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
https://res.cloudinary.com/engineering-com/image/upload/v1649269208/tips/CT_isglcb.pdf
RE: CT Shorted on Primary (secondary Impedance)
RE: CT Shorted on Primary (secondary Impedance)
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)
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)
RE: CT Shorted on Primary (secondary Impedance)
Page 4, first paragraph. The whole paper is good reading though..
RE: CT Shorted on Primary (secondary Impedance)
RE: CT Shorted on Primary (secondary Impedance)
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)