Shunt Reactor CT Ratio Testing Error

[gostop]

Hello,
We are commissioning our first 345kV shunt reactor and are finding 15% error in measured CT ratio tests on all bushing CTs.
Manufacturer states that when tested outside of the bushing, all CTs were good, but their testing when CTs were mounted showed same results we are getting. I am waiting for their test results.
345kV shunt reactor, 3ph, 40MVar
2CTs/bushing, 2000:5MR C800, Y-connected, connected and tested at full ratio.
Tests showing 347/1 ratio rather than 400/1.
Testing being performed at the reactor with secondary circuits disconnected, voltage method, shorting leads on bushings not under test, similar to when we test transformer CTs.
I think it has something to do with the reactance, similar to a transformer without shorting leads applied, but can't quite work it out. Has anyone seen this and know the why? I will have to brush up on some theory when I get home.
Thanks.

 

[DTR2011]

Interesting...

Do you have the ability to see the volts, amps and phase angle from your test equipment? Is the ratio error consistent at different test voltages? Have you performed excitation test prior to ratio test?

 

[gostop]

Saturation tests were performed and I was told they were good. I am waiting to see the test results to verify. Our test set will show voltage and current points as it is performing the saturation test, but that is all. They have not yet viewed the excitation current to verify there isn't a short in the CT. Phase angle is not viewable. I will have them check at different test voltages. Thanks.

 

[scottf]

Almost certainly you're getting some coupling effect. Most likely a grounding/non-grounding issue.

Have you tried testing with the other bushing not shorted, but not connected to the line?

 

[DTR2011]

"Testing being performed at the reactor with secondary circuits disconnected, voltage method, shorting leads on bushings not under test, similar to when we test transformer CTs."

Have you grounded the shorted bushings (not under test)?

I've seen results similar to yours in Auto Transformers with a delta tertiary and generally it is resolved via shorting and grounding.

 

[gostop]

Thanks for the responses, rain held up re-testing.
First tests were done with ground bar disconnected from neutral side on the phase under test (H03 terminal) and H1, H2, H3 terminals had no trainers connected. Error existed.
Same configuration but with H01, H02, H03 terminals all shorted and grounded, same CT ratio error.
Same configuration with neutral ground removed, error existed.
We were thinking of the reactor as an auto-transformer with no low-side tap and testing it as such. This is an oil-cooled, coreless reactor.

 

[DTR2011]

After solving your problem, probably a good idea to document exactly how your test is performed for future reference and NERC compliance.