Open Circuited Secondary Current Transformer

It is simple knowledge that if secondary of CT when not connected with a meter must be shorted other wise it may develop very high voltage and and is dangerous . So check up your wiring throughly to short circuit the non connected CTs. For a better consuptual knowledge about the subject read the link below.

http://www.eng-tips.com/faqs.cfm?fid=1189

My question is why didnt all six of the disconected CTs burn up when open circuited, they were in service for roughly 12 months? we have already found the wiring mistake and corrected it.

Luck.

Come on guru that's not much of an answer. There has to be someone out there smart enough to give me some kind of logical answer. These generators have all run for about 12 months now, each generator with one CT open circuited. The gens are all connected to a common 11KV bus, and load share equally. We have had three CTs blow in the past 3 months (prior to finding the incorrect wiring). First failure was on gen #6 while on transit departing the shipyard, we had it replaced under warrantee in Singapore. Few months later, we finaly departed Singapore and while on transit once again gen #6, same CT. Week later, still on transit gen #2.

With the secondary winding open on a CT we know that extremely high voltages will be produced. Would the CT be in constant saturation too? Why didn't all of the opened CTs fail? And why didn't they fail sooner?

It seems that the CTs are only failing while under transit, this is when the generators are loaded the greatest. The thing is they all share the same load.

Though the other CTs haven't visibly failed yet, I guess the insulation must have considerably deteriorated to cause trouble any time.

Hence, better to remove /replace the other three CTs also so as to minimise the problem while generators are running / ship is in transit.

For your question why these haven't failed yet, the answer can only be that - though the insulation is identical in all the CTs, there is bound to be some inherent differences in failure pattern due to many factors that are internal to CT manufacture as well as ambient conditions. Most likely, it is a matter of a few more days that these CTs to showup visible damage.

The open circuited CTs would have certainly failed instantaneously in the event of occurence of any current based faults.Perhaps such faults have not occured in your case.

The voltages developed across the open circuited CTs during the normal operation were perhaps not enough to burst them instantaneously.However, the stress due to higher voltages have led to gradual deterioration of the insulation.Hence,the delay in failure of some of the CTs.The other ones,which have survived,must have lost considerable part of their life span.

Out of interest have you shorted the other CT's yet? I would have thought it would be a matter of high priority to you to prevent further failures.

I wouldn't like to be on a ship knowing there's a fault lurking about waiting to bite me.

I'm a bit confused here: why didn't anyone notice that none of the metres were registering any current?

Or have I got the wrong end of the stick? (again)

Quite.

Good question Johny451, if they weren't connected how did we know that they had failed?

The reason we didn't pick up on the CT not outputting was because it wasn't used for metering purposes. There are three on each generator. One around each phase, one leg of each of the three CTs was tied together to create a star configuration. The remaining three legs are rectified and sent to the AVR as a Booster to help produce the excitation current. With the missing phase the AVR would have still received a DC to help with the excitation but it would have been messy DC.

When the CT burnt up it would also burn up our AVR, I'm not sure exactly how this would happen either? If anyone has ideas on that would love to hear them too.

Sorry about the small picts its the best I could paste them into this text box.

Adam, well Most of our friends have clarified that what might be the reason, why other CTs were not failed.

As per Mr. Raghun, It would be better to replace all other CTs to reduce interruption and failure in future. It would be possible that they may fail one after another and put your system in nuisance.

This is in reply to your secondary question about the AVR damage.

A CT failure doesn't always mean that the secondary becomes involved directly with the primary, simply that it develops an extremely high secondary voltage that damages the CT. If the CT faults in a way that connects the secondary with the primary however, your AVR will see the primary phase directly. Whereas it had been receiving relatively low DC secondary current rectified from a secondary side varying voltage of approximately 0 - 12 volts AC, I'd think that a sudden connection directly to the primary voltage would take out the rectification and hit the AVR with AC line-ground fault-value currents.