CT off by 26% 4

[stevenal]

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?

 

[MikeHalloran]

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



Mike Halloran
Pembroke Pines, FL, USA

 

[dpc]

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?

 

[stevenal]

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.

 

[dpc]

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?

 

[stevenal]

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

 

[jghrist]

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

 

[scottf]

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.

 

[ELEP]

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

 

[slavag]

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

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

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

 

[slavag]

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.

 

[scottf]

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.

 

[slavag]

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

 

[oldfieldguy]

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

 

[stevenal]

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.

 

[GTstartup]

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.

 

[GTstartup]

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

 

[stevenal]

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