Can differential protection of a transformer protect a downstream switchgear also ? 3

[bdn2004]

There is a new 12.47kV primary line being fed into a new 15kV power circuit breaker with a 751SEL relay. The power circuit breaker feeds a 2500kVA outdoor pad mounted transformer. The transformer is a 2500kVA, Delta-Wye, solidly grounded, 12.47kV-480/277V. The transformer feeds a low voltage switchgear that's located about 50' from the transformer inside.

We have a proposal where he's showing 3 - 200/5 CTs on the primary, and CT's on the secondary. And he's calling that differential protection that will protect the switchgear. We weren't expecting the secondary CT's. Is this a common installation now ?

 

[jghrist]

Unless the low side CTs are on the outgoing switchgear breakers, the switchgear is not protected by the differential relay. You could use the relays for LV overcurrent protection that would act fast for a switchgear fault and limit arc flash energy.

 

[RRaghunath]

Differential protection always is defined by a zone within which the protection is effective. The zone is defined by the location of CTs. In your case, the CTs are on either side of transformer, meaning the differential protection is strictly for protecting the transformer only.
Having said that the numerical protection relay 751 has many other protection elements. If the proposer is planning to deploy some other protection element for protecting the switchgear (which is possible), it is not obvious from the sketch you posted.

 

[Palletjack]

I assume you adding an SEL 787. The 751 won’t do 87 protection.
The 787 only has 6 inputs (+1 for N) for 87 protection. To attempt that you would need 9 or combine two sets of CTs on one input.
For a switchgear you could get arc flash detection on the 751 series. I *think* it’s the 751A.
Other wise forget the CT in the middle and make the entire area the protected zone.

 

[bdn2004]

We must be misunderstanding the purpose of this - that it's not differential protection of the transformer. These guys say they use this setup often on new installations. It is a well established company.

It is indeed a SEL 751A relay.

 

[bdn2004]

Although still a bit confused here if this is for arc flash protection - which is what he's saying. Looking at the manual - that's done with fiber optics installed in the switchgear - not current sensing through a CT.

 

[jghrist]

And he's calling that differential protection that will protect the switchgear.
We must be misunderstanding the purpose of this - that it's not differential protection of the transformer.

That's the confusion; there is no differential protection.

The design is improved overcurrent protection to reduce arc flash incident energy. The existing sub has high side fuse protection which will be slow in interrupting a fault in the switchgear. The modified sub adds a high side breaker and low side CTs to allow fast tripping for low side switchgear faults. This is a common solution for high arc flash incident energy situations.

 

[Palletjack]

751A has fiber optic sensor that will trip on arc flash

 

[cranky108]

I think most of us would prefer to use differential protection around the switchgear, and not depend on fiber optic sensors. The problem is that the fiber optic technology only started showing up in the say last ten years, and the differential technology is over fifty years old.
That said, the fiber optic can seem faster, but unless you have faster breakers to take advantage of it, it doesn't buy you much.

The age of fiber optic technology is not mature enough that we understand all the problems it can have.
Example, what if the fiber becomes covered in dirt? Who, and how often does it need to be cleaned by?
Yes we know of the issues it has with flash photos, but what else?

I had one of those relays on my desk for a while, and with the sunshine in my window the long fiber was receiving a full saturation of light. Will it still work then? What if a cubicle door is open with the day light?

 

[LionelHutz]

Is a salesman telling you this stuff?

The 751 and 751A can't do differential, so it's wrong to be claiming that. Both can only take 3 phase CTs and a zero sequence CT.

The 751A can do arc flash protection with fiber optics. We use it quite a bit and stock rolls of fiber for such installations.

I'm not seeing camera flashes being too much of an issue. We have to use a professional type add-on flash to get the relays to trip. An all-in-one camera flash won't trip SEL or Multilin relays.

I've never seen differential protection built into or around 480V switchgear.