Multi-tap CTs - Should the unused CT taps be shorted or not?

Multi-tap CTs - Should the unused CT taps be shorted or not?

by: K. Sivakumar, Head-Technical, Megawin Switchgear -P- Ltd., Salem

The purpose of any business is to earn profits. And, when one earns good profits in a business, one would, sure, want to expand one's business. When such an expansion happens in a production environment, the electrical load demand also would increase. Thus, the CT ratios - for protection and for metering - would also increase. One would have opted for a lower ratio CT - say, for example 500/5A, in an unexpanded production requirement. But, when the production capacity is doubled, the CT ratio may have to be changed from the earlier 500/5A to 1000/5A, say. So, the earlier 500/5A CT has to be disposed and replaced with a new 1000/5A. This would be necessary across the feeders in the network. This is uneconomical. So, to cater to such requirements, dual ratio (or, even multi ratio) CTs were introduced.

In the above example, if one had gone for a 1000-500/5A CT, initially, when the load is less, one would select the 500/5A tap, to cater to the initial load demand. And, when the production capacity is doubled, one need not replace the CT, but only to change the CT tap from 500/5A to 1000/5A. This would save cost & time.

Now, if one uses such a dual ratio CT of say, 1000-500/5A, with the secondary terminals S1-S2-S3. one would get a ratio of 500/5A, when one selects taps S1-S2 and 1000/5A, when the selected tap is S1-S3.

Here, when one selects taps S1-S2, the winding portion between S2-S3 is open.

Should this open CT secondary be shorted or should it be left open?

Many feel that this untapped portion shall be shorted, on the much-dreaded assumption that an open CT secondary would produce a dangerously large voltage, which could damage the CT itself, apart from causing other catastrophic effects on the installation.

But one need not fear about any open circuit high voltage in the secondary as the Ampere-turns (AT) balance would be achieved in the lower ratio taps itself, in the given situation. When such an AT balance in achieved, there would not be any stray flux to induce high voltages in the CT Secondary. Thus, any apprehension that such an open circuit would damage the CT is totally unwarranted.

Alternately, if such a shorting is effected, it would cause unacceptable current division and, thus, errors in measurement and protection. The intended protection would be jeopardized.

To quote from “Design of Electrical Systems for Large Projects” by Mr. N. Balasubramanian, (Published by NBS Consultancy, Chennai):

"In a secondary tapped, bar primary 1000-500/1 CT, if S1, S2, S3 are the secondary terminals at 1st, 500th, 1000th turn respectively, then the ratio of 1000/1 is obtained between terminals S1 & S3 and the ratio of 500/1 is obtained between terminals S1 & S2, with no short between S2 & S3.

If there is an inadvertent short between S2 & S3, then the required Ampere-turns of 500/1 tap may be distributed between the two windings S1S2 & S2S3. Since S2S3 is shorted, while there are relays across S1S2, the shorted section S2S3 may contribute more (say, 300AT), while the section of the winding in service (i.e.) S1S2 may contribute only 200AT (carrying 0.4A, instead of the intended 1.0A), causing an error of 60%. The remaining 0.6A (corresponding to 300AT) will circulate in the 500 turns of the shorted section S2S3".

Conclusion: In a dual ratio or multi ratio CT, the unused CT taps MUST NOT be shorted.