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CT TESTING

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?

 

RE: CT TESTING

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.  

 

RE: CT TESTING

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

 

RE: CT TESTING

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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RE: CT TESTING

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.

RE: CT TESTING

(OP)
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.

RE: CT TESTING

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.

RE: CT TESTING

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.

RE: CT TESTING

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

RE: CT TESTING

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.

RE: CT TESTING

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

RE: CT TESTING

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.  

RE: CT TESTING

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

RE: CT TESTING

(OP)
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).
 

RE: CT TESTING

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

RE: CT TESTING

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?

RE: CT TESTING

(OP)
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.

 

RE: CT TESTING

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

RE: CT TESTING

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.  

 

RE: CT TESTING

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

RE: CT TESTING

(OP)
Ok I don't want to preform an accurate test, I just want to know that the thing is in the ballpark (working or failed).

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

Is this a system that has been fully commissioned, has been in service, seen through faults, and is now subject to NERC 005? So far, NERC has not specified the testing interval, or the extent of testing needed. Do to the risk inherent in this type of testing, perhaps you might want to decide that a periodic visual inspection is all that is needed. This decision must be justified, perhaps by utility experience. I know of no standards that address periodic testing of CTs. We passed audit using this type of approach.

  

RE: CT TESTING

(OP)
Yes we have several installations that have been in service for several years. The question has been how do other companies test these.

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

Back into the fray...

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

(OP)
That is one nice thing about the 587Z relays is that a second setting can be applied at a much lower setting to provide an alert to some problems.

 

RE: CT TESTING

Not sure if this is acceptable for your requirements, but we periodically test the BBP CTs by ramping up voltage on the secondary terminals and checking the current.
This is done when each feeder is out for maintenance.

RE: CT TESTING

OK -

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

***********************************************************

NO I am NOT CRAZY - I know what I am doing

***********************************************************

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.

************************************************************

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.

***********************************************************

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.


************************************************************

All the best!

 

RE: CT TESTING

I am sure you are experienced and know what you're doing. But you're right... it sounds crazy.

=====================================
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RE: CT TESTING

smallgreek-

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

Besides it doesn't prove anything more than if you would just short the CT

RE: CT TESTING

(OP)
Shorting a CT on a high impedance bus differential is the same as taking the differential out of service. And in fact measuring a voltage on the differential circuit is a way to prove there isen't a ground connection.

RE: CT TESTING

I think if you shorted only one CT you would see that it's not the same as taking the relay out of service

RE: CT TESTING

High Z bus differential has CTs paralleled. Short one, and you've shorted all on the same phase. No voltage can develop at the relay, so relay is out of service.

RE: CT TESTING

Many of our relaying designs have the Metrosil close up to the relay. If you O/C the CT then the Metrosil doesn't clamp the voltage. A few years ago I inadvertently caused this condition on a very large Class X CT on a generator LV side diff scheme. I've described the circumstances in more detail on here before but suffice it to say that it was a brown trouser moment. There was a fair bit of damage to the lug and the terminals - hardly what you want during an online test.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: CT TESTING

Cranky,

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

Please ignore my second suggestion. Didn't notice that you were talking about bus diff. However electronic relays should provide you current on each line and the sum of all should still give you the number you are looking for.  

RE: CT TESTING

(OP)
We sort of decided on visual review of all CT's in the bus diff. that we can't verify with a clamp-on.

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

We @ TVA simply measure current and phase angle at relay with one of the Bus breakers current diconnected (Pull PK inside breaker cabinent). The measured current should be close to the current expected from that breaker except with opposite polarity. This is only true however when all Bus contributing currents are connected to Diff. circuit and the measured current is close to 0. Do this check first. You can do this procedure on all breakers (one at a time)to check C.T. integrity and it's associated wiring.

RE: CT TESTING

Just take a voltage across each input of the bus differential relay (if it's for example a GE SVD bus relay).  Any voltage substantially higher than zero (if your bus has tested out) will tell you if you have a failing CT - just maybe not which one.   

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