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.

 

[inpran]

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

 

[scottf]

inpran-

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

 

[odlanor]

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

 

[inpran]

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?

 

[scottf]

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.

 

[waross]

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

 

[odlanor]

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.

 

[scottf]

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).

 

[odlanor]

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

 

[odlanor]

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%

 

[scottf]

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

 

[leoliu]

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.

 

[odlanor]

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))

 

[stevenal]

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.

 

[odlanor]

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