132kV side of 11/132kV Generator transformer - OHL

[RRaghunath]

We plan to take the power output of 25MW generator to the 132kV switchyard that is about 2km away for evacuation to the grid.

I would like to know whether we can take the power through OHL directly from the generator transformer 132kV terminals (ofcourse, there will be surge arrestors across the transformer HV windings).

There is space constraint for providing full fledged 132kV switchyard bay with breaker etc. near the transformer.

The 132kV breaker in switchyard (2km away will be controlled from the power plant and there will be trip and interlock circuits will be extended to the switchyard breakers as required by the safety norms / codal requirements.

We can accomodate disconnecting switches, if the same is codal requirement (though I don't think it is).

I would be obliged if some one could tell me the practice / precedences / opinion.

Thanks in anticipation.

 

[buzzp]

Info is a little vague as far as what is happening on the generator side of this transformer. I assume you have a breaker between the transformer and generator?

The 132kV breaker in the yard, is this only hooked to your feed or is it on the main transmission line?

 

[RRaghunath]

buzzp,
Thanks for the attention.
There is no breaker on the generator side of the transformer, generator, generator transformer and unit auxiliary transformers are connected as one set with isolation envisaged at 132kV switchyard. The protections of these equipment trip the 132kV reaker as well as the excitation breaker of the generator.
The incoming transmission lines in 132kV switchyard have their own breakers and the generator transformer feeders have separate breakers.
Trust the above is helpful.

 

[buzzp]

I can see a need for a breaker or some other disconnect at the plant before the switchyard for maintainence purposes (transformer tests, generator tests, cable tests). It is just much easier to through a switch than it is to disconnect cable. We typically have a generator breaker between the generator transformer and generator so that we can be ready immediately to transmit power (generator turning but generator breaker not closed). I work with hydro electric plants so this is probably not a factor for you. Some plants have a disconnect after the transformer before the switchyard to better isolate the transformer for testing.
If you only have one generator to worry about then one breaker in the yard will suffice. But if you have multiple generators I would hope you have a breaker for each one so the plant does not have to be down in the event of one generator being off line. I am pretty sure you realize this but just checking.
I have not really checked into any 'best practices' on this subject from IEEE or similar since all of our plants were built well before I came on board.
Personally, besides the maintainence issues, I do not see any issues with your design except one: If the OHL or underground cable faults between the generator and the generator breaker in the switchyard your generator protection relays better pick-up. Presumably they are set for this type of fault. Then the excitation will have to be tripped, while the generator coasts to a stop you may still be feeding the fault until any residual magnetism is gone.
I hope this was of some help.

 

[RRaghunath]

Thanks buzzp for sharing your thoughts. It is certainly of help to me.

 

[busbar]

§5.9.3 “Transformer Ungrounded Neutral” in IEEE Std C62.22-1997 suggests application of an arrester to ungrounded-neutral situations. That may be a reasonable compromise with graded transformer insulation that may require only cursory relay studies.

It may pay to get an opinion from the original transformer manufacturer for their expectations for ungrounded-wye neutral overvoltage through lightning and switching events.