Can disconnect switch be used to energize transformer?

[QBplanner]

In the company I work, we used to have a case that in a weak system (138kV extising and 230kV ultimate) with fault level only 110MVA we plan to install two 120MVA auto transformers 230/138/25kV. I had a concern for transformer inrush current and teh impact to the distribution loads along the line. But our transformer expert mentioned that they can use disconnect switch to energize the transformer without causing inrush problems. I recall that Bpa (In North America) did some research to monitor the transformer residual flux when trying to close the CBs. However, I have no clues how the disconnect switches work on energizing transformers.

Could any transformer experts share some throughts of what will happen by using disconnect switch energizing transformers?

Thanks in Advance!

 

[davidbeach]

Disconnect switch? Anything I'm aware of in those voltage levels called a disconnect switch is strictly for unloaded operation to provide an air break around other equipment. Circuit breakers and circuit switchers are what you want for transformer switching.

 

[slavag]

Hi.
Maybe it's some misunderstanding.
This size of AT without CB???
Where is installation of CB, where is protection?
On other hand, once we had here topic on similiar case
I think Jghrist explained it.
Trafo with disconnecters instead CB's , but smaller size and about 110kV voltage level. It was some old standard:
Weak line with CB on the SS and transformers on the line w/o CB
in case of trip --CB open in out of current/voltage pause DS open and after this recloser send close command to CB.
But isn;t sutible for this size of AT.
Regards.
Slava

 

[QBplanner]

Thanks Davidbeach and Slavag:
That is right, disconnect switch not 138kV CB or circut switcher.
I don't think we can use CB to pick up Trx it will be a like short circuit to the system consider the fault level and the size of the Trx.

However, what I heard was that they said they will close disconnect switch several times in order to minimize the Trx. inrush.

I heard somebody used 25kV disconnect switch to pick up 25kV Trx. but don't know exactly what the theory behind

 

[dpc]

Single pole switching of breakers can be used to reduce inrush. BPA has experimented with that. Just using a disconnect switch instead of a circuit breaker will not reduce transformer inrush.

There is some confusion here.

 

[davidbeach]

Single pole switching of transformers can reduce inrush, but only because that switching is done at a very specific point on the wave of the incoming voltage. No way in he!! that you can get there using disconnect switches. Closing the disconnects several times will just mean that you have inrush several times. Time for some serious engineering work, probably starting with an EMTP study of the system and the transformer switching to determine just what can and can not be accomplished. Breakers can be closed at a specific point on wave, disconnect switches will close at random times. It could be that you just can't install those transformers on that system.

 

[QBplanner]

Thanks everybody! It seems somebody broke a new ground. Maybe next time I should ask how they can do it.
Regards,

 

[dpc]

They may have broken more than new ground. Better wear your hard hat.

 

[jghrist]

Inrush current is lower for a weak system. Maybe this is a factor.

It is not uncommon to energize a transformer with disconnect switches. De-energizing a transformer requires a switch with a proper magnetizing current interrupting rating.

 

[rbulsara]

Only time I have seen (year ago) Isolator (disconnect) switch used for energizing a transformer (while secondary side breaker was open), when there was a common oil CB feeding the two transformers (fed off a common bus) and it was not desirable to interrupt the service to rest of the system.

It has nothing to do with inrush, but if the quesiton is whether an isolator can be used to energizes a transformer, it believe it can be , at no load.

Regulatory and safety procedures may have changed some requirements today, that I am not aware of.

 

[QBplanner]

Thanks Jghrist and Rbulsara!
Could you be more specific of at what voltage level you have seen that people employ disconnect switch to energize teh transformers. I knew people can do it on distribution levbel below 33kV but have anybody tried 138kV and above?
Could you give some example if it is not uncommon to energize a transformer using a disconnect switch instead of a CB? What rationales behind it?
Thanks!

 

[QBplanner]

I don't see transformer inrush get much to do with system strength.Any thoughts?

 

[apowerengr]

We use gang-operated switches typically at 69kV and less, sometimes at 115/138 kV to energize SMALL transformers (12 MVA base-rated and smaller). The switch is in series with suitably sized power fuses where the power fuses provide high-side protection for the power transformer.

A 120 MVA power transformer is too large for this application in my opinion and as stated earlier the use of a switch or disconnect will have no bearing on the magnitude of inrush current.

 

[QBplanner]

I agreed that is why I was not so sure about using disconnect switch to pick up that 120MVA transformer.Good thing is that it has been canceled.

I agreed and it make sense since at that time, I asked what if during Trx energized using disconnect switch if there is a fault, how we have to deal with that. I think you very much answered my question in a detailed way.

I guess people using disconnect switch to pick up Trx mainly because no need to worry about CB tripping or P&C modifications or saving high side CBs. But as you or others mentioned (and I also believed ) it gets nothing to do with avoiding Trx Inrush
Many thanks!

 

[jghrist]

Where I have seen disconnect switches used to energize a transformer is on 100 kV stations where two transformers are protected by a single breaker or circuit switcher. Each transformer has a disconnect switch to allow isolation of the faulted transformer.

I think the practice of using disconnect switches for energizing is not so much a function of voltage or MVA rating directly, but of substation configuration. Higher voltage stations and larger transformers are likely to have their own circuit switcher or breaker. If they do, then this will naturally be used for energizing.

Disconnect switches don't have a problem picking up current, they have a problem breaking current.

 

[rbulsara]

qbplanner:

Sorry, I logged in after many days. The voltage level I was involved in was 66kV. As jghrist also indicates it is more a matter of substation configuration than the voltage level.

 

[QBplanner]

Thanks rbulsara! I thought people always use disconnect to pick up low voltage level Trx instead of beyond 138kV.

 

[RRaghunath]

Normally, disconnecting switches are not high speed switches. These have operating mechanisms with gears and motor and close / open slowly.
But, if the same are required to switch the transformer, I expect speed of operation is an issue in order to minimise the period of arcing.
For this purpose, the switches may have to be spring operated like circuit breakers.

 

[QBplanner]

Thanks raghun, what I heard is that they don't expect high speed switching all they used is close disconnect switch several times to distinguish the Arc. and then close it finally to pick up the Trx once the Arc is gone. they 've been using it on 25kV system in the past.

 

[jghrist]

Thanks raghun, what I heard is that they don't expect high speed switching all they used is close disconnect switch several times to distinguish the Arc. and then close it finally to pick up the Trx once the Arc is gone. they 've been using it on 25kV system in the past.

You only get a short buzz when energizing a transformer with a disconnect switch (at least at 100 kV where I've witnessed it). The arc that occurs with a short prestrike is extinguished when the contacts close. Closing the switch several times to extinguish the arc would mean opening it several times (you can't close a closed switch). Opening the switch will definitely create an arc as the inductive current is broken. I don't understand the logic behind this practice.