connection of dual core current transformer 3

[urjaengrs]

i would like to know the information about current trasformer. I have dual core current transformer with three terminals viz s1, s2 and s3. s1 is common and earthed. Between s1 and s2 i get ratio of 200/1 and between s2 and s3 i get ratio of 400/1. if i would like to use s1 and s2 with ratio of 200/1, what should i do with s3 terminal. is it nescessary toshort s3 and s2? or s3 should be earthed? please guide..

 

[RRaghunath]

Are you sure, it is dual core CT. It has only s1, s2 & s3 and is likely to have only one winding with centre tap. If true, the s3 terminal should be left unconnected when s1-s2 winding only is used in the circuit. This applies to s2 terminal when s1-s3 winding is used.

 

[urjaengrs]

Thanks Raghunath..
it is single core CT only. Can you any backup document which will support your answer. Because I am also of the same opinion but not getting any backup. Please provide if u have.. Once agin thanks a lot...

 

[bacon4life]

If you were to short s2 to s3, there would be no current flowing to your protective devices connected from s1 to s2.

You will also want to have one of the terminals earthed. See IEEE C57.13.3.

 

[slavagar]

Hi urjaengrs.
I'm also recommend connect S1,S2,S3 to terminals and earthed one of them on terminal, not on the CT.
In future:
1. You can change connection to relay ( or meter) easily.
2. You can disconnect earth easily (for megger test).
Earth of CT it is safety requrements.
You must put earth only in one point ( in case of differntial protection is critical).
Earth connection and option for short of CT terminals you must put in first star point of CT connection (near to CT).

 

[urjaengrs]

Thanks to all the memeber who had given their opinion. however, still I am not convinced. Can somebody therotically answer my query? can anyone have literature on the multitap CT secondary connection subject?

 

[scottf]

There is no doubt that you have a single core CT with a tapped secondary. For CTs, the number of cores is also the number of secondary windings, since (99.9999% of the time) each core has only 1 secondary around it. S1,S2,S3 is one tapped winding, with S2 being the center tap. As is said above, connect your device to 2 terminals and leave the other terminal unused. A ground should be applied to 1 point in the secondary circuit.

For a muli-core/winding CT, each winding would be marked with a different letter and there would have to be at least 4 terminals.

 

[JensenDrive]

One thing seems odd. You report:
S1:S2 is 200:1
S2:S3 is 400:1

Note S2 to S3 appears to be the full winding, which implies S1 is the center tap. I would have expected S2 to be the center tap. Or maybe you misreported the matter.

Nothing will be as definitive as going back to the mfr and asking for a document.

 

[scottf]

He probably made a typo, but taps don't 'have' to be center taps, although that's the most common.

In this example, the full winding 'could' be 600:1. We make some CTs like that for certain customers, although it's pretty rare.

 

[urjaengrs]

Thanks all.. there was typographical error. For S1 & S3 ratio is 400/1 and between S2 & s3 ratio is 200/1. however, still the question remains un-answered. Why unused terminal to be left open.. I hope i will get the answer

 

[jghrist]

You leave the unused terminal open because the total ampere-turns in the secondary have to equal the primary ampere-turns.

Assume the CT is a window or bushing type with one primary turn and 400 secondary turns, 200 from S1-S2 and 200 from S2-S3.

Say you have a primary current of 400A (400 ampere-turns)and you are using S1-S2 for a 200/1 ratio. With S2-S3 open, you will get 2A in the S1-S2 circuit, for 2·200=400 ampere-turns.

If you connect something to S2-S3, there will be current in S1-S2 and S2-S3, split in some manner based on internal and external burdens of each circuit. If S2-S3 is shorted (nearly zero burden) and S1-S2 has a burden significantly higher than the internal CT winding resistance, then all of the 400 ampere-turns will occur in the shorted section of the winding. You will get almost 2A (2·200=400 ampere-turns) from S2-S3 and practically no current from S1-S2.

 

[slavag]

Hi Urjaenrs.
I wouls like add to Jghrist explanation some pict.

http://tinypic.com/view.php?pic=6frr41s

Regards.
Slava

 

[lz5pl]

Nice explanation jghrist ! Vote to you.

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