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using 2 taps on a multi-ratio CT
3

using 2 taps on a multi-ratio CT

using 2 taps on a multi-ratio CT

(OP)
Will using 2 different taps of a multi-ratio CT work? Will it cause any problems?

RE: using 2 taps on a multi-ratio CT

No and yes. This is why you don't need to short circuit all taps when shorting a CT.

RE: using 2 taps on a multi-ratio CT


Dear Mr BrkfldEE (Electrical)(OP)29 Mar 21 15:31
B. " Will using 2 different taps of a multi-ratio CT work? Will it cause any problems? "
C. I may have mistaken your question. I try to answer as following. e.g. a CT with 200/1 and 100/1 toppings.
1. With present load is < 100A,
a) use 100/1 tapping for any load, depending on the CT Class,
b) at the same time, the 200/1 tap may also be be used for any load, depending on the CT Class; without any problem,
c) when the 200/1 tap is not in use, it SHALL be shorted.
2. With future load >100A but <200A,
a) use 200/1 tapping for any load, depending on the CT Class,
b) the 100/1 tap is not used for any load,
c) when the 100/1 is not in us, it SHALL be shorted.
Che Kuan Yau (Singapore)

RE: using 2 taps on a multi-ratio CT

Wrong.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: using 2 taps on a multi-ratio CT

Quote:

c) when the 200/1 tap is not in use, it SHALL be shorted.
c) when the 100/1 is not in us, it SHALL be shorted.

Wrong. Don't short unused taps.

RE: using 2 taps on a multi-ratio CT

(OP)
stevenal, I am confused by your reply, are you saying that it is OK to use more than one tap on a multi ratio CT?

RE: using 2 taps on a multi-ratio CT

Is this an ANSI MR CT (bushing or donut) or an IEC HV Type CT?

RE: using 2 taps on a multi-ratio CT

You asked 2 questions. Will it work? Will it cause problems? The answers are no and yes.

RE: using 2 taps on a multi-ratio CT

(OP)
The CT is an ANSI Multi-Ratio bushing type CT in a 115kV circuit breaker bushing.

So I am hearing that using 2 taps on a CT at the same time will NOT work correctly.

RE: using 2 taps on a multi-ratio CT

"So I am hearing that using 2 taps on a CT at the same time will NOT work correctly"- You are hearing correctly. Don't do that, it wont work.

You can run multiple relays in series with a CT on a single tap (ex X1-X2 or X1-X5). Multiple relays in series may work out, depending on the settings used and if it's a digital relay. If you put a diff relay in series with something else, put the diff first. It's possible that someone using a Doble with an FT paddle can inadvertently short out the diff relay, if the test paddle is plugged into the Doble first and short out the diff relay (possibly causing a trip), if the relay is in service.

RE: using 2 taps on a multi-ratio CT

Yes it will. Your putting your ct secondary in parallel. With 2 loads the current will be split and unevenly divided between the loads. Only 1 secondary per ct or put them in series.

RE: using 2 taps on a multi-ratio CT

You WILL have two conductors connected to the CT. This is NOT the same as using two taps. You choose the two taps that give you the ratio suitable for your application. All other taps are left floating.

old field guy

RE: using 2 taps on a multi-ratio CT

I'm not an expert on field wiring of CT's, but it seems to be if I have 5 taps let's say X1,X2,X3,X4,X5 then the every winding segment between taps should be included in one (and only one) external circuit loop or short. (Segment X1-X2 needs to be included in one and only one external circuit or short; Segment X2-X3 needs to be included in one and only one external circuit or short; etc).

Applying this rule:

If I feed an external circuit loop by connecting to X1 and X3, then I need to short from X3 to X5 so that no segment of the CT is open-circuited.

If you need to add more devices typically they would all be in series in that one external circuit X1-X3. In theory if the burden is getting too high on that circuit I guess you could feed another external circuit from X3 to X5 assuming your calculate the ratio correctly. You could not add another external circuit connected between X1 and X4 because then the segments X1-X2 and X2-X3 would be shared among multiple external circuits.

That's my take, I apologize if I'm repeating what others more knowledgeable than me have said. If I said something wrong please feel free to correct me, then I'd learn something.

=====================================
(2B)+(2B)' ?

RE: using 2 taps on a multi-ratio CT

Nope, two wires as stated by oldfieldguy. Even that means the two wires are connected to X2 and X3 with X1 and X4 & X5 floating. The taps work by because all of the amp*turns of the secondary occur between those two connection points. Fewer turns means more amps. If there are other possibilities for amp*turns on the secondary none of the currents will have the correct ratio and the actual ratios will be determined by how the burdens relate.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: using 2 taps on a multi-ratio CT

ok thanks David. You're right, I wasn't thinking about it correctly, wasn't thinking about the amp-turns balancing as the key aspect. It makes sense now that I think about it that way. (I think I knew that once upon a time but I got a little rusty somewhere along the way).

=====================================
(2B)+(2B)' ?

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