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CT off by 26%
4

CT off by 26%

CT off by 26%

(OP)
Tripped on bus differential for a through fault last week. When troubleshooting, we found one of the 1200:5 CTs was actually 1200:3.7. That explains the trip, but what would cause such inaccuracy? Demagnetizing did not help. All taps on the multi-ratio CT had the same error. Has anyone seen this?  

RE: CT off by 26%

Maybe you bought the wrong one?  Maybe it was mismarked?

Mike Halloran
Pembroke Pines, FL, USA

RE: CT off by 26%

Was it ratio tested during commissioning, or do you have factory test reports?  

Mis-marked taps perhaps, as Mike suggested?

Or sneak parallel path on the secondary side?   

RE: CT off by 26%

(OP)
It came in a breaker which indicates it has 1200:5 CTs on each bushing. The B and C companion units test fine. So the question is not if I bought the wrong one, but it the breaker manufacturer substituted the wrong one. But why would they have a 1200:3.7 or a 1622:5 tapped the same as a 1200:5 lying around?

No to the commissioning test. Next time, yes.
Factory curves are "typical" not actual.

I was hoping for mis-wired taps. No such luck. All tap positions had the same error in the same direction.

Test was done at the control cabinet after isolating the relay from the terminal block. Any sneak path would have to be at the CT itself, which seems unlikely unless it was left shorted; not the case.

RE: CT off by 26%

That's a weird one.  Internal shunt path for some reason?  I guess you could test that by varying the burden on the test circuit.  

Are these bushing CTs?

 

RE: CT off by 26%

(OP)
Yes, bushing CTs on an SF6 breaker. They are external to the tank under an aluminum shroud.

RE: CT off by 26%

Turn to turn short in the CT?  That wouldn't explain the error in all tap positions.

RE: CT off by 26%

2
jghriat-

I suspect it's a turn-to-turn short and/or multi-turn short to the core.

A turn-to-turn short could have turned into a layer-to-layer short, which could affect all winding segments. Localized saturation and resulting heat from a turn-to-turn short can do a lot of damage to winding insulation, especially if the primary current was high at any point.

RE: CT off by 26%

Interturn shorting possible. Measure the secondary winding resistance of this CT and compare with healthy one.

RE: CT off by 26%

Hi.
Stevenal, thank you for information.
I see in this case two very important aspects, what I try explain to many customers:
A-
1. CT must be check after installation ( ratio and Volt/Amper curves).
2. CT must be check once per 5-6 years.

B-
1. Protective relay must be include CT/VT meas supervision and send any alarm signal to system.
Possible several options: 100%*(Imax-Imin)/Imax (same for voltages) or NPS of current or voltages.
2. Modern BBP must be include option of differential current supervision with selectable option of blocking for prevent unwanted trips.
Regards.
Slava

RE: CT off by 26%

This COULD be a result of the dreaded "operate with open secondary" thing. The high voltages involved in the secondary may have caused internal shorts.

Have you checked every connection for good and reliable continuity?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: CT off by 26%

You stated that it sustained a through fault.
If it was an off ratio CT, you would have had a diff. trip fault from the first day.
A diff trip after a through fault strongly implies that the CT was damaged by the fault.
respectfully

RE: CT off by 26%

Hi Waross.
It's situation "yes and no".
It's BBP setting and possible option, Stevenal can explain to us, that setting of BBP are more then max. load current.
In this case protection are not sensetive to this error.
For example:
One infeed and one outgoing. Set of protection 1kA and load
is 500A, instead diff current "0" you see 150A<<<1000A.

This situation also possible in trafo protection.
Regards.
Slava.

RE: CT off by 26%

waross-

I tend to doubt the CT was damaged by high primary currents (i.e. a through-fault), in that the CT is a bushing CT. The breaker would fail long before the bushing CT would.

ELEP-

Unless the inter-turn short is across a substantial portion of the winding, it is not likely to be seen by a secondary winding resistance measurement.

