Current Transformer

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Electronaut Commentator Join Date: Dec 2009 Location: Mangalore, India Posts: 65 Good Answers: 4	Current Transformer 01/25/2010 10:48 PM
	The term Idyn (Rated dynamic current) and Ith (Rated short time thermal current) are specified on a CT that confirms to IEC Standards. IEC defines Ith as the current that can be withstood by the CT for 1s without getting damaged and Idyn as peak current that can be withstood by the CT without getting damaged, both with secondary short circuited. What is the significance and difference between these two specifications? Do these terms enter into any of the calculations involving CT's.
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Mitsurati Commentator Join Date: Mar 2009 Location: Southern California Posts: 97 Good Answers: 15	#1 Re: Current Transformer 01/25/2010 11:43 PM
	If you have a breaker protecting the CT that has an "LSI" trip, the instantaneous trip would be set to a current somewhat lower than the Idyn, and the short-time trip would be set to a current somewhat lower than the Ith. Generally the breaker protecting a CT is not going to be that sophisticated, though. Usually it's a standard thermal-magnetic breaker with a trip rating somewhat lower than Ith; that way even in the worst case you'll never get current greater than Idyn through the CT...at least, if the breaker trips fast enough.
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shanbhag Commentator Join Date: Dec 2009 Location: Bangalore,India Posts: 80 Good Answers: 1	#2 Re: Current Transformer 01/27/2010 1:35 AM
	Dear Electronaut, Normally, if you check in any electrical machines like motors, transformers or genertors, Ith is thermal rated current. It is the current allowed for the max temperature rise. the ratings of machines which produces heat due to iron laminations are decided by this thermal factor. In case of CT, since the operation depends upon the primary line current(and it doesn't drive any load, saturation of the core is a critical factor. so it is important to understand that the current doesn't cross above Idyn which can saturate(may be permanently) the core. The breaker should clear the fault before this pea current can be reached. Dear All, Please give your good advice on the above and correct if the above statement is not correct. Regards
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kvsridhar Guru Join Date: Sep 2009 Location: Temporarily at Ashburn, VA Posts: 2744 Good Answers: 164	#3 Re: Current Transformer 01/27/2010 4:03 AM
	As Shanbhag says Ith is the thermal current that any equipment including the CT should be capable of carrying indefinitely. So no problem there. The peak Short-circuit current can mechanically damage the CT. Hence i suppose it makes sense to specify the max peak that the CT can withstand, Idyn. Any short circuit current will almost certainly drive the CT into saturation, which will not have detrimental effect as long as the core material is made from good silicon steel i guess. __________________ Nothing worthwhile can ever be taught, it can only be learnt.
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kvsridhar (1); Mitsurati (1); shanbhag (1)

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