A comparison of low voltage and medium voltage wind

turbine drive trains

EWEC 2010, April 22, Warsawby

Anders Troedson, The Switch

Introduction/drivers for medium voltage (MV) drive trainsPerformanceTechnology issues

TopologiesDevices IGBT and IGCT packagingCooling

Cable cost comparisonMaintenance and serviceReliabilityPro’s and con’sCostsSummary

Presentation outline

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 304/22/23

Drivers for medium voltage power converters

Potential to reduce converter size and weightPotential for reduced cable cost unless power converters can be installed in the towerFewer electric componentsSmaller filters using multi-level converter topologies

Benefits with medium voltage converters come at a costHigher cost for equipmentOften more complex cooling system using press pack devicesCurrent LV designs are more robust Maintenance / service technician must be qualified for MV

Low voltage and medium voltage converters

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Medium voltage ( 2KV, 3.3 KV and higher)

Low voltage (400, 480, 500, 600 & 690 Volts)

kW0 1500 3000 4500 6000

Low voltage (400, 480, 500, 600 & 690 Volts)

Medium voltage (2 KV, 3.3 KV and higher)

Industrial drives

Wind turbine power converters

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

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

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 converter3-level capacitor clamped NPC converter3-level H-bridge NPC converter

Cascaded converters using multiple 3-phase modulesDifferent hybrid topologies

Converter topologies for LV and MV

690 V full-power converter system

Permanent magnet DD generator and full-power converter

Medium voltage full-power converter3-level MV full-power converter with IGCTs

3-level NPC full-power converter (3300 Volts) *

* Converter diagram courtesy ABB

Size comparison – 6 MW converters

PowerPoint Test 1104/22/23

Low voltage 690 V – 6.38 sq.m.(2.3 cubic meter / MW) **

Medium voltage 4160 V – 4.32 sq.m.(1.6 cubic meter / MW)*

MV & LV wind turbine converters

•Winkelnkemper, Wildner & Steimer, 6 MVA Five-Level Hybrid Converter for WindPower, IEEE 2008** The Switch 690 V Power Converter for 6 MW Turbine

690 V and 3,300 V full-power converters

Full-power converterfor 690 V from

The Switch

Inverter section of 3.3 KVABB* full-power converter

* Picture courtesy ABB

Press pack (MV) IGBT & IGCT

More compact converter design (packaging)More efficient heat transferHeat 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 plateHeat sink on ground potentialEnables simpler and more

rugged cooling system

Components for low and medium voltage

MV press pack IGBT / IGCT IGBT modules for LV & MV

Cooling system comparison – LV and MV

Flow diagram – Liquid-cooling system for MV power converter

Flow diagram – Liquid-cooling system for LV power converter

Cooling system considerations

• 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 1604/22/23

De-ionized water required with glycolMust maintain low coolant conductivity – needs special deionizer“Hot” heat sinks – potential for electrolysisMore stringent material requirements for cooling components

Low voltage Medium voltage w/ press pack

5 MW system - power cable comparison Tower to base

PowerPoint Test 1704/22/23

Reliability

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Factor Low voltage Medium voltage

Component count +

Corona effects +

Effects of possible condensation

-

Cooling system robustness +

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

Pro’s and con’s of LV versus MV

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Low voltage Medium voltageCost ++

Physical size and weight +

Cable cost incl. stress cones & lugs +

Control features & performance = =

LVRT = =

Efficiency Typ. 97.5 % Typ. 97.5 %

Partial redundancy +

Cooling system design requirements -

Servicing, safety & training requirements - -

Maintenance + -

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 2004/22/23

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 2104/22/23

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

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FinlandTel +358 20 783 8200

www.theswitch.com

Questions are welcome