Accuracy Class of Core Balance Current Transformer (CBCT)

..." both are giving less than 5% of error while measurement, 5P10 gets saturated over and above 10 times of rated current where as other one at 20 times"...

http://www.crompton-instruments.com/datasheets/2010/ctcore.pdf

https://support.industry.siemens.com/tf/ww/en/posts/protection-ct-class/94350/?page=0&pageSize=10

Dear Sir,

Thanks for the reply.

ALF of phase CT 5p10 or 5p20 is known to us i.e. 10 times or at 20 times of current CT should not saturate.

But I was talking about CBCT ! What should be its accuracy class to be considered so far the design feasibility of the CBCT is concerned.

Should it be class-1.0 or 5P10/5P20 ?

Any write up to support that cl-1.0 is sufficient to operate the ELR?

Kindly support to reply.

Regards,

If you're buying, then let the supplier come up with a solution.

If you're selling, the answer should be covered already by approved design procedures.

For metering purposes, the user is mainly concerned with accurate operation through the normal measurement range of the circuit. That is why metering accuracy CTs (such as the 1.0 accuracy class CTs you mention) are used for that. You don't care what the CTs do under high-current fault conditions - you need protection then, not metering. But you want them to be accurate even down to low currents.

For protection, the greater concern is that they are accurate at higher currents, such as overloads (several times normal) or faults (sometimes 20x or more than normal load current). In that case, the accuracy has to be good at high currents, which required a protection class CT such as the 5P10 or 5P20.

In the current balance situation, normally the current is balanced, and you may think a metering accuracy CT would work. But it is exactly in the fault condition that you need the protection class CT. In that case, the current is unbalanced, and the difference between phases can be very large, great enough to drive the CT into saturation if it is not capable.

So use the correct CT for the function you are doing. If for metering, use a metering CT. If for protection, use a protection CT. It's that simple - don't compromise. If you do, you will regret it later, when your installation burns down and you are left with the greater cost of rebuilding.

As per our knowledge the sensitivity of cl-1.0 CT is always better than 5P20 CT. Then how come 5P20 CT will give better signal to the ELR in case any unbalance?

In case of normal fault for residual protection, 5p20 or 5p10 CT will take care but what happened to cl-1 metering CT at that moment? Will it saturate? Certainly not!

Hence as per my opinion 5P20 CBCT is not justified so far the sensitivity of CBCT is concerned. During a fault condition how the resultant reflection at the secondary will vary during normal unbalance & fault unbalance ? Kindly discuss.

Your opinion please.

How can you say, "Certainly not!" that it will not saturate under a fault??? You have not provided any indication that you have calculated the maximum current that the CT is going to experience under a fault. It will NOT be balanced between the phases - that is what the current balance arrangement is supposed to detect.

Yes, the metering CT is more sensitive. That is why it is used for metering. The protection CT is less sensitive, but still accurate enough for its purpose, plus it is designed to handle higher currents before it saturates.

Why are you asking us to justify something that is standard design practice across the electrical industry, so that you can cheap out and use the incorrect CT improperly?

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There is much old research on the combination of certain protective relay coils and zero sequence current transformers. This came up as a result of trying to detect the lowest possible ground fault current, using resistance grounded medium voltage systems, as low as 400 amperes still allowed downstream breakers to sectionalize a ground fault before igniting the power resistor. Zero sequence CTs were used on the feeder breakers.

It was found that not only the CT affected the result, but the combination of a specific CT and a particular protective relay would also make the difference whether the relay actually picked up and operated. So not only do you have the consideration that Mr. PeterT puts forward, but also that you may not generate enough current to operate the relay.

Thanks to all for a constructive replies.

Still I am not clear on below points:

1. If I want to use CBCT for sensitive earth fault or leakage current protection through ELR, then what should be the accuracy class of CBCT to consider for a MV switch gear?

2.If the accuracy class of CBCT will be 5P20 or 5P10 then its design feasibility or size also to be considered for accommodation in a stipulated space availability. What do you say on this?

3.What should be a standard specification of a CBCT which will include MOPC , class etc.

4. When any through fault is there in the system the normal metering CT( not CBCT) whose accuracy class is 1.0 should be discarded or not as it may saturate or may not !

Kindly clear my doubts point wise which will enrich me.

Regards

1. you need to know the ground fault current, and you need to consider the relay and CT as a unit, to be sure the relay does not load the CT so it will not work on high or low current.

2. The size is what it needs to be to operate according to your needs.

3. Not sure what all of the abbreviations are, MOPC? See 1 & 2, in any event

4. No one cares if your metering CT saturates under fault condition, for a few seconds. Your protective functions must operate properly under fault conditions, so you need to check that with your fault current calculations, and ensure that your relays, CTs and wiring is sufficient to operate & clear a fault...