Fault Ride-Through Strategies for VSC-Connected Wind Parks

1. Fault Ride-Through Strategies for VSC-Connected Wind Parks Ralph L. Hendriks, Ronald Völzke, Wil L. Kling

2. Contents • Introduction • Technical requirements for grid connection • VSC transmission system outline • Influence of converter (de-)rating • Energy dissipation • Fast power reduction • Direct communication • Voltage reduction • Frequency droop • Design optimization • Conclusions

3. Introduction Future wind parks will be situated far from load centres, long transmission distances will have high power ratings (hundreds of megawatt) Application of HVAC transmission is limited by charging current of cables HVDC transmission can overcome these limitations. Two types: Current-source converter Voltage-source converter

4. Grid connection of wind power Transmission system operators require well defined technical behaviour from wind power plants During faults in the power system, wind power plants are usually required to: remain connected during and after the fault (fault ride through) support system restoration by supplying reactive current Wind turbine generators have been further developed to comply to these requirements Technical requirements

5. Grid connection of wind power Technical requirements Technical capabilities are required at the point of connection For VSC-connected wind power plants, the behaviour during faults is completely determined by the properties of the VSC transmission system Different types of wind turbines!

6. VSC transmission system overview • Two-terminal link connecting wind park to active network • WPVSC functions as a slack node, absorbs all power • GSVSC controls direct voltage • Converter type does not impact general applicability of presented strategies

7. Converter (de-)rating Power electronic switches have hardly over-load capability Current limit must be maintained at all times De-rating could improve FRT performance

8. Energy dissipation • Control of the direct voltage during faults using a dissipative device • Power electronic control is required (chopper) • Power electronic switches will constitute a high price for this solution • Thermal aspects need to be considered

9. Fast power reduction Wind-park side VSC signals power reduction order to turbines through a communication link Only applicable for turbines with controllable converters Typical time delay 10–100 ms Reliability is an issue Communication

10. Fast power reduction Wind-park side VSC sinks the AC voltage to reduce the incoming power Inherent reaction from directly coupled induction generators The success for wind turbines with power electronic converters depends on the ratings and controls of the converters Standard FRT methods need to be disabled Voltage reduction

11. Fast power reduction The frequency in the wind park network is increased to signal power reduction Inherent response from directly-coupled induction generators Additional droop characteristic in turbine control necessary Speed of frequency measurement is an issue, PLLs tend to be slow Frequency droop

12. Design optimization Converter de-rating and dissipation load to higher investment costs Strategies can (parly) be combined to realize reliable FRT solutions System can be optimized by formulating boundary conditions and optimization methods, such as linear programming

13. Conclusions The FRT behaviour of VSC-connected wind parks is greatly determined by the design and control of the VSC-system Grid-code compliance with respect to FRT could be achieved by de-rating, dissipation of excess energy and fast reduction of incoming wind power Fast power reduction methods yield lowest additional costs Optimized design could combine several FRT strategies