CT TESTING
CT TESTING
(OP)
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?
The ones with signific current on them is eazy, but what of the others?






RE: CT TESTING
I don't see how you are going to test a CT used in an active bus diff relay.
RE: CT TESTING
RE: CT TESTING
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RE: CT TESTING
RE: CT TESTING
I need at least 1/2 amp normally to verify a CT, but some feeds will not have that much load in normal conditions.
RE: CT TESTING
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.
RE: CT TESTING
RE: CT TESTING
If you need to test the relay, disable the trip, short the CTs, isloate and inject current
RE: CT TESTING
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.
RE: CT TESTING
RE: CT TESTING
RE: CT TESTING
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
RE: CT TESTING
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).
RE: CT TESTING
Test of CT by CT ( that mean, outage of feeders) with disconnection of CT input to the diff relay.
Regards.
Slava
RE: CT TESTING
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?
RE: CT TESTING
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.
RE: CT TESTING
RE: CT TESTING
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.
RE: CT TESTING
"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
RE: CT TESTING
It looks like there isen't a better way than the clamp-on measurment, and verify with another CT on the same breaker.
RE: CT TESTING
RE: CT TESTING
The other thought we had is that we verify that they have not misoperated on other system faults. Take a SOE log and calculate an equivlent short circuit study to show it would have seen current.
RE: CT TESTING
The big electrical utility for whom I used to work tested CT's periofically. I worked as a contractor for another larger utility conglomerate that did periodic testing of CT's.
One rationale for testing of CT's mounted externally to GCB's was that one manufacturer had problems with sealing the CT enclosures against water intrusion. Water in the CT steel corroded the steel and negated the effects of inter-plate insulation. This caused electrical paths to form in the steel which resulted in lower saturation voltages.
The second problem area was with CT's inside OCB's. OCB action causes a considerable mechanical movement. The jarring has caused CT's to shift on their mountings. I have found cases where this shifting caused the CT mounting plate to contact the bushing, resulting in a shorted turn in the CT parallel with the primary current path. This caused a severe drop in saturation voltage and is the cause of a differential operation on an out-of-zone fault.
In both situations the CT's accurately reflect the current passing through them. The problem is the change in saturation voltage.
That being said, a two-prong periodic testing approach might be prudent: Compare secondary currents with other metering to prove ratio and phase angle, and when the circuit is in an off-line condition, perform saturation tests.
Yes, it's a low-probability situation, but much of periodic testing is.
And all this verbiage assumes (!) that the circuitry ahs gone through proper commissioning tests and has been in service with no abnormal indications.
I won't tell you about finding a set of GE PVD relays with the trip lead lifted and taped because "they just rip at the most inopportune times"...
old field guy
RE: CT TESTING
RE: CT TESTING
This is done when each feeder is out for maintenance.
RE: CT TESTING
This middle aged field guy has an "in service test" for proving CT's and associated wiring for a Hi-Z Diff scheme.
I only recommend that this be performed by experienced and qualified field personnel who are good with hand tools.
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NO I am NOT CRAZY, I know what I am doing. :)
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I am assuming you have back up OC protection and some flexibility in switching. I would recommend setting the back up OC more sensitive. Any other overlapping Zone protection could also be set more sensitive.
The 87B trips from the lock out (86) relay will have to be disabled for the test. As a matter of fact, the audible sound of the 86 will speed things up. I would certainly verify that there are absolutely no 'sneaky circuits' in the 86 tripping scheme. It wouldn't hurt to verify that there are no DC grounds prior to commencing with the test.
#1 - 87Z theory - assuming that all secondary wiring and the CT ratios / polarity are correct, there will be no diff current at the 87 relay terminals, under load. The only way to prove this scheme (the anti test) is to create a false diff current / voltage and let the relay trip the lock out (86) device. Utilize load switching if you have a by-pass / maintenance breaker for double bus arrangements. This
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NO I am NOT CRAZY - I know what I am doing
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The idea of the test is to open circuit the CT at each summation point. Yes, I said open circuit the CT.
Use insulated needle nose pliers and a holding screwdriver. It only takes a second. Hold the lug / wire with one hand (in insulated pliers) and the holding screwdriver with the other - lift the wire just a little bit. You will expect to
see arcing and hear the 86 roll. The wire only has to be lifted just enough for an arc to develop. For those who are interested, there is a time constant for a dangerous voltage to develop. For the 30 cycles that you are lifting and the relay is operating only a few hundred volts will develop. I would guess that David Beech could fill us in on the exact formula. :)
For those still concerned about personal safety, the MOV's (or metrosils) will generally clamp at ~600V or so, which I believe will satisfy OSHA (or other local safety bodies) if you are wearing Class 0 hot gloves and properly rated insulated tools.
That proves the rest of the CT's in the scheme. Repeat for every CT that is included in the summation of the 87Z scheme. You may need a helper and two way radios if the CT's are paralleled in a marshaling cabinet in the switchyard, which is the proper location for this scheme. Have the helper confirm the trip and reset the 87 and 86 relays after each CT is tested.
In my experience, it takes longer to explain what you are doing to those who do not understand the 87Z scheme than to perform all the tests in the scheme.
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Just keep explaining that you are NOT CRAZY and understand what you are doing. :)
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The primary injection scheme proves restraint for through fault conditions, by making an open circuit, sufficient voltage and current is developed to test the MOV's, the relay 87H and 87L element and the trip to the 86 device.
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This was a 'patented trick' I learned from another OFG some years ago. :) I am passing this trick on as I enjoy the quality of the discussions on this board with some obviously sharp engineers and field technicians.
The first time I did it, my heart was pounding and I was sweating like a pig.
I commissioned 2 8 breaker ring buses with 587Z and MFAC relays using this method in about 4 hours some years ago.
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All the best!
RE: CT TESTING
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RE: CT TESTING
As someone who works for a CT manufacturer, I would say that testing scheme is dangerous.
Also, how exactly does the test prove the CTs to be in proper condition?
RE: CT TESTING
RE: CT TESTING
RE: CT TESTING
RE: CT TESTING
RE: CT TESTING
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If we learn from our mistakes I'm getting a great education!
RE: CT TESTING
It seems like you have answered your own question here :)
The most efficient way I have seen is to just use a clamp on ammeter.
If not available, then after disabling the diff trip take one CT offline at a time (short circuiting secondary side) and reading the high side and low side currents from the relay. Subtract one from another and the difference should be what your CT would put out.
RE: CT TESTING
RE: CT TESTING
These are mostly future and moble tie breakers (Yes we have breakers for future equipment. Makes good spares also).
Thanks very much.
RE: CT TESTING
RE: CT TESTING