Use of STATCOM for Offshore wind stability & grid compliance

1. Use of STATCOM for Offshore wind stability & grid compliance C Smith 15/10/09 This document contains confidential and proprietary information of Converteam and must not be used for any purpose other than that for which it is supplied by Converteam. Its contents must not be disclosed to any other person nor copied in whole or in part without the prior written consent of Converteam

2. Introduction • Grid code requirements • Options for VAr compensation for offshore windfarms • STATCOM • Case Study • Additional benefits of STATCOM

3. MVArs versus power current “ P “ 100 % 50 % 0 % - 50 % Lag - 30 % 0 % + 30 % + 50 % Lead Reactive Power “ Q “ UK Grid Code MVAr Wind Farm Requirements • Requirement is for a rated +/- 0.95 PF. • This is +/- 33 % MVArs at 100 % AC volts. • Below 20 % power MVArs can be zero. P MVArs versus network voltage “ V “ • Continuous control required • Requirement is for MVArs defined at a +/- 5 % AC supply voltage level. • Below 95 % AC supply voltage the MVArs are not defined but are to be at the plant’s ability. • For optimal system design the MVArs current should be constant for 0 to 110 % AC supply voltage. V UK grid rating point

4. Solutions to meet grid codes • Mechanically switched reactors/capacitors (MSR/C) • Cost effective  • Cannot provide continuous control  • Static VAr Compensation (SVC) • Cost Effective for large MVAr rating  • Not constant current below rated voltage  • STATCOM (Static Synchronous compensator) • Constant current down to 20% voltage  • Most expensive per MVAr of technologies  • Combination Switched devices/STATCOM • Cost effective • Keeps all the advantages of STATCOM

5. Converteam’s MV ProVAr STATCOM Controller PECe DC grounding switch IGBT phase power stack Terminals to motor Cooling unit with standby pump Terminals to transformer DC capacitors

6. Design decisions - example • Nominal offshore wind farm • 2 off 132kV step out cables of 90km each to meet UK regulator (OFGEM) recommendations • 72 turbines in 8 strings • Connection agreement of 260MW • Capacitive element of step out cables 99 MVAr • Two design options reviewed • Option 1 – MSR/C and Wind turbines • Option 2 - MSR/C, Wind turbines and STATCOM

7. Example – reactive power requirements • Load flows give following span • 99 MVAr reactors to compensate for step out cables • 20MVAr capacitive point A • 72MVAr inductive point C • Option 1 • Six 15MVAr & one 9 MVAr reactors • 72MVAr from Turbine (step out cables limited) • Option 2 • five 15MVAr and one 9MVAr inductor • One 15MVAr STATCOM • 72MVAr from Turbine (step out cables limited)

8. Options review-based on example • Option 1 – MSR/C and Wind turbines • Scheme Cost of £x • Option 2 - MSR/C, Wind turbines and STATCOM • Scheme cost of 1.5 x £x (50% cost addition) • Easy decision based on cost? • Cost difference compared to farm complete CAPEX is approximately 0.3% of total CAPEX • % Differential will reduce as wind farm becomes larger • Additional benefits for offshore

9. Offshore distribution system stability • Example from a study of a UK offshore wind farm • Long cables introduce resonances • Number of turbines connected alters circuit impedance. • Resonance point moves 12th harmonic 7th harmonic

10. Impact on offshore distribution system • Significant amplification of existing grid harmonics • Wind Turbine converters have very high control bandwidth • Possibility of random/nuisance trips At PCC Voltage At wind turbine converter

11. Solutions • Reduce wind turbine converter controller bandwidth • Reduces control performance • Could impact on Grid Fault Ridethrough capability • Passive Filter • Difficult to design for all combinations • Costly • Active Damping • Can be added to STATCOM with additional software Voltage Active filter off Active filter on

12. Frequency response • Grid generation/loads changing • Wind farms to play a more active role in Ancillary service • HydroQuebec grid code now calling for synthetic inertia from wind farms • At present requires derating of wind turbines output • Loss of revenue • Energy storage can be added to STATCOM • Possible technologies • Supercapictors • Flow cells • Flywheels • Technology only just becoming commercially viable • Our STATCOM example would give 5% at full generation or 15% capability based on utilisation factor (40%)

13. Super cap DC energy store Converteam ProVAr With Energy Store • Each cap store: • 250 units, 125 in series. • Net 2.6 F. • 11 MJ available. Provides fast MVAr and MW support for the AC supply down to zero retained AC supply voltage. Provides fast acting current support to assist in fault recovery and AC supply frequency control. AC supply Transformer

14. Conclusions • Temptation to choose least cost option of mechanically switched devices • STATCOM provide additional advantages: • Active damping to improve wind farm stability • Frequency response can be retrofitted when commercially viable

15. Thank you for your attention This document contains confidential and proprietary information of Converteam and must not be used for any purpose other than that for which it is supplied by Converteam. Its contents must not be disclosed to any other person nor copied in whole or in part without the prior written consent of Converteam www.converteam.com