Wind Generation and its Grid Conection

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1 Wind Generation and its Grid Conection J.B. Ekanayake PhD, FIET, SMIEEE Department of Electrical and Electronic Eng., University of Peradeniya Content Wind turbine basics Wind generators Why variable speed? Grid Code requirements Concluding remarks 1

2 Introduction Worldwide wind capacity as at mid of 2015 = 393 GW China USA Germany India Spain UK Canada RoW WIND TURBINE BASICS 2

3 How wind turbine works? 5 How wind turbine works? Air incident on the airfoil produces plenty of lift which aid the rotation of the blade. The drag tries to bend the blade towards the tower. 6 3

extracted by the aerodynamic rotor = C p x Power available C p is the coefficient of performance The Betz limit: Maximum

4 Power available in a wind stream The kinetic energy in a flow of 1 2 air = perunitmass 2 U 1 Mass flow rate= ρau 1 (kg/s) ρ istheairdensityinkg/m 3 Power available in the wind stream = ρau Energy-extracted by the wind turbine Power extracted by the aerodynamic rotor = C p x Power available C p is the coefficient of performance The Betz limit: Maximum value of the coefficient of performance C p is 59%. C p depends on the tip speed ratio ωr λ= = U Velocity at rotor tip Wind velocity 4

5 Cpvs Tip speed ratio Variable speed operation Generator speed in pu 1440/1800 = 0.8 pu 13 m/s Power (pu) Maximum power that can be extracted 10 m/s 11 m/s 12 m/s m/s m/s 6 m/s7 5 m/s 9 m/s Generator rotational speed (pu) To extract maximum power ω r should vary with the wind speed 5

Evolution of Wind Turbine Technologies Available wind turbines Turbine Capacity Generator Rotor diameter Geared Vestas V164 8 MW FPC 164 m Yes Enercon E126 7.

6 Evolution of Wind Turbine Technologies Available wind turbines Turbine Capacity Generator Rotor diameter Geared Vestas V164 8 MW FPC 164 m Yes Enercon E MW FPC 127 m No Repower 6M 6 MW DFIG 126 m Yes Siemens SWT MW FPC - PMG 154 m No Alstom Haliade MW FPC - PMG 150 m No Areva M MW FPC - PMG 135 m Yes Gamesa G128 5 MW FPC - PMG 128 m Yes 6

7 HTS -DD Power curve of modern wind turbines Power 2 MW Cut-in wind speed 3.5 m/s Rated wind speed 12 m/s 25 m/s Cut-out wind speed Wind speed 7

Input aerodynamic power is reduced by increasing the

8 Control of modern wind generators Wind speed 7 to 15 m/s Wind speed above 15 m/s Electronic control Pitch control Input aerodynamic power is reduced by increasing the pitch angle at high wind speed 3.5 m/s WIND GENERATORS 16 8

9 Fixed speed wind turbine Variable speed wind turbines Wound rotor induction generator Back-to-Back VSCs connected to the rotor Doubly-Fed Induction Generator Turbine Gear box Induction or synchronous generator Diode bridge and a VSC connected to the stator Geared Full Power Converter Back-to-Back VSCs connected to the stator Gearless Full Power Converter Multi-pole permanent magnet synchronous generator 9

10 Doubly-fed arrangement Variable speed operation is possible by absorbing or injecting slip power using an external means. The sub-synchronous or super-synchronous speeds can be obtained by feeding in or taking out electric power to/from the rotor. This is normally done by injecting a voltage into the rotor circuit through slip rings. Zero rotor injection 10 R r /s jx r 5 se 2 Torque (pu) Operates as a fixed speed machine Slip (pu) 10

11 Negative injected voltage R r jsx r ω r > ω s se 2 sf V r P Will deliver power from the rotor through the converters to the network. Negative injected voltage The DFIG wind turbine running at super-synchronous speed 10 Torque (pu) (2) A (1) Slip (pu) 11

12 Positive injected voltage R r jsx r ω r < ω s se 2 sf V r P The DFIG rotor absorbs power. Slip torque curve The DFIG wind turbine running at sub-synchronous speed 10 Torque (pu) (2) A (1) B (3) Slip (pu) 12

13 Doubly fed induction generator (DFIG) v s is i g i a P + jq g g i r v r v a C1 C2 ac / dc dc / ac Controller 25 DFIG Converter C1 inverts dc voltage into slip frequency ac It can inject both positive and negative voltages by properly controlling the switching signals Converter C2 maintains a constant voltage on the DC capacitor 13

14 Control of converter C1 By injecting proper voltage through converter C1 Speed can be controlled for optimum power extraction. No load power factor of the generator can be controlled. Terminal voltage of the generator can be controlled. Control of DFIG wind turbines Torque control in q-axis Voltage control in d-axis Maintaining the turbine operation point on the maximum power curve is by means of controlling the generator torque Generator terminal voltage is controlled by manipulating the reactive power supply from the generator Pitch controller ω rated + ω r K + I P K pitch demand s β Orientation of the turbine blades are Physically moved to control the aerodynamic torque. 14

15 Fully rated converter (FRC) wind turbines DC-link totally decouple the generator from the grid Gearbox can be avoided if a multi-pole Grid frequency is decoupled, wind turbine can synchronous generator is used, e.g. operate at any rotor speed Enercon turbines with 64 poles Grid voltage is decoupled, change in grid voltage does not affect the generator dynamics Control of FRC wind turbines Machine side converter control In order to ensure maximum power extraction and wide speed range operation, the controller of machine side converter varies the operating frequency. This shifts the torque-speed curve and thus moves the operating point to match the maximum power extraction curve. 15

