1. SLIM® and Bio-SLIM® The “Life starts at 36”-project:
development of wind turbine generator transformers above 36kV
Raymond Van Schevensteen
Technology Head Distribution Transformers
Co-authors:
Dr Jan Declercq (CTO) & Yves Vanlinthout (Mngr PL DT Renewables)
SLIM® and Bio-SLIM®
The new generation of compact transformers

2. Offshore WF components? X
MV cable
Switchgear
Transformer
HV cable
Power Electronics
HV cable

3. Carbon Trust - OWA
The financial balance needs a cost of offshore electricity to come down from €160 to €120/MWh.
Until 2008 the capital cost of an offshore WF farm was around €2m/MW, the cost has now doubled due to rising material prices, more complex sites and supply chain bottlenecks.
One way to reduce this cost is by increasing the output voltage of the WTGT above 36kV.

4. Carbon Trust - OWA
Moving into the high voltage (HV) range above 36kV directly at the individual wind turbine level has the following advantages:
The same cable cross section can transfer higher power at higher voltages. Alternatively use smaller cable section for same power.
Lower currents = lower losses in transmission.
The higher output voltage per turbine will reduce the number and/or size/mass of the required platform substation transformers, or even eliminate them in case of near-shore wind farms.

5. Life starts at >36
Until today, the existing wind farms are limited to the medium voltage (MV) range up to and including the insulation level of 36kV (AC70/LI170). Typically 33kV or 34kV rated voltage.
As per IEC , the insulation levels above 36kV are:
52kV (AC95/LI250)
60kV (AC115/LI280); unpopular
72.5kV (AC140/LI325); incl. common 66kV

6. HV Switchgear >36kV
HV switchgear and cable accessories are today commercially available for both 52kV and 72.5kV, and as well in dry air, vacuum or SF6 technologies from the major well-known companies for MV and HV installation materials.

7. HV underwater cables >36kV
Current unavailability makes HV cables the main barrier to move quickly to the >36kV solution.
New types of underwater HV inter-array cables are to be developed.
The financial evaluation depends on failure rate of the HV wet cables : 3 failures per year doubles the LCOE compared to 1 failure every 6 years.
Cable manufacturers report promising developments: cost of 72.5kV marine cable is some 15% higher than 36kV cable while its capacity in transmittable power doubles.

8. HV underwater cables >36kV
The next step is get this range of underwater
cables tested and certified…
.... and to define the proper accessories.

9. HV WTG transformers
Two main families of fire safe WTGT are used inside wind turbines:
Dry type cast resin transformers
K-type liquid-filled transformers (fire point >300ºC)
silicone fluid (SLIM®)
ester (Bio-SLIM®)
The main differentiator between the (Bio-)SLIM®-line and the dry type transformer becomes the voltage level.

10. HV WTG transformers >36kV
The range >36kV already “came to life” with pioneering compact Bio-SLIM® units of 52kV (onshore WF in Canada) and 72.5kV for stationary traction. WTG transformers are today commercially available for both 52kV and 72.5kV.
Design flexibility allows customized designs:
Cooling KNAN, KFAF and KFWF
Copper and/or aluminium conductors

11. HV WTG transformers
Example 6MVA KFAF design with similar efficiency level
Small impact when moving to 52kV.
Important impact when moving to 72,5kV.
Higher unit cost (impacted by use of copper to stay compact) adds to increased structural cost, but will be covered in the overall system cost reduction.

12. All components inside tower or (txf & SWG) outside
Installation options ...
Switchgear
Transformer
Transformer
Switchgear
Hatch
Power electronics
Power electronics
All components inside tower or (txf & SWG) outside

13. ... containerised solutions (OWA)
For the WTG 5MW solution, the cost for 3 container types in the comparative study was estimated to be:
Horizontal layout = 100%
Vertical layout = 110%
2 container solution = 115%
From a purely layout point of view the horizontal layout is recommended.

14. Moving into >36kV Benefits (savings)
More wind turbines per string = fewer strings, lower overall cable length and cost, reduced system losses and higher output.
Fewer or lighter main grid substation transformers.
Drawbacks (expenses)
Higher structural total cost per wind turbine installation.
Larger clearances in and around the HV equipment.
Challenges
Get HV underwater cable certified.
Find most suiting housing of the transformer and switchgear.
Find fitting accessories and interfaces.
Minimise maintenance & maximise reliability.
Facilitate 72.5kV adoption by standards and certification bodies.

15. Conclusion
Once proven to be competitive in total project cost to well established 36kV arrays, and based on the promising feedback from industry, it is expected to see the first 72kV pilot project within 2 years.
Question is: who will be the first turbine manufacturer or project developer to set this innovative step?
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