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Associate

Join Date: Mar 2013
Posts: 31

Interposing Current Transformer

09/09/2013 7:51 AM

The existing CT ratio is 400/1 and earth fault current is limited to 100A, now I want to set the relay to operate for 10A i.e. 10% of the value of earth fault current. Plug Setting range of relay is 0.1 to 0.4. If I set it 0.1 then relay will operate for 40A. Somebody suggested me to have ICT. Now my query is whether to have ICT of 1/4 ratio or 4/1 ratio?

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#1

Re: Interposing Current Transformer

09/09/2013 9:20 AM

Why is CR4 involved at all? Why can't the original equipment manufacturers provide advice over the telephone directly, for instance? Why can't the information be obtained directly from the <...somebody...> who offered the suggestion?

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Commentator

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#2

Re: Interposing Current Transformer

09/09/2013 9:47 AM

"ICT of 1/4 ratio or 4/1 ratio?"

Isn't that the same thing, depending how you wire it?

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rogerggbr
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#3

Re: Interposing Current Transformer

09/09/2013 6:48 PM

In theory, 1:4 and 4:1 ratio are the same.

In the real world, that is not enough, it is very important you make it clear which winding is rated to carry the 1 amp from the existing CT secondary.

That is why one usually writes, for example, "400 to 1 amp"which clarifies the secondary continuous current rating.

A 400:1 ratio CT could have a secondary rated for 1 amp or 5 amp to common practice. Plug setting ranges for relays are given in decimals of the relay nominal current rating (which could be 5 amp or 1 amp).

Only your statement "If I set it 0.1 then relay will operate for 40A." gives a clue that you have a 400 to 1 amp CT system.

You need to ask for a "1 amp to 4 amp" ICT.

But since you are down to a relay setting of 1/40 of primary current, the magnetising current of the CT is probably NOT negligible, so a 1 amp to 5 amp ICT (which is probably an "on the shelf" item) may be right.

I expect you know that the ICT must be placed as close as possible to the relay, not at the CT end of the system. cabling I2R loss at 5amps will be 25 times that at 1 amp!

To do the job thoroughly, you need to estimate the CT secondary voltage V needed to provide the voltage dropped by the CT cables, ICT and the relay (impedance R + jX)at the relay operate current, then test the CT secondary at V (or more) to find the actual magnetising current which can be compared with the relay setting to find the error. The actual primary current to operate the relay corresponds to the relay setting (secondary current) plus the CT secondary magnetising current.

67model

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#4

Re: Interposing Current Transformer

09/09/2013 10:36 PM

In case it is ring type CT and your earth conductor is not too thick (max. earth fault current = 100A) and is flexible enough to loop around the ring, loop primary conductor twice the ratio becomes 200:1 and once more it will become 133:1. May serve your purpose.

Otherwise pick up a ring type Class M1 transformer of 5:1 ratio (or 10:1 ratio and loop twice the primary current), what ever avaiable of the shelf use it and slove your problem.

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Anonymous Poster #1
#5

Re: Interposing Current Transformer

09/10/2013 10:46 AM

1)It looks like residual connection.ie 3 line cts and ef element connected in the neutral.

2)A lot depends on the accuracy of the line cts and the design of the ef element. If the e/f element is of. biased type then a ict of 1/4 or better 1/5A may be selected .Actual ef setting is generally more than that calculated.

3)There are many alternatives and optimising will require detailed checking.

4)If possible better provide a suitable sensitive ef element with the existing line cts.

5)Without detailed study all the suggestions are just guidelines. Better not take protection related

decisions casually.

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