Hi all, my company uses protection CTs for both protection and metering. Is it a common practice for utilities? How do I know its accuracy (e.g. <0.3%) of metering normal loading current (not fault current)? Thank you.
I know its not normally done, at least in my country (India). Hence was confused when it was told that it can be done. Anyway thank you guys for your time.
Leoliu,
Can you please throw some light on the discussion?
Is it safe to sum up by saying that both metering and protection CTs may have the same accuracy at rated current.
However, protection demands that the CT maintain accuracy at very high currents
Revenue grade metering CTs must maintain accuracy at very low currents.
Some CTs may meet both requirements.
Bill
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"Why not the best?"
Jimmy Carter
scottf,
I am understand that at same core of CT I can connect protection device(10B100= 4VA) and metering device(0.3B1.8 = 1.8VA).
this combination is very difficult to prove because the greater burden of the cable and will not serve the purpose.
I'm saying that the CT can be both rated with a protection rating and a metering rating. I didn't say you should connect one CT to both a meter and a relay (you can do that too, but that's not the focus of this thread).
It should show a routine test performed in 2009 year about CT-13.8kV. It's been a long time that the errors of protection CT shall not exceed 2%, far from the specified (10%).
15 (fifteen) Current Transformer,, 8000-5-5-5 A, 60Hz, 10B400, 10B400 and 0.3C50, 24kV, 110kV(BIL), 46.5kA(Isc).
1- Ratio 8000-5 , Rated burden 100VA , protection core
- 1.0 * I secondary rated , Maximum Current Error: -0.68 %
- 20.0 * I secondary rated , Maximum Current Error: 1.135%
2- Ratio 8000-5 , Rated burden 50VA , metering core
- 1.0 * I secondary rated , Maximum Current Error: 0.02 %
- 0.1 * I secondary rated , Maximum Current Error: -0.25%
Thanks everbody here. I have no light to throw ... I'm very confused too. My company uses protection CT for both metering and protection, while a neabouring utility always distinguishes protection CT and metering CT. I have no clue why.
scottf,
brazilian Std is like IEEE Std .
The manufacturer according to IEC TC projects but adequate to brazilian Std.
There a some TCs type TPY according to IEC. The accuracy of results are the same but the saturation curve
is different. The knee voltage of TPY are very high.
Too much information to sort through? The very first answer from jghrist is your key. You've never stated whether the metering you are speaking of is for monitoring or revenue.