A comparison of low voltage and medium voltage wind turbine drive trains

1. A comparison of low voltage and medium voltage wind turbine drive trains EWEC 2010, April 22, Warsaw by Anders Troedson, The Switch

2. Introduction/drivers for medium voltage (MV) drive trains Performance Technology issues Topologies Devices IGBT and IGCT packaging Cooling Cable cost comparison Maintenance and service Reliability Pro’s and con’s Costs Summary Presentation outline

3. General information The presentation focuses on the preferred drive train technology using full-power converters This material is particularly applicable for the preferred permanent magnet generator drive train Both, the low voltage (LV) and MV converters discussed herein are liquid-cooled. Liquid-cooling preferred for multi MW converters due to more compact size and easiness to adapt to a severe environment, particularly off-shore The MV power converter discussion focuses on so-called press pack power devices (either IGBT or IGCT), which are currently the prevalent MV device packaging PowerPoint Test

4. Drivers for medium voltage power converters • Potential to reduce converter size and weight • Potential for reduced cable cost unless power converters can be installed in the tower • Fewer electric components • Smaller filters using multi-level converter topologies • Benefits with medium voltage converters come at a cost • Higher cost for equipment • Often more complex cooling system using press pack devices • Current LV designs are more robust • Maintenance / service technician must be qualified for MV

5. Low voltage and medium voltage converters Wind turbine power converters Medium voltage (2 KV, 3.3 KV and higher) Low voltage (400, 480, 500, 600 & 690 Volts) Industrial drives Medium voltage ( 2KV, 3.3 KV and higher) Low voltage (400, 480, 500, 600 & 690 Volts) kW 0 1500 3000 4500 6000 PowerPoint Test

6. 4 MW direct-driven PMG and low voltage (690 V) full-power converter

7. 6 MW drive train comparison LV and MV 6 MW LV (690 V) full-power converter system for wind turbine 6 MW MV (3300 V) full-power converter for wind turbine

8. Low voltage (up to 690 Volts AC) 2-level converter with LV IGBT modules Medium voltage (>1000 V AC) NPC multi level converters (3 and 5-level topologies) 3-level diode clamped NPC converter 3-level capacitor clamped NPC converter 3-level H-bridge NPC converter Cascaded converters using multiple 3-phase modules Different hybrid topologies Converter topologies for LV and MV

9. 690 V full-power converter system Permanent magnet DD generator and full-power converter

10. Medium voltage full-power converter3-level MV full-power converter with IGCTs 3-level NPC full-power converter (3300 Volts) * * Converter diagram courtesy ABB

11. Size comparison – 6 MW converters MV & LV wind turbine converters Medium voltage 4160 V – 4.32 sq.m. (1.6 cubic meter / MW)* Low voltage 690 V – 6.38 sq.m. (2.3 cubic meter / MW) ** • Winkelnkemper, Wildner & Steimer, 6 MVA Five-Level Hybrid Converter for WindPower, IEEE 2008 • ** The Switch 690 V Power Converter for 6 MW Turbine PowerPoint Test

12. 690 V and 3,300 V full-power converters Full-power converter for 690 V from The Switch Inverter section of 3.3 KV ABB* full-power converter * Picture courtesy ABB

13. Press pack (MV) IGBT & IGCT More compact converter design (packaging) More efficient heat transfer Heat sink on voltage potential - “hot heat sinks” Requires own separate internal cooling loop with higher complexity Press pack versus IGBT modules • IGBT module (LV + MV) • Design with integral cold plate • Heat sink on ground potential • Enables simpler and more rugged cooling system

14. Components for low and medium voltage MV press pack IGBT / IGCT IGBT modules for LV & MV

15. Cooling system comparison – LV and MV Flow diagram – Liquid-cooling system for MV power converter Flow diagram – Liquid-cooling system for LV power converter

16. Cooling system considerations Medium voltage w/ press pack Low voltage • De-ionized water required with glycol • Must maintain low coolant conductivity – needs special deionizer • “Hot” heat sinks – potential for electrolysis • More stringent material requirements for cooling components Cooling system is more forgiving/robust Drinking water quality with glycol is typical Heat sinks on ground potential Aluminum or stainless steel cooling system preferred, but copper/ brass also possible PowerPoint Test

17. 5 MW system - power cable comparison Tower to base PowerPoint Test

18. Reliability Reliability (MTBF) is more than fundamental theoretical calculations – it is also a matter of design and consideration for ambient conditions, installation and service and the robustness of the technology PowerPoint Test

19. Pro’s and con’s of LV versus MV PowerPoint Test

20. Other design considerations Multi-level topologies, used primarily for MV, helps keeping filters smaller and reduces torque ripple Switching frequencies are lower for MV (IGCT 500 – 1000 Hz) while LV IGBT is switching at 3000 – 5000 Hz Higher switching frequency lowers filter (inductor) sizes Corona effect increases with the voltage – ex. properly made cable connections and use of stress cones required MV is even more sensitive than LV to condensation and moisture Effective anti condensation features becomes increasingly important with higher system voltages PowerPoint Test

21. Costs MV converters up to 5 MW are still significantly more expensive than corresponding LV power converters Premium costs for 5 MW medium voltage power converter is at least 25 % Costs are therefore NOT a major driver towards medium voltage power converters – size and weight are PowerPoint Test

22. Summary Full-power converters with permanent magnet generators is the preferred drive train solution for new turbine designs Full-power converters have superior fault ride-through performance (LVRT) and with permanent magnet generators offer maximum energy yield Performance for LV and MV are essentially the same Medium voltage power converters increased popularity above 5 MW Installed cost for MV exceeds that of LV converters LV power converters are available up to at least 6 MW LV converters offer partial redundancy MV power converters more costly, but reduced size and weight Low voltage converters are more service proven. LV technology offers a more robust design

23. Äyritie 8 C FI-01510 Vantaa Finland Tel +358 20 783 8200 www.theswitch.com Questions are welcome