16 Control of FRC wind turbines (0.46, -0.2) (0.65, -0.4) -0.4 (0.8, -0.6) Hz (0.9, -0.8) Torque, pu Hz (1.02, -1.0) Torque-speed curve for maximum power 39 Hz 45 Hz Speed, pu Hz Control of FRC wind turbines ref i ds v ds ω r i ds i qs ω v qs sω ω L siqs ωlssids i qs ω ω i ds ref i qs 1Te ref k ref ids ω r ref T e ω ω r sω ref 1 iqs Lrr ref i R ds r ref i qs ref i ds 16

17 WHY VARIABLE SPEED? 1.4 Variable speed operation 1.2 Generator speed in pu 1440/1800 = 0.8 pu 13 m/s Power (pu) Maximum power that can be extracted 10 m/s 11 m/s 12 m/s m/s m/s 6 m/s7 5 m/s 9 m/s Generator rotational speed (pu) To extract maximum power ω r should vary with the wind speed 17

Power output Forces on wind turbines Loads on Blades Blade Flapwise bending Aerodynamic forces in flapwise direction Blade Edgewise bending Gravity forces Aerodynamic forces in edgewise direction

18 Power output Forces on wind turbines Loads on Blades Blade Flapwise bending Aerodynamic forces in flapwise direction Blade Edgewise bending Gravity forces Aerodynamic forces in edgewise direction Loads on Rotor Hub In-plane bending moments of the blades Out-of-plane bending moments of the blades Drive train interaction torque fluctuations due to control action Loads on Tower Axial bending moment 18

19 Forces on wind turbines Blade x direction Blade y direction Fixed speed Variable speed Blade z direction GRID CODE REQUIREMENTS 19

20 Grid Codes Grid Codes specify the mandatory minimum technical requirements that a power plant should fulfil and additional support that may be called on to maintain the second-by-second power balance and maintain the required level of quality and security of the system. Grid Codes for wind farm connections demand requirements at the point of connection of the wind farm not at the individual wind turbine generator terminals. 39 General Requirements Voltage [%] V H V L A 30 s B 60 min Continuous operation 1 min As most of the controllers employed in modern wind turbines are digital, the maximum allowable voltage limit for ICT should be considered. As all the wind turbines and PV parks are connected to the collector network through a transformer, a typical transformer V/f characteristic should be considered Frequency [Hz] 20

21 General Requirements V/f characteristics of transformers ITI (CBEMA) Curve Active power control Ireland, India, Denmark and Germany UK PA PB Active power output as a % of available power PD Frequency Ireland 47 (min) Denmark 47 (min) India 50.3 Germany

22 Reactive Power and Voltage Control Reactive power requirements 22

23 Reactive power requirements Reactive power requirements If reactive power requirement is not satisfied then a source of reactive power is required. 23

24 With the penetration of wind generation increasing, Grid Codes now generally demand Fault Ride-Through capability for wind turbines connected to transmission networks. What is FRT? Fault Ride-Through (FRT) Pre Fault Post Wind turbines operate normally and generate electricity Fault occurs and power generated from wind turbines can not be supplied. Generator speeds-up or DC cap voltage rise Fault is cleared by a circuit breaker Generators should come back to normalcy Before a fault Grid disturbances and DFIG J J During a fault, terminal voltage of the generator goes to very low value. In order to maintain the power flows, the controller increases the rotor currents. Crowbar is employed to protect the converter when the rotor current increases beyond its maximum limit. 24

LVRT capability The wind farm and any constituent wind turbine generating unit must remain transiently stable and connected

25 Grid disturbances and Full range J During a fault, power to the grid is limited p RDC DC-link voltage rises rapidly p GE Input power has to be reduced or excessive power has to be dissipated Chopper resistor at the DC-link R DC v dc C p C p GR LVRT capability The wind farm and any constituent wind turbine generating unit must remain transiently stable and connected to the system without tripping for balanced voltage dips and associated durations anywhere on or above the heavy black line 25

26 LVRT capability requirements FRT capability of wind turbines Fast pitching Braking resistor De-loading 26

27 T max 07/04/2016 Primary response Frequency support from DFIG wind turbine Stored kinetic energy of the rotor is high and can be used to support the power system. J Inertia support T max ω r + T sp f f d dt K f 2 K f 1 T

Concluding remarks Penetration of wind generation is increasing

now expect wind farms to perform exactly like a large synchronous

of connection In turn the CAPEX will increase Thank you You are

28 Concluding remarks Penetration of wind generation is increasing Large wind turbines and new technologies are emerging Utilities now expect wind farms to perform exactly like a large synchronous generator This demands extra plants to be connected at the point of connection In turn the CAPEX will increase Thank you You are invited to 600 acres beautiful campus University of Peradeniya, Sri Lanka 28

EE 742 Chap. 7: Wind Power Generation. Y. Baghzouz Fall 2011

EE 742 Chap. 7: Wind Power Generation Y. Baghzouz Fall 2011 Overview Environmental pressures have led many countries to set ambitious goals of renewable energy generation. Wind energy is the dominant renewable