Determining CT ratio
Determining CT ratio
(OP)
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?
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RE: Determining CT ratio
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!
RE: Determining CT ratio
Empirically. TTR set.
RE: Determining CT ratio
RE: Determining CT ratio
RE: Determining CT 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.
RE: Determining CT ratio
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.
RE: Determining CT ratio
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.
RE: Determining CT ratio
RE: Determining CT ratio
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.