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Use protection CT for metering

Use protection CT for metering

Use protection CT for metering

(OP)
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.

RE: Use protection CT for metering

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.
 

RE: Use protection CT for metering

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.

RE: Use protection CT for metering

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

David Castor
www.cvoes.com

RE: Use protection CT for metering

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.

 

RE: Use protection CT for metering

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.
 

RE: Use protection CT for metering

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.

 

RE: Use protection CT for metering

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

RE: Use protection CT for metering

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.

RE: Use protection CT for metering

(OP)
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.  

RE: Use protection CT for metering

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

RE: Use protection CT for metering

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.
 

RE: Use protection CT for metering

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

RE: Use protection CT for metering

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.  

RE: Use protection CT for metering

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.
 

RE: Use protection CT for metering

@ 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?
 

RE: Use protection CT for metering

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.
 

RE: Use protection CT for metering

(OP)
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).  

RE: Use protection CT for metering

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.
 

RE: Use protection CT for metering

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.

RE: Use protection CT for metering

@ Scottf
Then if I understand it right, you cannot use a Protection CT for metering purpose in IEC world.Is it?

RE: Use protection CT for metering

inpran-

Depends on the specific meter I guess, but normally it's not done.

RE: Use protection CT for metering

Inpran,
my opinion was precipitated. I think it is up to Leoliu answer your question because it is who has the experience.
 

RE: Use protection CT for metering

Scottf & Odlanor,

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?
 

RE: Use protection CT for metering

Please note that I failed to re-state that it's not commonly done in "IEC" markets.

It is very common to have dual rated CTs (like C800/0.3B1.8) used for revenue metering in IEEE markets.

 

RE: Use protection CT for metering

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
--------------------
"Why not the best?"
Jimmy Carter

RE: Use protection CT for metering

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.

RE: Use protection CT for metering

Odlander-

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).

 

RE: Use protection CT for metering

Leoliu,
Can you please throw some light on the discussion?

RE: Use protection CT for metering

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%
 

RE: Use protection CT for metering

What standard are those CTs built to? Is that Brazilian?

RE: Use protection CT for metering

(OP)
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.

RE: Use protection CT for metering

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.

 E% = error protection
 Saturation current Io =
 Io <= F * isn * Cl / 100
 F = 20; Isn = 5A, Cl = 10%
 E% = (Io * 100) / (F * isn))
 

RE: Use protection CT for metering

leoliu,

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.  

RE: Use protection CT for metering

I'm totally disappointed. For years now I try to eliminate operating CT measurement and still not finish my thought.

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