Use protection CT for metering 1

[leoliu]

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.

 

[jghrist]

It is common (in the US at least) to use protection CTs for metering that is not used for revenue purposes. In this case, high accuracy is not required. Revenue meters should use metering accuracy CTs.

 

[cranky108]

Relay CT's work well above about 10% of there rating. They are used to give general power flow, and are much easer to specify, in that all the CT's on a breaker have the same rating.

 

[dpc]

It's generally not possible to get revenue-grade bushing CTs, at least in my experience.

David Castor

http://www.cvoes.com

 

[scottf]

Protection class CTs will often perform well (accuracy) at rated current, however, the CTs aren't tested for metering accuracy as part of the factory routine tests. In general, the performance can vary widely based on design and core material used. The accuracy performance tends to fall off more steeply as the current goes below rated current as compared to a metering class CT.

It is common (IEEE/CSA markets) to have CTs with both protection and metering ratings, e.g. C800/0.3B1.8.

DPC...bushing CTs with metering ratings are very common (we make a lot of them). The problem generally comes when a ratio below around 600:5A is needed. That's pretty much the cut-off where metering accuracy can be achieved with conventional core material. We do make some bushing CTs with high-accuracy (0.15 class or better) with ratios as low as 400:5A, but either nickel or nano-cores must be used and the price goes up substantially.

 

[odlanor]

IEC 44-1 protection CTs with accuracy class 5P and 10P at rated frequency and with rated burden connected, the current error can not exceed 1% and 3% respectivally.

I have seen tests of protection CTs with accuracy of 1% at normal loading current.

 

[scottf]

odlander-

You're exactly correct on the requirement, but in actuality, the performance will likely be much better at rated current.

For instance a CT rated 5P20 - 60 VA, will likely have 0.5% or 0.2% performance at rated current.

 

[odlanor]

Scottf,
during CT routine tests I always found 10P20-(0.8%,1%).
My problem is that I do not know requirements of operational measuring.

 

[scottf]

Depends on the burden rating of course.

I just did a quick design calc on a normal 5P20 - 60 VA bushing CT about the size needed for a 115 kV bushing and it's about 0.5% current error at I nom and at 60 VA. Of course, a metering circuit (and a protection circuit for that matter) doesn't typically have 60 VA burden for a 1A rated secondary.

 

[leoliu]

Stottf, the operational measure is 0.3% at rated current according to our supervising authority. I doubt that our protection CTs can meet the requirement.

 

[leoliu]

Sorry, I have to make correction - the operational measure is 0.6% at rated current.

 

[odlanor]

scottf,
We always use 10P20-5A secondary.In ANSI std 10B200,10B100 for protection are very common.
I wonder how you calculate it, if it is appropriate to disclose the calculation.

 

[odlanor]

leoliu
how about protection CTs for others operational measuring: governor, AVR and synchronization? Do you use it too?

 

[scottf]

odlanor-

We have calculation software that gives all kind of parameters are various current levels. Not something I can really post on here.

However, C100, C200, C400, C800 are the common protection classes in IEEE. The common metering rating for MV and HV units is 0.3B1.8.

Almost any C100...C800 CT will also meet 0.3B1.8. The trick is that it's not measured as a routine test, so you don't really know for sure.

 

[odlanor]

scottf,
1- about calculation, no problem.
2- The trick is that it's not measured as a routine test, so you don't really....
I guess I can investigate this . We have many CTs in inspection now.
Any how , I did not find any Std of operational measuring.

 

[inpran]

@ Odlanor & Scottf,
I haven't come across such a case as of now. But I was wondering as to what will happen to the metering instruments when you use a protection class Core for a metering one. The metering core saturates much before a protection core does. And most of the meters have a amplitude limiting factor of 1.2. So, aren't there any possibilities of damage to the metering instruments when a protection core is used for a metering purpose?

 

[odlanor]

inpran,
No problem. In this case metering instruments are the relay equipment. Numerical digital relays can record all electrical quantitaties during normal operation and short circuits.

 

[leoliu]

odlanor, my company does not own generators so we don't use it for governors,AVRs etc. The operational measure of 0.6% is requirement by ISO (Independent System Operator).

 

[scottf]

inpran-

That depends on the standard to which the meter is built. IEEE/ANSI meters are required to have a much higher surge rating than IEC meters...typically. That's why you often see IEC metering class CTs specified with a security factor (factor by which the CT must saturate), whereas, such a rating doesn't exist under IEEE for metering class CTs.

 

[odlanor]

scottf,
I do not understand your answer. We're talking about measuring instruments connected to CT of protection. That CTs must have factor by which the CT must saturate about 20.