For 1 1200:5A ratio, there is 240 turns for the full winding. If 1 turn is shorted, that's only a change of 1/240 (0.4%). Even if the turn is on a smaller winding segment, it's unlikely to show on a resistance test.

The only real way to see a shorted turn is by performing an excitation test.

To have the secondary current be off by so much at all taps, it's got to be something major.

RE: CT off by 26%

Just idea.
Maybe CT is O.K. and it's mistake of CB supplier.
and this CT is 1600/5A and only plate is 1200/5A.
Meas data on all taps is 1622/5A.
I'm not so believe in such major fault.
Gunnar, open secondary was damage only one tap and usually
terminals and wiring burn.
Regards.
Slava

RE: CT off by 26%

Usually a turn to turn short on a CT will be easily detected by a saturation (excitation) test.  The CT will have a noticeable difference in saturation voltage when compares to its sisters on the other bushings.

I've found more than a few unhappy CT's over the years, and one resulted in a misoperation on a 230 kV line.

With a former employer we had such a bad track record with the bushing-mounted CT's on one particular manufacturer's SF6 breakers taht doing a saturation test was part of three-year maintenance.

old field guy

RE: CT off by 26%

(OP)
Warross,

Load current would be insufficient to cause the differential operation, and from the records this was the first through fault that involved the phase in question. I suspect problem existed from first day.

RE: CT off by 26%

Saw something like this and turned out to be an external wiring problem not discovered during new comissioning.  Didn't cause enough of a mismatch to trip.  Was discovered later due to some other problem.  Double check all the jumpers on the terminal strip and shoot the wiring.

RE: CT off by 26%

If you ratioed the CT with no wiring attached, then of course, the above doesn't apply

RE: CT off by 26%

(OP)
Update:
Factory wants us to do a full excitation test up to 10 A secondary to ensure full demagnitization. Will do so this afternoon.

RE: CT off by 26%

Was this on a phase that saw fault current?  

RE: CT off by 26%

This is getting very interesting. Exciting, I would say. No pun meant.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: CT off by 26%

Stevenal-

The factory actually thinks magnetization is causing a 26% ratio error? Wow.

Also, if it is a protection core, I'm not sure taking the secondary up to 10A is going to work to demag it.

Perhaps they mean take the excitation current up to 10A? Do they have you apply voltage across the secondary with a variac and then raising it until there is 10A flowing? That sounds a bit on the high side. Also, once you hit the level you want, make sure you lower it back down slowly.

RE: CT off by 26%

(OP)
Scottf,

Yes, excitation current. Tried the variac method before. Once you get above the knee, a slight change in voltage by the variac causes a huge change in current and it is easy to trip the breaker on the test set. Our new unit is automatic, set the parameters and start it. But it maxes out at 5 amps. This amount is well into the saturation area and should be sufficient. Don't know why they want ten. Five looks almost like ten anyway on a log log scale.

dpc,

Yes, this CT saw fault current from the line to ground through fault. Relay records indicate no other through faults involving this phase. Other identical CTs on this phase test okay.

Skogsgurra,

We're a sick bunch, aren't we? Those that find this stuff exciting.

All,

Will a turn to turn short show up in an excitation test by lowering the knee voltage detectably?  

RE: CT off by 26%

Yes...knee point voltage will lower and Iexc at knee-point will increase.

As for the test method...I was surprised by 10A, because 2-3A Iexc is deep saturation (probably) and is plenty to demag core.

RE: CT off by 26%

Stevenal.
What is a ratio ( multi-ratio). of this CT.
How much cores?
Are you check also metering core?
Regards.
Slava

RE: CT off by 26%

(OP)
Slavig,

The taps are ANSI standard for a 1200:5.
X1-X5   1200:5
X2-X5   1000:5
X3-X5    900:5
X4-X5    400:5

The other possible ratios are derivable from the above. We are using the full winding for the differential protection.

An identical CT sits above this one on the same bushing, that is used for metering. It tests fine.

