Knee point Voltages (Metering CT's) 2

[CraigMW]

I work for a utility group and had contractors perform Mag. curves on our 110KV post CT's with the following results:
CT ratio & Class - 1600/1200/1000/400/1A 1.0M - 10VA
''A''Phase Tap (8.23Ω) ''B'' Phase Tap (8.25Ω) ''C''Phase Tap 3.45Ω
X1-X2=240.37V X1-X2=239.57V X1-X2=40.32V X1-X3=606.81V X1-X3=601.27V X1-X3=103.26V
X1-X4=724.03V X1-X4=719.78V X1-X4=126.42V
X1-X5=956.51V X1-X5=957.84V X1-X5=176.57V

I contacted the maufacture regarding the knee point voltages & measured resistance of AB phases verses C phase and was told that,“If the core complies with its class then there is no issue regardless of the mag curve reading.
Mag curves are used to determine the knee voltage of PX cores, the metering cores could also have high knee voltages.”

They have indicated that the Mag curve results are not critical to the function of the metering cores (only relevant to protection cores) as long as the metering core is within the required class of accuracy.

Please comment/advise as to your thoughts on this.
Thanks

 

[Marmite]

We just had a very similar issue where the CT's met the spec but one of supposedly identical set of three CT's had vastly different results. In that case the manufacturer said that although the CT had passed its tests within limits it should not be installed on site, and they are replacing it.
Regards
Marmite

 

[collies99]

Multi core free standing CTs often are provisioned with protection and meyeting class CTs.

during faults the metering ccts do not see the possible voltage outputs that may be required yo drive the burden on the protection cct. Accuracy is better are rsted sec current fir metering functions.

 

[scottf]

If the CTs are supposed to be identical designs (same number of primary turns and same core size/material), then there appears to be a problem with the C-phase CT.

If the CTs are not identical designs, then the C-phase CT may be fine. The C-phase CT could be using a different number of primary turns and different core size. That would explain the different Rct and Vkp readings, but you need to make the manufacturer explain it.

 

[collies99]

Should be a nameplate that identifies the class / type somewhere on the termination box
... That is what is typical.

 

[dfdt]

we specify ISF (Instrument Security Factor) for metering CTs as this defines when the metering CT shoulds saturate.We specify ISF of 10 for metering cores.

 

[cranky108]

And the core could be magnetized, so it would not give the right results.

 

[CraigMW]

Thanks for the info.

We have now matched all CT's.