Can disconnect switch be used to energize transformer?
Can disconnect switch be used to energize transformer?
(OP)
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!
Could any transformer experts share some throughts of what will happen by using disconnect switch energizing transformers?
Thanks in Advance!






RE: Can disconnect switch be used to energize transformer?
RE: Can disconnect switch be used to energize transformer?
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
RE: Can disconnect switch be used to energize transformer?
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
RE: Can disconnect switch be used to energize transformer?
There is some confusion here.
RE: Can disconnect switch be used to energize transformer?
RE: Can disconnect switch be used to energize transformer?
Regards,
RE: Can disconnect switch be used to energize transformer?
RE: Can disconnect switch be used to energize transformer?
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.
RE: Can disconnect switch be used to energize transformer?
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.
RE: Can disconnect switch be used to energize transformer?
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!
RE: Can disconnect switch be used to energize transformer?
RE: Can disconnect switch be used to energize 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.
RE: Can disconnect switch be used to energize transformer?
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!
RE: Can disconnect switch be used to energize 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.
RE: Can disconnect switch be used to energize transformer?
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.
RE: Can disconnect switch be used to energize transformer?
RE: Can disconnect switch be used to energize transformer?
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.
RE: Can disconnect switch be used to energize transformer?
RE: Can disconnect switch be used to energize transformer?
RE: Can disconnect switch be used to energize transformer?
The motorised switches which operate slowly I have come across at higher voltages but these are not used for transformer energisation (pure disconnectors).
About closing repeatedly, I agree with you, jghrist.
RE: Can disconnect switch be used to energize transformer?
thanks for your response. Do you have any recorded inrush currents when using disconnect switch picking up Trx at 100kV level.
I would like to see the differences between CB and disconnect switch when picking up Trx.
Thanks!
QB
RE: Can disconnect switch be used to energize transformer?