CT TESTING 1

[cranky108]

Is there a good method of testing CT's in bus differential circuits without taking an outage?

The ones with signific current on them is eazy, but what of the others?

 

[dpc]

If you have backup overcurrent protection, you can take the bus diff relay out of service.

I don't see how you are going to test a CT used in an active bus diff relay.

 

[scottf]

In short, there is no way to really test an in-service differential CT.

 

[electricpete]

One test I remember seeing was a test of the magnitudes and phase angles of the various currents fed from the secondary of the CT's. Can detect some problems.

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[davidbeach]

Assuming a low impedance bus differential, not necessarily a good assumption, you can compare the metering from the relay on that winding with metering from other CTs on the same circuit.

 

[cranky108]

Actually I'm trying to find a better way test the CT's on a high impedance transmission differential with out taking an outage of the bus.

I need at least 1/2 amp normally to verify a CT, but some feeds will not have that much load in normal conditions.

 

[slavag]

Hi Cranky.
Please more information about relay.
Actually, no chance test CT on the live line, but as David said dood option compare CT meas. with other meters/relays.
Best Regards.
Slava
In the newer 87B, bus diff relay, we used option "bay out of use", that mean, diff relay meas current from this bay, but dosen't include this current in the zone diff current.

 

[stevenal]

So you need to set up the abnormal conditions. We tested a bus differential just last week, by switching source and loads around so that every path had enough current to verify, two or more at a time. A time consuming process, but we have confidence now that the system will not trip on load or through faults.

 

[GTstartup]

Are you trying to test the CT or the relay circuit? If its the CT I don't understand your comment about needing 2A normally.

If you need to test the relay, disable the trip, short the CTs, isloate and inject current

 

[stevenal]

GTstartup,

Individually testing CTs and relay don't proof the system. I assume Cranky is trying to verify there isn't a polarity reversal somewhere. Not exactly what he said, though.

 

[GTstartup]

You can check CT polarity at a relay with a jumper to ground and a clamp on ammeter. Of course disable the trip first

 

[slavag]

Maybe use some phase angle meter with clamp on current input and clump on the BUS voltage referece. You know, what line is in to bus and what is out.

 

[oldfieldguy]

stevenal has the best idea so far. Referring to a good drawing, 'plug' the relay with a test plug and use a phase angle meter to verify proper relationships of 'in' versus 'out' and expected current levels.

However, if the circuit has been in service with expected loads already, the chances are slim that you will have a problem with polarity or ratio because you'd have already tripped.

Following successful commissioning, the only failures I've seen on bus diff circuits were CT's that changed saturation due to water incursion (iron corroded. saturation voltage went down) shifting CT ( completed a second path through the CT window. Saturation went down) and bad drawings.

In the last a relay technician inadvertently pushed current into the CT circuit of the LIVE bus and tripped the main substation off line.

The first two were found during offline saturation tests. While on line, the currents read correctly. In the second, we were investigating a misoperation on a through fault. The CT we found with the lowered saturation level went into saturation and operated a differential.

All thsoe words are basically to say this: Online tests won't hurt, except for exposing your system to the possibility of an operation caused by your testing effort, but the tests may not provide you with much more than the ability to say you did something. Your best tests will be performed while off line by knowledgeable technicians and field engineers.

And ONE more caveat: If you add or remove anything from the differential circuit, you REALLY want to do a thorough set of recommissioning tests.

old field guy

 

[cranky108]

I'm not concerned about the relay, as I can open the test switches and test that.
In some instalations we have PVD's, and others we have a 587Z.

What I want to do is prove that the CT and circuit is operating correctly. The 0.5 amp limit is to see that there is real current and not noise from the world.

My other option is to take a bus outage and test each breaker CT to the relay, and it is difficult to get these kind of outages.
In order to comply with NERC PRC-005-1 each CT and circuit must be perionically verified (tested).

 

[slavag]

Only one way, what I see.
Test of CT by CT ( that mean, outage of feeders) with disconnection of CT input to the diff relay.
Regards.
Slava

 

[davidbeach]

Good reason to switch from high impedance bus diff to low impedance bus diff; use of relay metering (and getting good metering values) is a means of complying with PRC-005 (provided that's what your test procedure says you do).

Couldn't you use a clamp on ammeter and measure the secondary current on each leg ahead of the point you parallel all the CTs and compare the results to other metering?

 

[cranky108]

Couldn't you use a clamp on ammeter and measure the secondary current on each leg ahead of the point you parallel all the CTs and compare the results to other metering?
Yes that is the intent, except I need 0.5 amps secondary as a minmum, and with 2000/5 CTR's, that means the loading needs to be greater than 200 amps primary, which is 40mva at 115kv or 80mva at 230kv.

 

[davidbeach]

In that case stevenal had your procedure in his 19 Aug 08 11:39 post.

 

[dpc]

There's a difference between testing CTs and testing the secondary wiring of the installed CTs and relay.

If you just want to check the CT wiring into the bus diff relay, you follow the procedure described by stevenal. This will give you a high degree of confidence that the CTs are wired correctly (phase and polarity) into the relay.

When your titled you question "CT Testing", I assumed you wanted to perform current injection testing of the CT itself.

 

[slavag]

Hi.
"I assumed you wanted to perform current injection testing of the CT itself".
I understood same.
We prepare now test procedure for the some BBP ( LI, but isn't important).
We'll use trafo 22kV/0.4 ( about 400-630kVA) and will feed it from 400V and short it on the 22kV side ( BTW, if you have some trafo is connected to the bus, calculate SC current on the HV side and feed it from low side). And we will check pairs of connection with moving of shorts.
Best Regards.
Slava