Three winding transformer

[elektroeng]

Hi Gents,

We are going to install a gas turbine in our refinery (36kV infeed and 6kV distribution with 15MW peak load and two transformers 36/6kV, one 15MVA and one 25MVA) with a power of 14,5MW. This transformer needs an insulation transformer to reduce short circuit power added to the 6kV network and to insulate from high earth fault currents (700A with two transformers parallel). Now my idea was to replace the 15MVA transformer with a three winding transformer :
- 1 36kV side 25MVA with load tap changer controlled by the 6kV voltage
- 1 6kV side 25MVA (parallel with existing 25MVA)
- 1 10kV side 20MVA (GT connection)

What do you guys think of this scheme ? Are there things that i have to take special attention of ?

Thanks for your suggestions.

 

[swn1]

Overall availability and reliability will be higher with a separate transformer for the generator. First cost and contingency replacement costs will be lower with two-winding transformers.

Generator control as well as protection are far easier with significant impedance between the grid and the generator. Order as much as is practical in your application.

 

[motorspert]

hi
I have seen this done before and never heard of any problems. The split of the transformer reactances was quite strange I seem to remember - there is a value allocated for each winding and you can specifiy a high reactance for one winding (say 6kV) and lower for the other (10kV).

Differential protection becomes interesting, but is just a matter of selecting the right ratios.

You've obviously looked at in-line reactors and split bus system and rejected them?

Rich

 

[elektroeng]

Hi Rich,

Yes, we've thought about in-line reactors but they don't solve our ground fault problem, 700A will damage the generator windings too much. For short circuit reasons they would be ok.

Regards,
Jan Bosmans