wind farm earthing

[royclh]

i have a question and hope somebody can enlighten me about an offshore wind farm with a three winding transformer(yd1d1) on the offshore platform. The client requested the star winding of the 150/33kV offshore transformer NOT to be unearthed as the system is already earthed at the onshore substation about 30 miles away. delta sides (33kV) connected to the wind turbines are however grounded via the earthing transformers. I wonder what is the purpose of leaving the neutral unearthed? I normally dealt with earthed systems where the earthing point is generally close to the transformer. This time, the earthing point is 30 miles away from the transformer.. how does that affect the winding insulation level, fault current etc?

look forward to your reply. Many thanks!

 

[rbulsara]

I take it, you mean Not to be earthed on 150kV side.

My opinion is based on my recent invovement with a WTG project of much samller scale compared to yours, but principles may still be the same.

WTG do not always generate power, while the interconnect transformer remains always energized from the grid. So for the grid, it is no different than other load transformers. WTG are certainly treated differently than regular generator plants, that is as induction motors regardless of direction of power flow.

Since the grid is always on, it is supposed to clear any earth faults.

Also typically wind turbine interconnect xfmrs are Y on the WTG side and Delta (unearthed) on the grid side. Your case seems to be an exception. Most WTG are 3 wire machines and and grounded through wye side of the interconnect transformers.


Rafiq Bulsara

http://www.srengineersct.com