Multi Ratio CT Testing Question
Multi Ratio CT Testing Question
(OP)
Greetings-
We have a customer that insists that all ratios of a MR CT be tested. We normally test all of the main taps, ie x1-x5, x2-x5,x3-x5,x4-x5. We are being asked to measure the inter tap ratios (x2-x3, x1-x2, etc). It is my understanding that the inter tap ratios are obtained via calculation.
I can not find any information in the IEEE C57.13.90 (Field Guide for CT Testing), that states if the inter tap ratios are to be calculated or not.
I am wondering if any CT OEM's could chime in, or provide some useful insight on this.
Thanks
We have a customer that insists that all ratios of a MR CT be tested. We normally test all of the main taps, ie x1-x5, x2-x5,x3-x5,x4-x5. We are being asked to measure the inter tap ratios (x2-x3, x1-x2, etc). It is my understanding that the inter tap ratios are obtained via calculation.
I can not find any information in the IEEE C57.13.90 (Field Guide for CT Testing), that states if the inter tap ratios are to be calculated or not.
I am wondering if any CT OEM's could chime in, or provide some useful insight on this.
Thanks






RE: Multi Ratio CT Testing Question
RE: Multi Ratio CT Testing Question
x1-x5:1000/1 (suppose 1000 turn and check it)
X2-x5:800/1 (suppose 800 turn and check it )
x1-x2:200/1 ((1000-800) nothing new to check)
RE: Multi Ratio CT Testing Question
We perform ratio / polarity on main taps and excitation on the full tap. Insulation resistance is performed on the full tap. Burden tests are not in the specification. Winding resistance is not in the specification.
I agree that nothing new is found in the inter-turn taps. We are testing with the voltage method, as the CT's are internal to the power transformer.
My question relates to why these tests are requested (inter turn taps), with the only response being that "that's the way we have always done it". As the utility client is paying the bill, I am looking for some justification one way or the other, as the additional tests add a significant amount of time. We have about 400 CT's per substation and 2 more substations to commission.
Thanks
RE: Multi Ratio CT Testing Question
You can get voltage value to be injected for between whatever turn we want to test.
Atually this voltage testing methode is adopted in turret CT which are inside the Bushing of the tranformers.
During field testing there is no need to perform interturn - ratio test but when you are facing any problem with regular turn ratio like s1-s2, s1-s3, s1-s4 and if you have any problem with s1-s3 then it will help you to make easy identification of faulty winding section.
Hope this will help you.
RE: Multi Ratio CT Testing Question
RE: Multi Ratio CT Testing Question
The question regarding calculation is confusing, since the manufacturer information should show all ten possible ratios. If you don't have the information for some reason, the intended ratios are calculable.
RE: Multi Ratio CT Testing Question
My understanding is that the requirement has more to do with labor rules than an actual technical requirement. For the additional time it takes, I'd rather perform a DC winding resistance test and a burden test and provide the customer with more "bang for the buck"
Additionally, my concerns is that the ratios deviate by more than 5% in the inter tap tests, yet come out right on the money when calculated.
I am interested in what CT manufacturers have to say about how they are tested in the factory.
Thanks
RE: Multi Ratio CT Testing Question
Add them all up, and they will equal X1 to X5, and the proportions will be correct for the reduced winding ratios.
For other combinations, you simply add the voltage measurements, i.e., X2-X4 equals the sum of X2-3 and 3-4. You're treating the secondary winding as an autotransformer.
For that matter, you can apply voltage to any two terminals and read proportional voltages on the other terminals. Using full winding, though, guarantees you don't apply 100 volts on two terminals and end up with a thousand volts across the full winding.
But we're engineers and we're smarter than that, right?
It adds a few minutes to the tests.
It's a good test, too. I've found more than one CT installation where the secondary wiring was incorrectly terminated on the terminal block, and a couple where the internal connections were incorrect.
old field guy
RE: Multi Ratio CT Testing Question
I attended to routine test of current transformer for once. And they tested ct according to nameplate (only highest tap for each core). However if you use inter tap in your scheme you somehow have to test it for your client.
RE: Multi Ratio CT Testing Question
For HV CTs, an inter-core coupling test is also done to check to make sure there isn't a shorting path for the primary winding. This is done by applying a voltage to one secondary winding and checking the induced voltage on a different secondary winding.
Of course, all of the other routing tests called for in the standard are also performed (e.g. PFWV, induced test, etc...).
I'm not sure about your reference to "calculating" the taps. Normally when one uses the term "calculated" in regards to protection CTs with multi-ratio windings, it's in regards to the protection performance at tap. For instance, a 2000:5A MR CT with a C800 rating can be calculated to have a C400 performance at the 1000:5A tap. The "C" in the C800 rating stands for "calculated", which means that the secondary winding is applied in such a way that performance at the lower taps can be calculated based on the ratio proportion. The other type is "T" as in "T800", which stands for "tested" and means that performance at lower taps must be tested to confirm the rating. CTs with a "T" class generally indicate that each winding segment is not distributed about the core, which introduces significant leakage reactance.