1. Overview OF MULTI Mega Watt WIND TURBINES and wind parks1

2. CONTENTS Introduction Wind Turbine System Overview
Conventional Generators
Control systems
Energy storage and power smoothing
Power-Converter Topologies
Park Connection Systems
Grid Connection Issues
Conclusion
References 2

3. INTRODUCTION Energy crisis
Importance of renewable energy resources like
Electricity can be produced from wind energy by Multi MW wind turbines
Wind turbines can be constructed offshore or onshore
High wind energy potential of offshore wind parks 3

4. 4

5. conventional generators
Doubly Fed Induction Generators (DFIG)
Squirrel Cage Induction Generators (SCIG)
Synchronous Generators (SG) 5

6. Doubly Fed Induction Generators
Widely used for variable speed generation
Reduced power converters rated 30% of nominal power
Stator is directly connected to the grid
Speed range is limited and slip rings are required to connect the converter
Gearbox combined mechanism is required
Fault handling capacity is poor 6

7. Squirrel Cage Induction Generators
SCIGs are of robust construction and mechanically stable
Rotor consist of metallic bars, resistant to dirt and vibration
Two full scale power converters are required
Fault current is limited by grid side converter
Variable speed operation cannot be obtained using reduced size power converters
Cannot be used in direct-driven WECS 7

8. Synchronous Generators
Most efficient Synchronous Generator is direct drive PMSG
They have lesser power losses
Noise reduction is achieved as gear boxes are eliminated
For offshore applications increased oil spills from gear boxes are eliminated
More reliable
Cost , weight and size is more than DFIGs 8

9. CONTROL System Electrical control system Mechanical Control System
Supply of active/reactive power to the grid
Overload protection
Mechanical Control System
Power limitation with pitch adjustment
Maximum energy capture
Speed limitation
Reduction of acoustical noise 9

10. ENERGY STORAGE & POWER SMOOTHING
Wind speed variability produce unacceptable variations on the power
Power smoothing is done by supplying a compensating power Pc from an energy storage system
Pgrid = Pw + Pc
Energy storage may be based on flywheels, super capacitors, lead acid batteries etc.. 10

11. POWER Converter topologies
POWER CONVERTORS FOR DFIG
A back to back converter with converter bridges are used 11

12. CONVERTERS FOR OTHER GENERATORS
Full power converters or full scale converters are used. 12

13. A parallel array of Converter modules are used for reliable power output13

14. PARK CONNECTION SYSTEM
Parallel Connection
Series Connection
Cluster Connection 14

15. Parallel connection of turbines
Figure shows a parallel connected offshore wind park in which a high voltage power converter is employed 15

16. High voltage DC transmission is employed with the help of converters
Conventional scheme in which turbines are connected to a low voltage grid
A high power transformer is needed to increase voltage to transmission level
High voltage DC transmission is employed with the help of converters
Advantages :
More reliable
Can be used for both DC and AC offshore grids 16

17. Weight is more and thus offshore platforms needed
Drawbacks :
Weight is more and thus offshore platforms needed
Investment cost is high
Lesser efficiency due to losses in low voltage offshore grid
Introduction of medium voltage power conversion systems in WT can eliminate transformers and increase efficiency. 17

18. SERIES CONNECTION OF TURBINES
The figure shows a series connected offshore WECS which employs HVDC transmission 18

19. Requires lesser cables and investment cost
Output of each wind turbine is converted to DC and then connected in series via HVDC(High Voltage DC) cables
Advantages :
Requires lesser cables and investment cost
High power transformers are not needed
Least losses in transmission lines and offshore grid since HVDC is transmitted 19

20. Losses in power electronic converters
Drawbacks :
Losses in power electronic converters
Variation in wind velocity will cause variation in output
Less reliable 20

21. Cluster connection of wind turbines
Figure shows the cluster connection of wind turbines which holds the advantages of both series and parallel arrays compromising between reliability and efficiency. 21

22. GRID CONNECTION ISSUES
1. HARMONICS
At control level modulation techniques and harmonic controllers are used
At hardware level filters are used
Introduction of filters may cause stability problems
2.FAULTS/UNBALANCES
May create negative sequence components in voltages or currents
Reduces life span of generation system
Controlled by connecting generator stator to grid using full power converters 22

23. CONCLUSION
Most adopted generator system is DFIG equipped with a back to back converter since less weight and cost
For large wind energy systems, direct drive PMSGs are preferred due to better reliability and efficiency
Full power converters can reduce the effects of grid voltage unbalances in the generator
For offshore wind parks, cluster connection combines the advantages of both series and parallel connections 23

24. References
A.Mogstad, M.Molinas, P.Olsen and R.Nilsen, “A Power Conversion System for offshore wind parks”, IEEE transactions on Industrial Electronics, vol 58, no.4, Nov 2008
Kaigui Xie, Zefu Jiang and Wenyuan Li, “Effect of Wind Speed on Turbine Power Converter Reliability Wind”, IEEE transactions on Industrial Electronics, vol 27, no.1, March 2012
B.Rabelo and W.Hofman, “Control of an Optimised power flow in wind power plants with doubly fed induction generators”, IEEE on Power Electronics, June 2003 24