CT Currents Splitting Between Phases in Relay

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vijaymail_electrical Commentator Join Date: Jan 2011 Location: India Posts: 98 Good Answers: 2	CT Currents Splitting Between Phases in Relay 07/09/2020 8:01 AM
	Dear Engineers, We were testing Siemens 7SJ66 relay for measurement circuit verification by primary injection. We were using ISA T3000 current injection kit. Attached is the scheme of how CT wiring is done. While we inject current in Y phase CT like 300A, we measure in IL2 and Io as 300A in relay display. But while we inject same 300A in R phase, we see as below. Same is observed when injected in B phase with only difference is IL1 is 0A and IL3 will be 300A IL1: 250A IL2:50A IL3:0A, Io:300A I1:250A I2:50A We checked all wiring tightness, CT secondary winding resistance, all found OK. But when we try same current injection from secondary injection kit (omicron CMC256+) directly to relay terminals by injecting 5A, we don't see this issue. We even tried with new wirings from CT to relay terminals but same issue repeats. Any one can help us out why current is splitting in Y phase? Thanks, Vijay __________________ You don't understand anything until you learn it more than one way.
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PWSlack Guru Join Date: Jan 2007 Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on. Posts: 32175 Good Answers: 839	#1 Re: CT Currents Splitting Between Phases in Relay 07/09/2020 11:19 AM
	One might be tempted to look for a phase-to-neutral connection in the downstream power wiring, and eliminate it when found. __________________ "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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Rixter Guru Join Date: Apr 2010 Location: About 4000 miles from the center of the earth (+/-100 mi) Posts: 9910 Good Answers: 1141	#2 Re: CT Currents Splitting Between Phases in Relay 07/09/2020 3:32 PM
	Here is what I think is happening: When you inject current with the ISA T3000 current injection kit, the CT under test actually has 3 windings, your single wire injection winding (primary) and two secondary windings, the CT winding going to the relay and the single winding that is the bus phase. If the bus phase is open, no current will be induced into that "winding", and the output to the relay winding will give the correct reading (300A). If the phase is connected to a load, there is a complete circuit and the induced flux from your test set drives current in this circuit instead of the current in the winding to the relay. Bottom line is you have a connection (either load or short) between Blue and Red.
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vijaymail_electrical Commentator Join Date: Jan 2011 Location: India Posts: 98 Good Answers: 2	#3 In reply to #2 Re: CT Currents Splitting Between Phases in Relay 07/22/2020 8:12 AM
	I checked same current injection by laying new cables from CT secondary to relay terminals and still the same issue happens. This CT has only 2 secondaries, metering and protection cores. __________________ You don't understand anything until you learn it more than one way.
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PWSlack Guru Join Date: Jan 2007 Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on. Posts: 32175 Good Answers: 839	#4 In reply to #3 Re: CT Currents Splitting Between Phases in Relay 07/22/2020 8:22 AM
	...then the cause of the problem has not yet become apparent. There's only one thing to be done: Keep looking. __________________ "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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vijaymail_electrical Commentator Join Date: Jan 2011 Location: India Posts: 98 Good Answers: 2	#5 In reply to #4 Re: CT Currents Splitting Between Phases in Relay 07/23/2020 4:01 AM
	The subject feeder is now loaded and all currents are balanced. It is a mystery why the CT behaved during our testing. Anyways thanks for your help. Will keep you posted if anything comes up in my mind regarding this. __________________ You don't understand anything until you learn it more than one way.
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