CT Ratio Test Results Distorted By Connected Cables
CT Ratio Test Results Distorted By Connected Cables
(OP)
Who can tell my why we were seeing 5.0% ratio errors on new ABB SCV CT's with load cables connected? When we disconnected the cables the error was 0.1%. BTW the test set is a Vanguard EZCT.






RE: CT Ratio Test Results Distorted By Connected Cables
If so first thought that comes to mind is how much impedance is presented by those cables. Trying to push too much current through too much secondary impedance of course pushes you into saturation where you see a ratio error.
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RE: CT Ratio Test Results Distorted By Connected Cables
Funny, we tested many CT's in this facility and the CT's in this line-up are the only units that reacted this way? The other CT's are older GE units that have been in-service for 30 years. They all ratio-ed > 1.0%.
RE: CT Ratio Test Results Distorted By Connected Cables
Regards
RE: CT Ratio Test Results Distorted By Connected Cables
Bill
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"Why not the best?"
Jimmy Carter
RE: CT Ratio Test Results Distorted By Connected Cables
Nothing making sense.
RE: CT Ratio Test Results Distorted By Connected Cables
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If we learn from our mistakes I'm getting a great education!
RE: CT Ratio Test Results Distorted By Connected Cables
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: CT Ratio Test Results Distorted By Connected Cables
RE: CT Ratio Test Results Distorted By Connected Cables
RE: CT Ratio Test Results Distorted By Connected Cables
A current loop on the primary, though, is usually detectable as a downward change in saturation voltage. You DID check saturation voltage, did you not?
old field guy
RE: CT Ratio Test Results Distorted By Connected Cables
I looked over the circuit again this am and there it was, grounded cables. Lesson learned.
OFG, the saturation voltage was about 130 volts. I suppose the ground loop could influence the saturation voltage. I'm not really clear on that. Explain.
Thanks
RE: CT Ratio Test Results Distorted By Connected Cables
If there is a current path, a loop through the primary, as voltage is raised on the secondary to perform saturation tests, as soon as the secondary voltage is sufficient to flow current through the inadvertent primary path, current will flow. On the secondary winding, this will appear as if the core has reached saturation. This value will be noticeably lower than the actual saturation voltage.
That is why saturation tests of installed CT's are important. I have found these unintended paths on installed CT's that have resulted in mis-operation on out-of-zone faults in differential and distance schemes.
old field guy
RE: CT Ratio Test Results Distorted By Connected Cables
RE: CT Ratio Test Results Distorted By Connected Cables
RE: CT Ratio Test Results Distorted By Connected Cables
With the primary current loop in place, the stacked CTs did influence each other. Normally, they should work independently.
As a general rule, I would keep the untested CT shorted for safety.
RE: CT Ratio Test Results Distorted By Connected Cables
Regardless of the question of mutual influence, CTs not under direct test should always be shorted. That's a basic safety precaution.
RE: CT Ratio Test Results Distorted By Connected Cables
Sorry I'm a little late in commenting, but it sounds like the situation at hand may be describead in IEEE C57.13.1, IEEE Guide for Field Testing of Relaying Current Transformers, ¶ 11.3 Intercore Coupling Check.