Determining CT ratio 1

[Shavner]

I'm doing a SC and Coor. study on a large system and ran across a 13.8KV switchgear dated prior ot 1960. What is the best way to get CT ratios?

 

[jwerthman]

The most accurate way is to inspect the CT's, but this can't be done without at least opening the breaker and drawing it out of the cubicle. Unless your're doing this during a normal maintenance outage, that is probably out of the question. With many of the older switchgear designs, the CT's aren't easily accessible even with the breaker out.

If you can locate the drawings, that is a good source - if you have a reasonable degree of confidence that they are up to date. That is a big if, and depends entirely on the diligence of the plant staff. Some are very good, others are very bad.

Many times on older switchgear there are analog ammeters in the same circuit with the relays. The full scale of the ammeter should be equal to the CT primary rating, unless it is some oddball setup. 99% of the time that will give you a valid ratio.

If there have been periodic maintenance tests on the switchgear, they may have done CT ratio checks. If you can locate the test reports, they are a good source of data.

Good luck!

 

[busbar]

Empirically. TTR set.

 

[Shavner]

JW, Thanx for the quick response, i'm going out today to inspect the SWG. I'll check the AM and see if i can find a plant electrician.

 

[RajT]

Primary injection tests as stated by busbar is the only option for accurate data

 

[trevorm]

Primary Injection is the only accurate way to determine the ratio.
Using the ampmeter sometimes is an option, but often the Protection and Metering circuits have different ratios. You also have to be aware that Protection and Metering CTs have different characteristics and different classes. Magnetisation Curves should also be done.
Try reading current in the secondary circuit and comparing to indicated primary current.

 

[electricpete]

I don't think primary injection is the only test method. There is a voltage test method. Volts per turn is same on primary (one turn) and secondary. Apply what you think is 1 volts per turn on the secondary. Measure voltage on the primary (ie between bus locations at either side of the ct... be careful about ground locaitons which can alter the test... but also maintain safety of course).

Alter the voltage to make sure you are on the linear portion of the curve. Ratio is secondary voltage over primary voltage.

My experience agrees with jwerthman's. Take the full scale reading of the ammeter and divide by 5A and usually that is your ratio.

 

[electricpete]

maybe there was a distinction drawn about accuracy. Volt test method alone will not tell you anything about ratio correction factor... although that can be calculated fairly easily without primary injection by measurement of secondary current vs secondary voltage.

If I'm reading the question right, accuracy is not a big concern.

When I saw the test done it was kind of a home-made test setup. I think busbar is saying the voltage method can be implemented using a TTR test set. That seems to make sense.

 

[bass1mj]

Usually in Switchgear they put the CT rating plates inside the cubicle, behind the test block, against the back wall because the CTs are hidden away at the back.

 

[trevorm]

Maybe the 5A secondary is standard in certain parts of the world, please bear with me, my experience is only on systems in South Africa. Depending on the system, the secondary could be 1A or 5A. Many protection relays also give the option of selecting 1A or 5A.

Whether a voltage TTR or primary injection is done the test can not be done live. Definitely not at 13.8kV

Not everbody has access to a TTR test set. I usually use a primary injection test set, but if one is not available, most secondary test sets can circulate 50A. If all else fails, try using a variac to circulate current in the primary and calculate ratio by comparing primary to secondary current.

I have used the voltage method by checking the primary voltage while doing magcurves on the secondary side. On a class X CT it is fine, but not too accurate on other. There again we were doing commissioning tests on 400kV CTs with 2400:1 ratios. Smaller ratios should be fine.

Thanks for a stimulating forum. Especially electricpete, I enjoyed following the one on drying out motors with a megger.