TRANSFORMER TRIPPING 1

[tszpun]

Hello Experts,

Could somebody guide me on the following issue :

I have a transformer with circuit breakers installed at both upstream and downstream switchgears.
Transformer has many protections like, winding temperature trip, oil temperature trip etc...
And I have several options for the tripping of this transformer, in case of a fault :
1. To trip the downstream circuit breaker
2. To trip upstream circuit breaker
3. To send the trip signals to downstream circuit breaker, and then with intertripping cable to trip the upstream circuit breaker.

Could somebody explain me, what are the pros and cons of the 3rd option?
Can i trip the downstream CB first, and then the upstream CB ? If not, could somebody explain me why ?

Thank you.

 

[davidbeach]

For transformer faults you want to be tripping a lockout relay. You may also directly trip both breakers, but the lockout should trip both and block closing of both. For cases where you are tripping to clear a through fault, just tripping one of the breakers is sufficient. If you just trip the downstream breaker you leave the transformer energized and don't have to deal with the inrush later. If a though fault winds up tripping the lockout, then you have a bunch of work to do to prove that it wasn't a transformer fault.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[bacon4life]

For new installations I would trip/block-close both the high and low side circuit breakers with a lockout relay. This does take a little bit more cabling and may require a larger lockout relay. It also requires a bit more testing.

Older stations may have only tripped/block-close on the high side breaker for a radial installation. With the expected growth of distributed generation/storage, I think it makes sense to always trip both circuit breakers.

I assume you are only asking about faults actually in the transformer. We typically use a microprocess based transformer that has a differential element that trips the lockout, plus a low side overcurrent element that backs up the feeder breakers. The low side overcurrent elements protecting the feeders will only trip the low side breaker.

 

[cranky108]

I would think you would want an over-current on the high side, as differential relays can fail, or be taken out of service for stupid reasons. But you may not want it to trip a lockout. Just trip the high side breaker for what might be just an overload.
This same relay can also offer a ground protection on the XO bushing. The problem is in cases you can have too many over curves to properly space out.

 

[Palletjack]

Agree with above. Let the secondary 50/51 handle faults downstream of that relay. No need to trip a transformer if it isn’t faulted.
Same with primary protection. Use it to trip 87/50/51 for the transformer or protected zone only. If the secondary protection isn’t tripped you know the downstream is good, manually trip it to energize XF after testing.
DTT schemes are ok for certain things, but they can create confusion if they are used simply because…

When a sub is tripped out, it makes troubleshooting much easier if every breaker in the sub isn’t tripped open.

 

[VTer]

The consideration on tripping operation should depend on the system arrangement. Tripping of the downstream devices may be required for multiple sources or parallel operation.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[cranky108]

That said, if there is generation on one or more incoming feeders, it may be required to trip those feeders.