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CT Shorted on Primary (secondary Impedance)
3

CT Shorted on Primary (secondary Impedance)

CT Shorted on Primary (secondary Impedance)

(OP)
Just wondering if anyone on here has thought about this scenario playing out. When our maintenance crews perform routine maintenance on HV circuit breakers, they typically throw up safety grounds on the HV drops to guarantee the equipment de-energized. The CT circuits remain in service. If the breaker is closed, then we have created a primary loop for current to flow. Due to this primary loop, the impedance of the secondary winding collapses. I included a graphic for any interested. If the CT is incorporated into (for ex) a HV bus differential protection scheme. Is there a possibility of the bus protection not operating due to differential (operate) current diverting thru the CT? I haven't seen any work practice to deal with this, so I'm wondering if what I am describing is actually a concern or not.



https://res.cloudinary.com/engineering-com/image/upload/v1649269208/tips/CT_isglcb.pdf

RE: CT Shorted on Primary (secondary Impedance)

(OP)
I just realized in my eq circuit diagram, my Zoc should equal to R // jXm not R + jXm. my bad

RE: CT Shorted on Primary (secondary Impedance)

if CT is connected to the dedicated analogue input of protection terminal, protection works properly, but if there is an electrical sum of some different CTs and a single relay, i suppose that differential protection can fail, because part of differential current will flow through the primary-shorted CT.
So you need to disconnect secondary current circuits (with disconnection switches which performs disconnection and shorting of CT secondary winding)

RE: CT Shorted on Primary (secondary Impedance)

Not exactly picturing what your describing, but..
If your bypassing a breaker and grounding out the primary side as you describe and there should happen to be a fault down line of the differential zone you could have nuisance tripping due to the grounded primary circuit.

Want to see exactly what’s happening?
Next time this is done, record IOP and IRT values before bypassing and grounding and after it’s done.
When we bypass for breaker maintenance we disable the 87 protection just to avoid nuisance tripping should something happen a mile or two down line.

RE: CT Shorted on Primary (secondary Impedance)

(OP)
Thank you for the replies retriever & Palletjack, pretty much confirms what I was thinking. Unfortunately there was no event to go off of, one where I can analyze relay records. It's interesting how Protection folks (engineers and the like) kind of have a blind spot to what the maintenance crews are doing and vice-versa, leading to this issue never being addressed properly. Will push to have maintenance request auxiliary isolation for doing this work to avoid misoperation of 87 prot when CT circuits are summed electrically. Thanks again.

RE: CT Shorted on Primary (secondary Impedance)

(OP)
Hey Palletjack, the paper describes a slightly different scenario than the one I am describing. In our case the breaker is not bypassed, it would be switched out of service, ie, the breaker disconnect switches would be opened and the breaker terminals would be grounded at both sides. So there wouldn't be any primary current flowing here as is described in the paper. But instead, my concern is that during an internal or external fault, the CT secondary current (fed via other CT's in the CT summation circuit), will be diverted thru the shorted CT due the the CT secondary impedance collapsing (due to the primary circuit loop). I hope that's clear enough of an explanation!

RE: CT Shorted on Primary (secondary Impedance)

It has been standard practice where I work to disconnect the CTs from the busbar protection for this type of switching set-up, but the main focus is the grounds create a path for fault current and cause the busbar protection to incorrectly trip (which has happened to us). I guess the reasons you provide may also be true too (i.e. it may not trip for a bus fault)

This work practice is not without risks though and the field staff need a good knowledge of which side of the CTs to short, correct sequence to make sure protection isn't accidentally tripped or disabled.

RE: CT Shorted on Primary (secondary Impedance)

(OP)
Hi Discop, Thanks for your comments, I never thought about the risk of fault current passing through the closed breaker ground loops, but that is very interesting, another reason to isolate the CT's! Cheers

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