34.5kv Equipment, Highest Ampacity 3000A? 2

[cyriousn]

I've been looking into this a bit and it seems like the highest ampacity of readily available equipment you can get for 34.5kv is 3000A. I'm looking at a collector substation for a renewable projects where they are trying to produce 200MW and I'm wondering if there are alternative solutions to having multiple Main Power Transformers to step the voltage up to 115kv. Any insight on this is much appreciated as I'm currently planning on having two 34.5kv busses with a tie breaker with each bus going to an MPT and then combining them at 115kv.

 

[davidbeach]

150MW is probably the highest you're going to reasonably get. Nothing says you can't parallel 150MW blocks though...

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[cranky108]

It is also possible to have a transformer with two secondary's. Each with half the capacity of the primary. It will cost more, as many 115/34.5 transformers are auto's, and a dual secondary will be a Y/Y/Y.
But such a transformer will be a custom, just like a breaker with a higher rating.
Also, it will be difficult to get switchgear with a rating higher than 3000 amps.

That said, there is something to be said for using common equipment, like two transformers, and two collector buses.

 

[cyriousn]

Agreed that 150MW at 0.95pf capability at the POI would limit things. I've seen 4000A bus at 115kv so I'm not sure why we couldn't have 4000A at 34.5kv to parallel into a single MPT. I'm assuming we could get the transformer differential protection to work between a high side breaker and two low sides?

 

[cranky108]

Yes you can get the bus at 4000 amps, but good luck on a breaker. Many transformer differentials will work for a three winding transformer, or even four windings.

 

[bacon4life]

Isophase bus available up to at least 28,000 amps per

https://www.technibus.com/products/isolated-phase-bus/.

Just using dual secondary windings or separate transformers sounds a whole lot simpler than messing with isophase bus.

 

[prc]

The transformer rating is limited by the 33 kV breaker capacity. I understand that at a 3kA current rating, the maximum breaker rating is 750 MVA. In India, a major oil refinery has a couple of dozen 174 MVA 220/34,5 kV Transformers with an impedance of 23 %.

 

[FPelec]

Hi cyriousn, we adopt a very similar solution for our "standard" renewable energy collection substations:
we use three-windings transformers, rated at 150 kV / 36 kV / 36 kV, YNdd, with a power rating of 250/125/125 MVA, with the two MV winding connected each one to a distinct 36 kV busbar.
As prc mentioned, the short circuit rating of the breaker is also a concern. Although modern 40.5 kV class vacuum circuit breakers can easily achieve a short circuit breaking current of 31.5 kA, it's important to consider also the contribution from generators connected to each busbar. Wind turbines and innovative "grid-forming" inverters for PV systems can ,to some extent, contribute to the short circuit current.
Therefore, it may be beneficial to utilize a three-winding transformer (as also bacon4life said) to reduce the short circuit contribution from the grid-side without increasing the transfomer short circuit impedance, as a much higher short circuit impedance could compromise reactive power support from Renewable Energy Sources.
A three-winding transformer of this type is not particularly complex; indeed, compared to a two-winding machine of equal power, it presents fewer issues for short circuit withstand.


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