Primary Current Injection Test on 67 Relay
Primary Current Injection Test on 67 Relay
(OP)
We have a 67 relay (directional relay) and a power circuit breaker with 3-CTs connected in wye-grounded of 500:5 Amps tap used. Kindly see attached diagram for details.
We injected current at the primary side of the breaker specifically at phase B where the phases A & B CTs were shorted. We assumed that only phase B will have a reading even the other phases are shorted. But, when we injected current and reached to 300 A, we noticed that phases A & C registered about 67 A.
We believe that all CTs should not be shorted. It was just an experiment out of curiosity. Does anybody give an explanation or illustration how this happened...and help me establish an equation to describe this scenario?
We injected current at the primary side of the breaker specifically at phase B where the phases A & B CTs were shorted. We assumed that only phase B will have a reading even the other phases are shorted. But, when we injected current and reached to 300 A, we noticed that phases A & C registered about 67 A.
We believe that all CTs should not be shorted. It was just an experiment out of curiosity. Does anybody give an explanation or illustration how this happened...and help me establish an equation to describe this scenario?






RE: Primary Current Injection Test on 67 Relay
RE: Primary Current Injection Test on 67 Relay
What is your reading on the 67N relay?
RE: Primary Current Injection Test on 67 Relay
regards, rasevskii
RE: Primary Current Injection Test on 67 Relay
RE: Primary Current Injection Test on 67 Relay
What are you using for polarizing during the test?
RE: Primary Current Injection Test on 67 Relay
Go through the shorts, then back through the A & C phase elements, or go up through the IN element.
They will then summate and go back up through IB and back to the B phase CT.
Do you have a measurement for In, also do you have phase angles ?
I would think the arrows through Ia and Ic would be in the opposite direction - the phase angle measurements would prove it.
RE: Primary Current Injection Test on 67 Relay
The readings i got from the display for Ia & Ic were about 67 Amps while Ib = 300 Amps. I was not able to capture the In. Likewise, with the phase angles. My apology for lack of data.
For your information, the relay has a multi-functions and we disabled the 67 & other functions and tested only the 51 (TOC).
RE: Primary Current Injection Test on 67 Relay
With the shorts applied, there are a number of paths for the current to flow, but it will all end up going back through the IB element.
If is 67 Amps in A and C, the remaining 166 Amps is going through IN
Without the shorts all seconardy current goes through IN and IB only, they will both read 300 A.
The shorts have no effect on the current in IB, but I believe the shorts are not required. If that was what was happening in real life, no current in A or C would appear on the secondary circuit.
There is no safety issue, the overcurrent relay elements are the "shorts".
RE: Primary Current Injection Test on 67 Relay
With the shorts, and if the relay coils have equal impedances, IA=IC=-1, IN=1.
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.
RE: Primary Current Injection Test on 67 Relay
As pointed above, you don't need short "unused" current terminals, not on the relay and not on the CT.
Please take in account Q's of jghrist :
"What are you using for polarizing during the test? "
You can't test polarization with such test, on this moment you test only wiring.
Good Luck.
Slava.
In the dynamic test, with real load, you must test with shorting of two phases and disconnection of one voltage phase.
For example current A-C shorted, voltage B disconnected.
Of cource if you use internal calculated brken delta voltage.
RE: Primary Current Injection Test on 67 Relay
I understand about the polarizing of the relay that should be used. But what we did was a current injection without the polarizing or reference voltage from the PTs. Its like we're testing the overcurrent functionality since the relay has a multi-functional features where 67 and other parameters are disabled. Meaning to say, regardless of current direction, the non-directional overcurrent relay element will be triggered.
regards,
RE: Primary Current Injection Test on 67 Relay