Current Transformer

Yes! I do confirm that a series of pre-commissioning tests are mandatory for any equipment. For CTs, I would suggest:

i) IR Value (Pry to earth, each secondary to earth, pry to each secondary and between secondaries)

ii) Polarity (of each secondary)

iii) Secondary winding resistance (of each secondary)

iv) Excitation charectersitics (Mag-Curve)

v) HV Test

vi) Ratio

And compare the results with the manufacturers' test certificate to enusre that there are no damages during transit.

Thanks for your input

Please give or quote the relevant BS/IEC for these tests and the test voltages recommended.

IEC 60044

I don't think you need to check all these functionalities to verify the damage during transport. The transport damage is generally happen due to shock or mishandling. You can see this sort of damage just by physical looking. You can do the insulation test and the continuity check for both primary and secondary. It is very unlikely that the transport damage alters the CT ratio, polarity, magnetization curve etc. However, if you do the test for other reason, then it is a different story.

- MS

Yes I do agree it is unlikely there would be any damage in transit but it's all about covering your backside.

Any idea why the old transformers failed? I would be hesitant to replace any three failed parts with identical units without an understanding of the cause of the failure.

One possibility is that they had been open-circuited and the insulation failed.

Yeah, that's one possibility. There's also the possibility for miswires or the infiltration of moisture. Then we can also go into the realm of plausible yet absurd. "Our welder will never cut open a transformer again. He's very sorry." A lightening strike would more than likely toast something else, too. But accurate predictions of what a lightening strike will toast is virtually impossible. I'm sure others here can come up with many more possibilities.

I'm just apprehensive about three identical failed components being replaced without an understanding of what went wrong.

Quite.

I agree totally with redfred . It is the correct practice to check the cause for failure before replacing the new one. If the failure was due to any fault it can damage the newly replaced CT also.The probable cause may be open circuit CT.

The reason the CT's are being replaced is they are cracking and the internal brass sleeve on each CT is slipping down.

Whoa! This is an interesting piece of information that needs to be more fully explained. It sounds like a teardown is in order, or was one made? Maybe a photo or two? Also, we need to know the type of CT, manufacturer and model numbers.

Sorry don't have name plate with me at the moment but the CT's are 11KV.

Thanks for the photo.

OK, now I'm clearly out of my league - I can't even tell what I'm seeing. That can't be a brass ring next to an 11kV bus bar?

You said something about an "internal brass sleeve?"

(as used for damping magnetic collapse in solenoids)

First, we have to say it's highly unlikely that you were sent three defective transformers. Yes, it could happen, but it's not likely (if the supplier was sending out severely-defective parts, he'd be called out on it really fast, and it would stop).

It's more likely that you might have somehow damaged the transformers.

There are two possibilities. One is the transformers aren't defective at all, and your test is defective. The second is that you have damaging them.

How does one damage a current transformer? Not from over current. They saturate and are happy enough, even if you aren't happy with what they say they see. How about over-voltage? Yes, damaged insulation. Gonzo. That can happen if you run the current transformer with a modest or high current, but without an adequately-low-resistance load. The secondary sees too high a high voltage and breaks down. End of story.