RE: CT off by 26%

(OP)
The tests were very interesting. Both the CTs on the bushing had no knees. All was linear up to 5 A. On a hunch we reran the test on one CT with the other's 2ndary open circuited. Now we reached the 500V voltage limit without going above 2A on the C400 CTs. They seem to be sharing a magnetic circuit. I'm beginning to believe the CTs were damaged from a known lightning incident a few years back. Following the strike, the opposite pole from the one in question faulted to ground, and had to be replaced.

RE: CT off by 26%

Say that again. Sharing..?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: CT off by 26%

(OP)
Okay, sharing. Instead of two individual two winding transformers with their primary windings in series, we seem to have a three winding transformer where the state of one secondary affects the other. Acts like they are sharing a common core. We can dump 5 amps into the 6Y secondary without developing the expected voltage, or getting the nice hum from a normal excitation test. With the primary open circuited, the only available path is through the shorted 6X secondary under this hypothesis. Open circuit the 6X, and the excitation test on 6Y looks and sounds normal.

Or maybe they are sharing the secondary circuit, as in a short circuit or crossed wires between the CTs and the the terminal stips in the breaker cabinet. No continuity foud yesterday with an ohmmeter, though. Forgot the megger.

Going for a visual inspection today, as well as further testing.

 

 

RE: CT off by 26%

Stevenal.
Why you don't replace this CT?
Regards.
Slava

RE: CT off by 26%

Several years ago we had a simular problem wth the outer most CT on a GE gas breaker. The explanation we recieved from GE was the shroud was effecting the magnitics. There answer was to use plasitc, or something like that, bolts to hold the shroud over the CT's.

Hope this helps.

RE: CT off by 26%

I would like see reaction of Scottf to this recomendation of GE.
Slava

RE: CT off by 26%

Hmm...sounds like the shroud was somehow making a completed loop around the CTs. Obviously, the breaker design would have measures taken to prevent that.

Something like that would also explain some of the issues in this case.

Also, could be some secondary wiring errors, i.e. crossed wires between the 2 windings.

Another, more remote scenario, would be an insulation failure between adjacent CTs, where the outer most layer of one becomes shorted to the other.

I highly doubt that the 2 secondaries are wound around a common core. Would be highly unusual and wouldn't really work.

RE: CT off by 26%

I vote with GTstartup.
I have seen the note "Short unused CT terminals" on enough drawing and specs to know there are some people who do it know matter what.
If your connecting X1-X5, X2,X3 and X4 should be open.

RE: CT off by 26%

Yes, BJC you are right, but short of unused CT terminals in multi-ratio type of CT are not damage it. You see lower current, in Stevenal's case we see 1600 instea 1200.
Regards.
Slava

RE: CT off by 26%

slavag
They don't necessarly damage it but if shorted the currents can affect the accuracy.  

RE: CT off by 26%

(OP)
Scottf,

I didn't mean to suggest the CTs were actually wound around a common core; they're just acting that way. The shroud extends down to the CT support, which is made of plastic.

This morning we pulled the shroud off to get a look. No obvious problems. We had a meggar today, so began testing with it. 6X and 6Y were both shorted to ground. 4Y was okay. Thinking we may have a problem in the conduit, we pulled the leads out. With the leads going directly to the CTs, we again read a dead short to ground. No ground leads go to the CTs, and they appear to be on non-conductive spacers. We propped up the 6X CT on some rolls of electrical tape and retested it. We now had some measurable Meg-ohms on it while 6Y below still read a direct short. We removed the CTs from the bushing and put them directly on a metal grid. Meggaring between the grid and winding, again showed a direct short. Raising the CTs above on lineman gloves yielded acceptable results. After reconnecting the ground lead to the wrapping of the CT, we again read a direct short from winding to wrap. We clipped the meggar leads across some of the spacers that we assumed were meant to be insulators. Instead we found zero Meg-ohms.

At this point we left the station, and brought the CTs back to our warm shop for further testing. Here they test fine, passing all ratio tests and meggar tests. We also have respectable Meg-ohm readings between windings and wrap.

