Transformer Protection for Overcurrent

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kvramesh

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Hi,

I have a 2500KVA, 13.8kV - 480 V, delta/star neutral solid grounded transformer. It is protected by a MV breaker on the upstream and closecoupled to LV main breaker on the secondary side. The trafo is also provided with a temperature monitor device which can send alarm/trip signals based on the sensor. But, the device comes with only one set of contacts by which I could trip either MV bkr or LV bkr but not both.

My question is, what is the safest way of protecting the system? Additional contacts on the temp monitor device can be obtained but at an additional cost. Is it a common practise to trip both primary and secondary of the trafo during overcurrent? or primary bkr alone would be sufficient?

The reason why I said primary alone instead of secondary is: I have another trafo with same rating but secondary main circuit bkr is not provided where as only the feeder distribution bkrs are provided. I cannot keep on tripping all the secondary feeder bkrs and MV bkr, which is not economical is what I think. So, please advise. Thanks in advance
 
I don't think it is a matter of overcurrent protection. It sounds like you are set up to add a differential relay for the transformer. I am not sure if it trips both breakers. Differential protection is used to guard against internal problems with the transformer. It can also be used to reduce the number of OCPD's needed, which makes coordination easier, however this is not really related to transformers, it is more used in switchgears and distribution busses. I think that is called "bus protection."
 
Thanks.

But the bus protection is different. The differential protection is expensive and is used for large transformers in general and not for 2500kva dry trafo's. I am actually talking about Winding temperature protection...I should have called it overtemperature instead of overcurrent...sorry about that
 
kvramesh said:
Thanks.

But the bus protection is different. The differential protection is expensive and is used for large transformers in general and not for 2500kva dry trafo's. I am actually talking about Winding temperature protection...I should have called it overtemperature instead of overcurrent...sorry about that
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For most of the installations I have seen, winding temperature protection has been only a visual or audible operator warning. When used for tripping, only the primary has been involved.
 
jim dungar said:
... When used for tripping, only the primary has been involved.
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That is also my experience. The few that I have seen that trip both Primary and Secondary CBs, the secondary opens (not "trips") on a Primary CB "trip". A set of contacts on the primary protective relay (say like an 86T) provide the trip output.

carl
 
A lockout (86) relay is the way to go. The lockout relay can trip and block close the MV breaker and the LV breaker. At a minimum, you want to trip the MV breaker.
 
kvramesh said:
... My question is, what is the safest way of protecting the system? ...
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You will need to rephrase your question. Safest for what? Personnel? Equipment? Continuity of service? What exactly are you trying to do with your protection scheme? Most of these are not code questions, they are design questions.

kvramesh said:
... I cannot keep on tripping all the secondary feeder bkrs and MV bkr, which is not economical is what I think. ...
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I'm curious about this. Not economical because:
1. Of the wear and tear on the secondary cbs? I would ask, "Are you running so close to the xfm temp limit that the OT sensor is regularly triping the CB? If so, You likely have other issues - like budgeting for a capacity upgrade and having it ready when the transformer burns up.

2. You don't want to pay for the primary protection relay? That is generally a design issue - not regulatory at all. You and the people you work for decide those issues. After you meet the regulatory criteria, then you decide what is most important - choose from issues like:
Continuity of service
Minimize maintenance
Life cycle cost

Pick your protective relay scheme to meet your selected criteria.

carl
 
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jbt260 said:
...At a minimum, you want to trip the MV breaker.
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Another method of selectively tripping secondary CBs is to use a UV relay on the Secondaries you wish to trip. Leave the ones needed for continuity of service closed.

carl
 
coulter said:
You will need to rephrase your question. Safest for what? Personnel? Equipment? Continuity of service? What exactly are you trying to do with your protection scheme? Most of these are not code questions, they are design questions.


I'm curious about this. Not economical because:
1. Of the wear and tear on the secondary cbs? I would ask, "Are you running so close to the xfm temp limit that the OT sensor is regularly triping the CB? If so, You likely have other issues - like budgeting for a capacity upgrade and having it ready when the transformer burns up.

2. You don't want to pay for the primary protection relay? That is generally a design issue - not regulatory at all. You and the people you work for decide those issues. After you meet the regulatory criteria, then you decide what is most important - choose from issues like:
Continuity of service
Minimize maintenance
Life cycle cost

Pick your protective relay scheme to meet your selected criteria.

carl
Click to expand...

Carl, Thanks for your reply. I am looking at the equipment protection. I am requesting an advise for design not with codes.

2. This is a new installation and hence we will take care that the trafo is sized properly and dont heat up and cause tripping. I'm asking about the too many OT sensor trip contacts may become expensive in order to trip all the LV feeder breakers along with MV bkr.
 
If you want continuity of service, simply alarm only. You just have to decide where the signal is going to indicate, usually it's local only unless there is a control/operations room that you can remote it too. If you want protection of equipment, then you could trip the MV breaker. If you for some reason need the LV breakers to trip, then they can be wired to trip on loss of voltage (27). But then they will have to be reset after the MV breaker is reclosed. Unless you have special operational requirements there is no need to trip the LV breakers.
 
Thanks for the suggestions by king and djohn, both are helpful. I did some google and realized that this is not as big a deal as I thought....appreciate your replies
 
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