CT accuracy
CT accuracy
(OP)
10P20 CT it means a protection CT with composite error of +/- 10% and that error will be maintained up to 20 times the rated primary current of the CT. Is it correct? Is it constant?
More and more current measurements/monitoring using the modern protection relays instead of dedicated instruments + dedicated measurement CTs are being used.
Are the measured values, even close to the rated primary current of the CT, subjected to the +/- 10% error?
Thanks
More and more current measurements/monitoring using the modern protection relays instead of dedicated instruments + dedicated measurement CTs are being used.
Are the measured values, even close to the rated primary current of the CT, subjected to the +/- 10% error?
Thanks





RE: CT accuracy
RE: CT accuracy
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: CT accuracy
I hope so. Is it possible the evaluation of this error at normal operation condition?
Is there a standard or a paper to be consulted?
Thanks
RE: CT accuracy
In installations where the client required both metering apparatus and microprocessor-based relays with extensive metering functions, I've not noticed appreciable difference in measured data over a period of time. However, I do recognize that this is anecdotal. However, the accuracy of microprocessor-based equipment is causing a lot of people to rethink the traditional protection ves. metering line of demarcation in non-revenue venues.
old field guy
RE: CT accuracy
I will continue to use protection CTs (5P20 - 15VA instead of 10P20), Multilin / SEPAM series protection relays and no measuring CTs. The problem is how to prove to the client the guaranted accuracy. To find a safe and simple way to get additional measures (to be compared) in a 15kV class Swgr is not so easy. I hope I can find related papers or articles before the comissioning.
A Happy New Year.
RE: CT accuracy
Happy New Year
RE: CT accuracy
RE: CT accuracy
RE: CT accuracy
I always did it but the modern protection relays feature a lot of measurement/monitoring related to current measures and there are just inputs for protection CTs. My question was derived from this reality. As oldfieldguy said above: "However, the accuracy of microprocessor-based equipment is causing a lot of people to rethink the traditional protection ves. metering line of demarcation in non-revenue venues."
Anyway, thanks.
RE: CT accuracy
Also, as you probably know, for a multi-ratio CT, the stated accuracy only applies across the full ratio. If it is tapped down, the accuracy goes down.
RE: CT accuracy
RE: CT accuracy
Have you had any problems with meters being damaged by through-faults? I'm no metering expert but I always understood that protecting the meter was the reason why saturation not too far above nominal current was desirable, which is obviously a conflicting requirement with a protection CT where reasonable accuracy is required when fault current flows in the primary. Or do you mean that the dual-rated CTs used with protection relays having sufficient accuracy to be used for measurement applications but which can handle the high secondary currents during faults? Apologies if I'm just not reading your post correctly.
RE: CT accuracy
I don't believe ANSI has anything like the IEC "instrument security factor." My assumption is that ANSI meters are as rugged as relays and are not damaged by fault current. We have not had an issue with meters on protection class CTs or with multifunction instruments on dual rated CTs.
RE: CT accuracy
The question it was: A feeder protection relay, GE Multilin 350, as an example, performs IOC, TOC, a lot of protections based on current measurement. Also provides metering of power, phase currents, thermal model, oscilography etc and displays some of them.
This relay has just 03 phase inputs from CTs and one more for ZCT. Of course these CTs shall be protection CTs, as for example, 10P20 (+/- 10% up to 20 In). I asked: Which is the accuracy of these CTs when the current is close to the nominal value? Can I trust on the normal running results or I need to provide a warning label:"Do not consider some readings of current. The composite error is +/- 10%". Of course the answer would be no but I was asking for a number.
Runsor answered that 5P20 has less than 1% and 01 degree. 10P20 less than 3%.
For me, a correct specified protection CT 5PXX will be enough and where accuracy is not critical I will not use measurement CTs and dedicated meters.
Thanks
RE: CT accuracy
RE: CT accuracy
RE: CT accuracy
Dual rated IEC CTs might be tough to find, since I understand metering CTs are required to saturate under fault conditions in order to limit current to the connected meter.
RE: CT accuracy
I think that is a valid concern. I once started a fire in 11 kV switchgear when I caused a 1600 kVA transformer to saturate (malfunctioning softstarter drew DC from transformer secondary) and draw a lot more current than anyone thought possible. With a DC saturated core, around half of the uk was lost. If that CT also had saturated, we wouldnt have had the fire. Fire started in an old Ferraris meter for internal use, no energy vending, in series with protective relays.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: CT accuracy