This station is close to the beach, and experiences salt fog conditions. The salt gets everywhere, and attracts any available moisture. I suspect there may be some salt under the wrappings of the CTs, since they do not appear to be sealed in any way. The shroud can keep the rain off, but not keep the fog out. Don’t know why only one bushing of one breaker of one station would be the only one affected, though. The weather has been clear during all of our outdoor testing.

RE: CT off by 26%

Stevenal-

Hmm....

Normally internal low meggar readings on BCTs is due to moisture intrusion. The moisture soaks into some of the insulation material and brings down the dielectric strength.

Normally salt deposits alone wouldn't do that and if they did, you would expect to see it in lab conditions as well.

Definitely a head scratcher.

With the relatively low cost of wrapped BCTs, you may want to consider just replacing them to be safe. (disclaimer...I work for a company who makes BCTs :)

RE: CT off by 26%

(OP)
scottf,

What insulation materials are used that are subject to moisture intrusion and resulting dielectric strength reduction? And is there a better material for this application? Or should we be specifying sealed units, rather than wrapped? I don't mind replacing the CTs, just would like to think we're fixing the problem rather than just postponing it.

We'll be setting the CTs outside this evening. Under cover, but subject to the morning dew. We'll re-test 1st thing tomorrow.

RE: CT off by 26%

The material really depends on who the manufacturer is. Tri-Flex is the normal material used that is subject to absorbing moisture. In our designs we use a mix of tri-flex and mylar.

The best way is to specify that the wrapping be a heat-cured fiberglass tape, as opposed to the "electrical tape" type vinyl tape. The cured fiberglass tape is much better against moisture intrusion.



 

RE: CT off by 26%

(OP)
Update:

Manufacturer suggested baking the CTs. Also said they normally test by placing on a metal plate and meggering from winding to plate. We did so, and the X CT had a high reading while the Y CT read zero. The external wrapping is a fiberglass tape. Will be contacting the manufacturer for a replacement.

RE: CT off by 26%

If the secondary were 2.88A instead of 3.7A, I would have thought that the CTs are supposed to be connected in delta (2.88multiplied by Sqrt3 is 5A). But, looking at the thread of mails, it seems to be CT accuracy/saturation issue.

RE: CT off by 26%

(OP)
Update:

Manufacturer sent replacement CT at no charge, even though breaker is out of warranty. The two CTs on the bushing still interact. Waiting for better weather to troubleshoot.

I've attached a picture of the one that failed. We believe the problem may be right here where the external wires are connected. Uninsulated butt splices are covered with a cloth tube. The dark spots are mildew stains. We're thinking we can replace the splices with insulated butt splices, and have a good spare. Maybe we'll get some heat shrink butt splices from the local marine supply. Whatcha think scottf?

RE: CT off by 26%

Was the sleeving shown not covering the crimp?

In my experience, it's not normal to use insulated crimps (most manufacturers don't like them as they're more likely to be crimped poorly), but rather the crimps shown and then cover with sleeving. That kind of sleeving is pretty good stuff.

Are you at liberty to say whom the manufacturer is?

My gut feel is that the crimps probably aren't your problem, but it's hard to say.

RE: CT off by 26%

(OP)
scottf,

The sleeve was pulled back for the picture, but the loose fitting sleeve does nothing to keep water out. The mildew stains indicate the area has been continuously wet. I think if the connectors were water tight, we'd pass the meggar test with the whole CT submerged.

Meramec CT on an Alstom breaker. We have many similar units with no problems. Our latest delivery came from the manufacturer with plastic covering the poly part of the bushing and no other protection. Breakers were wet and filthy from road sand and salt after going over mountain passes in our recent challenging weather. CTs tested okay straight off the truck.

  

RE: CT off by 26%

-scottf
Is it possible to identify interturn shorting by excitation test ?. This is is new information for me. I think excitation test is perform to idetify CT saturation ?

We generally use precision widing resistance meter to identify interturn shorting.

