1. IV th International Conference on Advances in Energy
Research,
Indian Institute of Technology Bombay, Mumbai
LVRT for Wind Power System
[Paper Code: 254]
Shrikant Mali1
Ishwari S Tank2
Steffy James2
1Dept.
of Electrical Engineering, IIT Bombay
2Dept. of E&TC Engineering, SITRC

2. Scope of Presentation
• Introduction
• Wind Energy Conversion System
• LVRT requirements
• System Characteristics: SCR
• Validation of LVRT
• Conclusion

3. Introduction
• Behavior of WECS should be similar to that of conventional power plants.
• Grid code(IWGC) has been revised.
LVRT (Low Voltage Ride Through) : wind farm connected to 66kV and
above should fulfill LVRT requirements.

4. Wind Theory
Power contained in Wind:
P = ½ ρ A v3 Watts
The power extracted from Wind:
P = ½ ρ A v3Cp
P= ½ ρ A v30.59 Watts
Where, Cp= Power coefficient
(Betz limit)

5. Cp v/s TSR
• Power coefficient (Cp) indicates the aerodynamic efficiency.
• Ratio of extracted power to the power contained in wind.
• Maximum power coefficient occurs at a specific TSR.

6. Power v/s rotor speed for MPPT

7. Wind Energy Conversion System

8. SCIG
• Wind turbine is connected to SCIG through a shaft and gearbox.
• This topology represents a fixed-speed wind turbine system.

9. PMSG
• Variable speed operation.
• Generator is connected to the grid through full converters.
• This allows the control to maximize performance.

10. DFIG
• Variable speed Topology.
• Stator is directly connected to grid.
• Rotor connected to grid through converters.

11. Effects of voltage sag on Different WTG
Topologies:
• Effects on SCIG:
Generator gets demagnetized and speeds up.
• Effects on DFIG:
The generator gets demagnetized and speeds up and converters may fail
due to current limitations of the converters.
• Effects on PMSG:
Generator speeds up and converters may fail due to current limitations of
the converters.

12. LVRT Requirements
During sag condition, the WTG should:
1)
Remain connected to the grid and stay operational.
2)
Support the system during fault condition by injecting reactive power.

13. LVRT Specifications
• Wind farms connected to 66kV and above can be disconnected if the operating point
falls below the line in figure.
Where,
• Vf =15% of Nominal System voltage
• Vpf = Minimum voltages mentioned in IWGC.

14. The fault clearing time for various system nominal voltage levels:
Nominal
system
voltages(kV)
Fault clearing
time, T(ms)
Vpf (kV)
Vf (kV)
400
100
360
60.0
220
160
200
33.0
132
160
120
19.8
110
160
96.25
16.5
66
300
60
9.9

15. System Characteristics
• Grid, can be modeled as a voltage source in series with impedance ZGRID:
SCC = Vbase2/|Zbase|
SCR = SCC/Sbase
• Short Circuit Ratio (SCR) = 20 (as per German grid code) or 6 (as per IWGC).

16. Solutions for LVRT
• Chopper resistor.
• Energy Storage System: Battery bank, Super capacitor etc.
• Excess energy can be stored in turbine- rotor inertia.
• Non-MPPT operation.

17. Validation for LVRT

18. Control scheme: Normal condition
• Generator side control:
Controls speed of generator.
Generator operated at maximum power point with the help of
modified Hill Climb Search.
• Grid side control:
Maintains DC-link voltage constant.
Controls flow of active and reactive power.

19. Control scheme: Fault condition
• DC-link chopper resistor:
Maintains DC-link voltage constant.
Active power dissipated in the form of heat through chopper resistor.
• Grid side converter control:
Reduces active and reactive power to zero.
• Generator side control:
Generator is not affected by grid fault.
Continues to produce expected power.

20. Graph 1: Wind Speed
Graph 4: Electromagnetic and
Mechanical Torque
Graph 2: Generator Speed
Graph 3: Generator Current
Graph 5: Generated Power

21. Graph 6: Grid Voltage
Graph 7: Grid Current
Graph 8: Grid Active Power
Graph 9: Grid Reactive Power

22. Graph 10: DC-Link Voltage
Graph 11: Transformer Secondary Voltage

23. Conclusion
• LVRT requirement is a major leap in the integration of Wind Power System
with the grid.
• We can implement the LVRT requirement combining 2 or 3 technologies.
• Control can be modified to implement reactive power compensation and
support the grid during fault condition.
• Not only the LVRT requirement but we should consider other grid
connectivity issues as well.

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26. Thank You!