Substation/Switchgear capacity

[Adam1980]

Dear members,

when considering a substation in the following configuration:
HV 380 kV breaker and half switchgear with 2500 A rating
2 transformers 380/115 kV each 500 MVA
HV 115 kV breaker and half switchgear with 3150 A

is it correct to say that at 380 kV i can only serve a load of sqrt(3) x 2500 x 380 = 1645 MVA
even with two transformers in parallel (i.e. 2 x 500 MVA) i can only serve sqrt(3) x 3150 x 115 = 627 MVA

in case of correct staggering of generation at both voltage levels, in my opinion it is still important to take only the rating of the busbar as the limit since a failure of a generator/load will change the flow across the switchgear and the rating might be exceeded.

Any opinion about that.

Thank you.

 

[ScottyUK]

Post a single line diagram. Much easier than trying to describe your arrangement in words.

 

[Adam1980]

Hello Scotty,
i am asking in general how to consider the capacity of a substation with the previously mentioned configuration/rating. In my opinion the rating of the busbar is the decisive factor.
Thanks.

 

[dpc]

Capacity is limited by the equipment with the lowest current rating. If you have a situation where a transformer cannot be operated at full MVA rating due to a limitation of the bus current rating, that is not a good situation. Bus is cheap, transformers are expensive.

 

[ScottyUK]

I think you're correct in you reasoning and that the 115kV bus is your constraint. I just hate trying to convert text into a picture!

 

[Adam1980]

Hello Scotty,

i attached the concept i am talking about. as you can see the busbar at the lower level has a cpacity of 600 MVA for a certain rated current and voltage level while the connected transformers aggregated rating is 1000 MVA. Lets consider the case where both transformer are always operating.

The question is: is there anyway where the placement of the loads and the transformers feeders will allow the supply of loads which are more than 600 MVA?

According to my beleive in order to secure operation in all cases the lowest rating of any equipment (transformer, switchgear, feeder) would be the bottleneck in the substation. However there is also the argument that if the supply, in this case the transformers, and the loads are properly staggered where the current in all sections of the busbar doesnt exceed the rating, the sum of the loads connected to the busbar could be higher than 600 MVA.

Hope i was clear.
Thanks.

 

[rterickson]

I believe you are getting hung up in trivia.

First you need to determine your substation load (initial and ultimate).

Then size your transformers to said load (also initial and ultimate). Only you know whether your load is critical enough to size your LV bus such that you can support your load with loss of one transformer, though in general it is a good conservative approach.

Then size your bus accordingly, again taking into consideration loss of incoming and outgoing lines. As others have said, substation bus is cheap in comparison to transformers and transmission lines.

In your sketch, you have a total of 1520A of primary current, and you've chosen a bus good for 2500A. Your LV bus is carrying a total of 5020A, and your rating is 3150A. You want flexibility, but what you've chosen is somewhat limitted at by the 115-kV bus.

 

[Adam1980]

Hello rterickson,

i agree with you completely that if i need to serve a load of 5020A at the LV side i need a 3150A busbar.

however i was challenged that if the transformer/load bays are staggered in a way where the current in any part of the busbar is less than 3150A I can still supply a total of 5020A with a 3150A busbar rating.

Again it seems a bit of trivia but by practice is that actually done or considered as a planning criteria because my personal opinion is that for some reason at some time in the service life of the substaiton the load flow will shift or increase and the operator will forget the staggering issue and the busbar will be overloaded. So to have a good sleep at night i would load a busbar maximum with its rated current.

Thank you.

 

[Adam1980]

sorry i meant 5020A instead of 3150A in the first sentence.

 

[marks1080]

I think you are going to want to consider running all loads with only a single transformer in service... That is the proper way to build a station. So no, there would be no way to stagger your transformers or loads in anyway that would allow for what you want with just a single unit in service.

 

[bacon4life]

In your diagram, losing the middle breaker in all but the rightmost 115 kV rung would limit you to 600 MVA. Bus is cheap, but not so cheap that you should overbuild for this sort of low probability situation.

By serve load, it depends which outages you want to keep serving load. In the diagram, the worst case single outage is loosing a single transformer which would limit you to 500MVA. If ignore the rating of your transformer, the main bus rated at 600MVA could supply 900 MVA to the loads shown. For all N-1 contingencies, the main busbar in your diagram could be sized as small as 400 MVA as long as the transformer bay is 500 MVA. A more reasonable main bus rating might be equal to the short term emergency rating of the transformer. Sizing the main bus at 1000 MVA would be overkill.

Since it is good practice to stagger loads and sources, the bus rating doesn't usually limit power flow unless multiple elements are out of service. In order to evaluate the flow through bus elements during multiple outage situations, Dispatchers are moving towards modeling individual node and breakers instead of representing the whole voltage level as a single bus. At this voltage level, your operators should be well trained and should have engineering studies to guide them.

 

[Adam1980]

Thanks for the reply.

It means that if for some configuration i have 1000 MVA transformer capacity and 600 MVA busbar capacity at the LV side i can still connect 1000 MVA with the correct staggering. I understand from you that in simulation i should consider the busbar beween the bays as a cable with 600 MVA capacity and run simulations for the different possible/probable operation modes and if those sections are always loaded less than 600 MVA I can connect a load with a cumulative sum of 1000 MVA.

I always had the impression that one shouldnt be cheap in designing a substation and if i am to serve a load of 1000 MVA then i should install a busbar with a rating of 1000 MVA.

It would be interesting to me to know if this is a common practice in substation design/ equipment sizing.

Anybody have information about the difference in cost, i would expect for different current ratings the difference is not so big?

Thank you.

 

[dpc]

Anyone who designs substations that have bus ratings lower than the transformer capacity will not be designing substation for very long, or will not repeat the same mistake too many times. The marginal cost for increasing the bus rating is pretty low. Of bigger concern would be the switch ratings for the disconnect switches, etc.

 

[Marmite]

Don't forget to consider the transformer short term and long term overload ratings which may be as high as 1.5-1.8 pu. The transformer is the major capital purchase, in your case several million Dollars. You wouldn't want to limit its throughput in any way by the selection of an inadequate busbar rating, which might save you a few $K's.
Regards
Marmite

 

[Adam1980]

thanks for the replies.

I see that most answers go in the direction of coordinating the equipment rating of the equipment in the substation which i agree with completely.
Any other opinions to include a busbar rating lower than the load rating the substation will serve?
I believe in case generation is included in the substation, i.e. for example at the LV side of the example attached above, the LV busbar rating could be lower than the agreggated sum of the load served espcially if generators and loads are connected to the same bay! however all possible/valid operating configurations should be investigated thoroughly!
Thanks.

 

[jghrist]

You have to look at the maximum current each section of bus can carry under all conceivable switching/loading conditions. The bus rating must exceed this.

If this is a new design, then size all of the main the bus to carry all of the transformer load from both transformers at their emergency rating. Only reduce the size to actual loads if you're talking about a major expense to replace existing bus.

 

[m3ntosan]

What would you expect to happen for a bar fault on the LV side? With 1 and 1/2 arrangement is it OK to assume you won't lose any demand or infeed from 380 kV and post fault you'll continue feeding the same demand via the two transformers all selected to the remaining busbar?

Is the infrastructure different for lower rating busbars? Are you going to have any fault level issues if you'll end up adding an extra transformer or generation in the future? It would be inconvenient to have the CB fully rated and be restricted by the busbars infrastructure...

Not seeing the big picture, this running arrangement looks a bit tight under outage conditions, but maybe you can transfer demnad out of the group or run interconnected on the LV ...

May you grow up to be righteous, may you grow up to be true...