Let's have a look at the stevenal question one more time: difference in CT ratio; 1200:5 CTs was but actually 1200:3.7;

Do you think 1 or 2 no. of interturn shortings ?.

RE: CT off by 26%

ELEP-

I doubt a single turn short on a 1200:5A CT would show up on a secondary resistance reading. 1200:5 = 240:1, therefore 1 shorted turn would be approx. a 0.4% difference, which is well inside the difference you would see between like units, i.e. manufacturing tolerance.

A shorted turn will show up on secondary excitation tests. The knee-point voltage will drop noticeably and the excitation current will increase.

Note that a shorted turn won't simply result in secondary current changing by a factor of 1 turn difference. That's because the shorted turn causes local saturation of the core and throws off the magnetic properties.

RE: CT off by 26%

Re: the mildew near the tap connections. I found out the hard way that mildew is conductive.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: CT off by 26%

The shorted turn will quite often show up in the excitation test, especially compared to similar CT's.  The one with shorted turns will have a lower saturation volfor a given current.  Connect a variable voltage source to the secondary terminals and raise the voltage while monitoring the current from the source.  Stop at one amp and record the voltage.  Compare with the others.

It is not unusual for the "shorted turn" to be a conductive path through the window of the CT brought about by the shifting of conductive supporting members.  I've found those inside OCB's and transformers.

old field guy

RE: CT off by 26%

(OP)
Update:
Since February, we've been waiting for weather and schedules to come together. Finally got out this morning. The problem is that CTs that test fine in the shop do not test okay once they are installed on the breaker. This morning we found ratios were unacceptable on both CTs. As we prepared to begin disassembly again, I noticed the arrester bracket touching the shroud creating a ground loop. Although the factory (and cranky108 and scottf) told us way back to look for ground loops, I failed to notice this contact previously. The breaker had been retrofitted with factory supplied arresters and brackets following a damaging lightning incident. The other two phases had brackets that were still very close, but not touching. A small adjustment, and the ratio was correct on both CTs. I wonder why the manufacturer built these brackets to sit so close. In any case, I think we finally have this thing solved. Not sure at this point if the original problem had more to do with the loop or the moisture intrusion, or if the loop even existed at the time of misoperation. We've torn the thing apart and put it back together a few times since then. Thanks for all the help.

RE: CT off by 26%

Stevenal-

Glad you found your problem.

I've seen such problems in the past and once I read your last post I thought "I should have told him to check the arrester bracket a lot earlier". Sorry I didn't pass that along sooner...it's a somewhat common cause of creating a loop with the shroud around bushing CTs.

 

RE: CT off by 26%

I'm not picturing this at all. How does the arrester bracket interact with the bushing CT?   

RE: CT off by 26%

The shroud that covers the CTs in this design is aluminum. The shroud is designed to be isolated from the base that supports the CTs, such that it does not form a loop around the CT.

The arrester bracket touched the shroud and base, thus connecting them together and forming the loop.

In theory, the loop forms an additional 1 turn secondary winding and would carry (or attempt to) the primary current flowing in the bushing. This has a countering effect to the CT secondary winding and produces errors as mentioned above.

 

RE: CT off by 26%

I have found this problem before in brand new S**mens GCB's.  there was a bit of casting flash completing the circuit.  We discovered the problem in normal commissioning tests and corrected it by knocking the offending bit of metal down with the blade of a screwdriver.

Another incidence of similar nature was a found in an OCB that tripped on an out of zone fault.  In that case the CT retainer plate had shifted during normal operation and contacted the metal portion of the bushing, completing the offending loop.  It saturated at less than half the expected voltage as compared with similar units on the other phases.  Again, easy fix, but big problem before it was found.
 

old field guy

RE: CT off by 26%

Yep!  That'll do it.  I'd hate to think of what an inadvertent contact from vibration would do during normal operations...

old field guy

RE: CT off by 26%

Scott and Steven,

Thanks for the explanation and the photo.  I was thinking about an internal transformer CT under the oil.   I should have re-read the thread.  

Cheers,

Dave

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