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Power-User
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Current Transformer Accuracy

09/15/2019 11:00 PM

fellow experts, i would like to solicit ideas regarding accuracy of current transformer for revenue metering use. the case involved the CT operating at an intermittent open-circuit condition due to a "snipped" conductor at the terminal block (say test block). i have already tested the CT using a specified testing equipment for metering instruments and have met the required values stating that the CT is still in good condition. however, my gut tells me to do otherwise that is to not re-install the CT. i want to perform another test but i dont know if what i am going to do will be justifiable since i have not seen the procedure in any written literature. hope the experts here could enlighten me with this problem. thank you.

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#1

Re: Current Transformer Accuracy

09/16/2019 6:49 AM

This is NOT my field, so please regard the following as "discussion starter" only.

If the current transformer is "open circuit" (Or feeding a really high impedance measuring device) and thus providing a voltage indicative of the current, then it's probably not such an issue provided the wire terminations are outside the coupled circuit.

If the CT is feeding a current to a voltage sensing device, then it might be of concern since the removed portion of conductor would represent a reduced resistance relative to the calibrated situation.

What is the cost comparison for a new CT (including replacement downtime) compared to the lifecycle cost of potential error in metering? This answer might lead you to the optimum solution.

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#3
In reply to #1

Re: Current Transformer Accuracy

09/16/2019 9:56 AM

A current transformer is never used open circuit. You connect the current transformer to an ammeter, a low impedance meter.

For example, if a current transformer around a conductor has 100 turns, then the current read with the ammeter will be 1/100 the current through the conductor.

https://www.electronics-tutorials.ws/transformer/current-transformer.html

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#8
In reply to #3

Re: Current Transformer Accuracy

09/17/2019 4:24 AM

Thanks, Was over 40 years since I've been in that field.

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#10
In reply to #3

Re: Current Transformer Accuracy

02/28/2021 6:31 AM

Ratio tests are performed by applying a suitable voltage (below saturation) to the secondary of the CT under test while the primary side voltage is measured to calculate the turns ratio from the expression above. In this way one can check the accuracy of the current transformer suitable for metering or not https://favreadblogs.blogspot.com/2020/01/what--current-transformer-meaning.html?m=1

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#2

Re: Current Transformer Accuracy

09/16/2019 9:33 AM

A CT does not meter anything. A CT is a transducer that proportionally converts current into a voltage. The instrumentation that measures this voltage performs the metering task.

Like any transformer, a CT can be made non-linear or more subtly change its transfer ratio due to a number of influences (eg. core saturation, bandwidth limitations, extraneous B fields, heat, capacitive coupling), every measuring system has its limitations.

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#6
In reply to #2

Re: Current Transformer Accuracy

09/16/2019 11:47 PM

A current transformer is different than a current transducer. Current transformer converts current into current, always a very low impedance path.

Everything else is ok...

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Guru
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#4

Re: Current Transformer Accuracy

09/16/2019 10:08 AM

So what's wrong with getting the supplier of the <...CT...> on the telephone?

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#5

Re: Current Transformer Accuracy

09/16/2019 11:19 PM

Open circuiting affects the CT core as well as the electrical insulation of the wiring.

CT core damage would show up as increased magnetising current in tests which means increased errors for a metering CT.

Insulation damage will get exposed when IR test is done with a 500V megger on secondary wiring (Sec terminals to earth). If the CT is Bar primary type with window to pass the primary busbar, then IR test is not relevant on primary side. However, checking IR on Primary to Secondary is required.

It is possible that there is no damage to CT if the open circuiting was only for a short duration (overall). Considering the metering CT is designed to saturate just above the rated current (ISF<5), the voltage developed is also unlikely to be excessive that can damage the insulation.

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#7

Re: Current Transformer Accuracy

09/17/2019 12:00 AM

I agree with Raghunath, it is likely there was no damage if it passes your ratio tests. I have inadvertently opened CT circuits with nominal line current for a short time, horrified I put the shorting screws back in, but never lost a unit in the 8 years or so of subsequent experience. Not that I was actively checking, but if a 35kv gas breaker at that plant had to be disassembled and have a bushing CT replaced, I would have heard about it, if I didn't have to do the job myself.

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#9

Re: Current Transformer Accuracy

01/25/2020 5:36 AM

If the revenue meter is microprocessor based electronic meter, you can take the MRI which will reveal whether the measured current was zero at any instant. If there exist current when load is available, the CTs connection is perfectly OK. The looseness in the wiring can be checked by re-tightening at test terminal block. In neither case CT shall be open circuit. If CT is open circuit, the net Magneto Motive Force(MMF) in the core of the CT will be enormously high which cause CT core saturation. The CT core saturation will lead insulation failure of CT winding and moreover, the high voltage induced at the secondary terminal may cause insulation failure/spark over at the test terminal block.

For more details you may refer below given article:

Why CT secondary shall not be never kept open?

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#11

Re: Current Transformer Accuracy

03/01/2021 8:16 AM

My greatest concern for the CT if it experienced an open-circuit situation is that the insulation was damaged by the overvoltage it experienced. If your ratio and excitation test results (that's what I assume your "specified testing equipment for metering instruments" was) were as required, then I would recommend that you do a DC insulation resistance ("megger") test to verify the insulation condition. If it's a 600V insulation class CT, then 1000VDC is probably adequate, 1500V better. If you have already done this test and the CT passed, I would say it is probably acceptable to be returned to service.

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