Question about: IEEE C57.13.1-2006, clause: 16.3 Inter-core coupling check

[JimmyXWang]

thread238-283362 talked about CT inter-core coupling which is special issue in IEEE C57.13.1-2006 clause 16.3, and Mr. lindsayspeters (Electrical) at 7 Nov 10 21:33 had replying: This problem may be due to the core problem because of the air-gap. There is some leakage at the gap. The leakage flux will induce a voltage around windings. If the gap is the cause, the induced voltage can be eliminated by rotating the CT.

My querstion:
1. Does IEEE C57.13.1-2006 considered air-gap CT situation?
2. Is it worthy to check air-gap CT inter-core coupling situation on site? Or manufacture should have done it in the shop to provide data for site re-verification if required?
3. How to quantify the testing about air-gap CT inter-core coupling situation, instead of using voltage to verify according IEEE C57.13.1-2006, since we alway usually get high volatge due to the air-gap CT inter-core coupling.
4. We receive the the CT whatever bushing CT or other types as a whole entity. (1). How can we or (2). do we need to fix it if inter-coupling happens without manaufacture involed?

We have done this test and proposed alternative mehtod & cretiea for verifying inter-core coupling isuse, but here I want to hear the opions from guys have experienced issue and manufacture guys like Mr. Scottf and Mr.Randyman. Thanks.

Appedix: IEEE C57.13.1-2006, clause: 16.3 Inter-core coupling check:
In many cases, such as circuit breaker bushings and separately mounted extra-high-voltage CTs, several secondary cores are mounted in close proximity on the same primary lead. It is possible to have coupling between these cores that may not appear as a short-circuited turn in the excitation test, Clause 12, but which can cause a detectable imbalance in a bus differential relay circuit. Inter-core coupling occurs when a spurious metallic conducting path is established that encircles more than one CT. It may not be detectable with the excitation test if enough resistance is present in the conducting path.
Inter-core coupling will occur if one of the following conditions is present:
a) If the CT support is in contact with the bushing ground sleeve, making a single turn conducting path around the bushing CT.
b) If a surge protector across the H1–H2 terminals of an oil-filled CT is short circuited or if the H2 insulation fails.
c) If the insulation of grading shields surrounding the cores of an SF6-filled CT fails.
d) If the insulation on the metal support for the primary insulation on an oil-filled CT fails and establishes a conducting path through the support.
To determine if there is coupling between cores, the excitation test should be repeated, and the voltage across the full winding on each of the adjacent cores should be measured one at a time with all other current-transformer secondary windings shorted. A high-impedance voltmeter (20 000 [1]/V or greater) will read less than 1 V or 2 V if there is no inter-core coupling. If there is coupling, the voltage will be substantially higher.

 

[randyman]

Hello JimmyXWang,

Let me first comment on the referenced thread from Nov 10, 2010 - It looks as if the OP was not performing the test correctly and may not have been using a high-Z voltmeter like C57.13.1 suggests, but who really knows as there was very little discussion on the method. As for the replies from lindsaypeters, I don't agree with them, especially the air gap statement. What air-gap is he refering to anyway?? Most C class CTs are non-gapped cores and even if it did have a remanence controlling gap in the core, it would not cause "inter-core coupling" as defined in C57.13.1.

Now, let's talk about inter-core coupling - in C57.13.1 is says that "it occurs when spurious metallic conducting path is established that encircles more than one CT". I have seen this happen many times in both power transformers and HV power breakers. Every case it is due to shifting of a mounting plate or some external object connected close to the BCT housings. In every case it affected every CT on that bushing as would be expected because of the introduction of a low or high Z artificial primary loop that parallels the main primary conductor. I am not sure that i agree with the term "inter-core coupling" in this scenario.

All that being said I would like to understand more about your specific problem. Are you saying that in the case of gapped-core CTs on the stack, you are performing test per C57.13.1 section 16.3 and encountering high voltage readings on an adjacent CT (gapped or non-gapped) while exciting the other?? Can you elaborate more on your procedure and findings and provide some details as to the ratings of the CTs in question so I can gain a better understanding of what you are seeing.

I do agree that the test procedure in C57.13.1 for detecting this is a bit lame. In my opinion IF I were to use this test, it would be as a troubleshooting method ONLY if suspicious results were obtained during excitation test and turns check on ALL CTs on same bushing AND I couldn't visually see the shorted external loop.

If "inter-core coupling" were found to be the case then the equipment OEM should be able to correct/repair in field without involving the CT manufacturer.

You are proposing alternate method, are you making a formal proposal to the IEEE working group of that standard?? I would be interested in learning more about your findings and what is driving this requirement.

 

[scottf]

Jimmy-

Your method described is exactly how we conducted factory inter-core testing for HV CTs.

It's common to have multiple gapped-cores (like TPY, etc...) in an HV CT and I've never heard of any issues of those gapped-cores impacting the inter-core testing.

 

[randyman]

Greetings Scottf,

The words Jimmy used to describe the test are verbatim as in C57.13.1, in fact it is entire 16.3 as published. My question to you is in the test itself. At what levels of induction do you take the CT while performing this "inter-core coupling test"?? The standard doesn't say, only to repeat the excitation test. So do you pick an induction level arbitrarily?? And I could see where in CT assemblies such as used in HVCTs this would be a needed factory test since it is a quite bit different than BCT assemblies on breakers and transformers.

Did you read the thread 328-283362?? Just curious what your thoughts are on the reply comments, especially regarding air-gaps and non-distributed turns inducing high voltages??

 

[scottf]

Hi Randy-

For the factory testing of HV CTs, the purpose was to make sure there wasn't a connected loop in the grading layer (conductive layer around the outside of the head of the insulation system) or an improperly connected multiple-turn primary winding. If memory serves we didn't apply that high of a voltage to the secondaries during this test. We didn't do it in conjunction with any excitation testing (as many of the units had metering accuracy only so there was no production excitation test). I'm thinking we did the testing at on 20-30V or so. A lot of the units we were building had nano/nickel cores and had a pretty low knee-point voltage.

With the 2 scenarios described above, I think induced voltage would show up on the other cores with only 20-30V applied. I'm on my phone now, but if I get a moment, I'll try to read the past thread.

 

[JimmyXWang]

Parasitic circuits described in "IEEE C57.13.1-2006, clause: 16.3 Inter-core coupling check" is not my concern. My question is reagrding to air-gap CT situation, since the air gap reduces the magnetic permeability of the flux path of the core, and the leakage flux increases accordingly. The leakage flux will induce a voltage on an adjacent CT winding if two CTs are mounted in close proximity on the same primary lead. The voltage induced by the flux from other CT is also known as inter-core coupling voltage.
We have measured the inter-core coupling voltage of bushing CBs at several 230kV stations, the voltages have never been found to be this low@1-2V level. The induced voltages are normally around at 30V when a 900V voltage was applied across the secondary winding of a CT. For some CTs the induced voltage has increased to as high as 42V, which stands for a 4.67% of the saturation voltage of a CT: the level had exceeded quite much.
Our further anaylis indicates that the inter-core coupling effect between two adjacent CTs is negligible to the Secondary Current of CT, therefore, there is no adverse impact on protection systems. And Clause 16.3 of IEEE C57.13.1-2006 standard is not applicable to air-gap CT.
Go back to my questions at the begining of the thread, please give me some hint/clue from the point of views of manufacture.