therefore, the generator parameters of a motor-type slip-ring rotor machine is described below for different slip behaviors, which can be adjusted with the help of additional resistances in the rotor circuit. Figure 5.15 shows a comparison of the relevant parameters for different levels of slip. The machine in question was operated with a short-circuited rotor at approximately 3 % nominal slip or with additional resistors in the rotor circuit, which are selected to achieve 7 %, 14 % or 21 % nominal slip at nominal load and voltage.

18 105 Figure 5.14 Circle diagram Figure 5.15 shows an increase of input power at higher slip values due to increased losses in the rotor circuit and thus a worsening of the machines efficiency, particularly in the full-load range. The mechanical input power increases with increasing slip. Stator current and power factor values, on the other hand, show only small differences. They are almost identical for all slip values. Finally, the slip level illustrates the very strong voltage dependence of these variables and their significant increases as the voltage decreases. This trend is highlighted in a very striking manner by the levels at the highest slip

19 106 values (21 % operating at the nominal rating). It is thereby possible to achieve the desired degree of slip by the selection of Rotor resistance in conjunction with Rotor leakage S = R X σ and k ' 2 S k = R R + X X R X ' ' σ 1 and the specification of Stator voltage As was clearly shown at the beginning of this section, with the help of the derived circle diagram for induced voltage, very different behaviors can be expected from the motor-type design compared with generator-type dimensioning. Figure 5.15 shows marked trends and constructive areas of influence, by direct comparison of the relevant parameters. Mechanical input power and, in particular, stator currents are higher for the motor-type machine than for the generator-type machine across the entire generator operating range. The excessive increase in current at high voltages clarifies the high saturation dependence of the motor. Efficiency and to an even greater degree, power factor values are much more favorable for machines of generator dimensions, particularly at higher voltages. At low voltages, however, only small differences are apparent. The generator-type machine with 3 % slip compared with 4 % nominal slip of the motor demonstrates significantly more rigid behavior, due to the more efficient design, resulting in a more rigid grid connection. In fine, we can state that the asynchronous machines used in wind turbines should be designed to achieve a low degree of saturation. Figure 5.15 illustrates this point, since the generator always achieves higher induced

20 107 voltage values and thus higher saturation states in normal operation than in motorized operation. This achieves A low mechanical input power and Stator currents as low as possible for the prevailing load state (Siegfried et al 2006). Figure 5.15 Comparison of performance of a motor type asynchronous machine with slip ring rotor The fault condition may last upto 150 m sec or even more. The grid side controller senses the DC link voltage error and stator terminal voltage magnitude error and develops the current commands to compensate them while the rotor side control senses the stator active and reactive power errors and generates the rotor current command to compensate them.

21 REAL, REACTIVE AND APPARENT POWER Electrical power engineers think in terms of real, reactive and apparent power flows in a power system. Real power (KW or MW) is the capacity to do useful work such as pump water or rotate a shaft against a load. Reactive power (KVA or MVA) is drawn by transformers or induction motors as magnetizing current for their iron cores but is generated by capacitors. Finally, the apparent or complex power (KVA or MVA) is the vector addition of real and reactive power. The power triangle of an inductive load is shown in Figure Hence, the load takes real power P to provide heat or work and some reactive power Q to energize the magnetic circuits. The power factor is defined as the ratio P/S or cosθ. Figure 5.16 Power triangle of an inductive load where, P = Real Power (W), Q = Reactive Power (VAr) and S = Apparent Complex Power (VA) Impact on the Distribution Systems Figure 5.17 shows a single representation of a fixed speed induction machine with only limited power factor correction, connected to a radial distribution circuit. The wind turbine exports real power P, and imports

22 109 reactive power Q. The reactive distribution system is represented by a single series impedance of resistance R and inductance reactance X. The bus bar V O represents the connection to the transmission system; it is called as infinite bus bar because its voltage is assumed constant under all conditions. Figure 5.17 Single line diagram of fixed speed induction generator by: The voltage of the wind turbine may be calculated approximately V 1 = V 0 + [(PR-QX)/V 0 ] (5.5) Real power P, passing through the network resistance R, causes a rise in the voltage of the wind turbine, whereas the reactive power Q, with the inductive reactance of the circuit is called the lowering of the voltage. The electrical losses in the circuit may be computed from W = [(P 2 + Q 2 ) R]/V 0 2 (5.6) Calculated the Power Output of the Wind Machines The operation and maintenance costs of the wind power system, the expected lifetime of the systems, the interest rate on investment capital and the price of competing terms of energy are calculated.

23 110 The power output P 0 of a wind machine can be expressed in the following terms: P o = 6 2 ( E)( Dr )( Pb)( Vr) { Cp( Vr) } T (5.7) where, E = Efficiency of power train, Dr = Diameter of swept area of rotor, P b V r T = Barometric pressure,. = Instant wind speed at rotor hub-height above ground level, = Ambient temperature, Cp (V r ) = Power coefficient of the rotor, V r = V a (H r /H r ) 1/2 V e H r H e = Instant wind speed at anemometer height above ground level, = Rotor hub-height above ground level and = Anemometer height above ground level. The above algorithm is based on the use of instantaneous wind speeds (i.e. where the wind speeds are averaged over a period comparable to the response time of the wind turbine generator). Mass density of the air, ρ m = P/R T a Kg / m 3 = 1.29 Kg / m 3. where, P = Absolute pressure of the gaseous system, R = Gas constant for air, T a = Absolute temperature, T = Ambient temperature and P b = Ambient barometric pressure. ρ m can be obtained in terms of the number of Pascal s of ambient barometric pressure.

24 111 Fuel savings or energy credits are the major benefits from the wind power. These savings result from the reduced need to run other generating plants. This in turn results in lower fuel and related variables costs, including maintenance and staff costs. 5.7 RESULTS OF SIMULATION The output power of the rotor-resistance-controlled wind energy conversion system is shown in Figure The output power variation is about 5 % and the settling time is about 5 secs. Thus, the performance of the system is greatly improved compared to the pitch controlled system. The rotor speed is plotted in Figure The speed is restricted between 1.0 p.u and 1.1 p.u as designed. The rotor speed indicates the power dissipated in the rotor circuit. To obtain a stator output power of 12 KW (or 1 p.u). The electromagnetic torque has to be 1.0 p.u. (using equation (5.3)). Thus, the rotor power loss has the same profile as the rotor speed and the rotor losses are found to be about 10 % of the stator power at wind speeds of 16 ms -1. Figure 5.18 Output power of the rotor-resistance controlled generator

25 112 Figure 5.19 Rotor speed variation of the rotor resistance controlled system Thus, at low and medium wind speeds (where the slip is large); the system has a low overall efficiency (Anderson and Anjan Bose 1983). 5.8 SUMMARY The rotor resistance controller was designed and the performance of the system was tested in the presence of disturbances. The controller was found to have a smaller output power variation and faster response compared to the pitch controlled system, at the expense of large rotor power losses at low wind speeds. The method using external resistance for fixed speed wind turbine with a variable pitch wind turbine or variable slip generator has been discussed.