pushing currents thru CT secondary with Live voltage

[Uquresh1]

If I push currents from CT secondary to test my relays for overcurrent, with the substation energized (26KV) with no load, does that CT secondary current show up on the primary? Even if it does or doesn't , is there any other thing i should be worried about?

 

[rcw retired EE]

I wouldn't do it.

Do you have any test switches or shorting terminal blocks in the CT circuit where you could isolate the relay while still shorting the CT?

Is the relay in a draw out case so you could bench test it or at least remove the test plugs and inject currents at the relay?

I don't have time now to think this through, but I would be very concerned about damaging the CT, the switchgear or yourself trying to inject current in a live CT. I'll let some other experts suggest ways of doing it.

 

[pwrengrds]

I doubt if you could push enough into the primary via the secondary while it's attached to the relay, which is almost a dead short. To push current into the primary would require a lot of voltage on the secondary.

If you connect up to the secondary almost the current would go toward the relay, not the CT primary.

 

[GTstartup]

Agreed, I've pushed current to the relay with the CT secondary wires disconnected and also connected. I couldn't measure a difference. The impedance of the CT is much higher than the relay and any current going to the secondary of the CT is negligible. Depending on the type/size of the CT of course

 

[racobb]

Hopefully you have shorting blocks, as has already been mentioned, which would allow you to short the CT and disconnect the lead to the relay for testing.

Alan

 

[rasevskii]

Of course you should have test blocks installed that short circuit the CT itself, and allow current to be injected to the relay with the CT out of the test circuit. Many types available, from many suppliers. Such as States test blocks, made by ABB in Florida.

You should absolutely not mess around with CT secondary leads even if there is "no load" on the primary. You might just accidentally get onto the wrong CT or be into the wrong panel and open circuit a secondary and get killed. I know of a site many years ago where this actually happened to a young engineer.

Regarding back energizing of CT secondaries: It will take a up to a couple hundred Volts to get a enough mA through if the pri side is open. See tests for "Mag Curves" and "Knee Point" where such a test can be required for certain protections. i.e. Generator Differential.

Be very safe in HV installations. Only one mistake is allowed, better none.

rasevskii

 

[jghrist]

It's OK to leave the CT connected if the primary is open, say if you're testing a MV or HV breaker with relays in the breaker control cabinet where there are no test switches. If the breaker is isolated (disconnects on both sides), then practically no current will flow through the CT and all the current will flow through the relay. In this situation, there is no possibility of opening up the secondary of a CT with current flowing in the primary.

 

[Uquresh1]

Thanks guys...Yes I inject currents thru the relay and short the CT by opening the test switches....Live equipment is always dangerous and I usually keep my hands in my pockets if I am not sure about something.