Connection of current transformers in star/delta question 3

[WHiPCPL]

Hello

I've recently been reading about current transformers and their application in differential protection.

From what I can understand if you're protecting a star connection, you have to connect the current transformers in delta and vice versa. What I don't understand is why this is the case and I can't seem to find a proper answer on google.

What I have been able to figure out is that it's to prevent zero sequence currents from appearing, but what I don't understand is why this is desirable.

Can someone help me shed some light on this?

 

[FreddyNurk]

As you noted, its only relevant for differential protection for a transformer.

The reason, as I understand it, is to account for the phase shift that occurs for a star - delta arrangement. You'll get a 30° phase shift across the transformer, arranging the CTs the opposite way negates this. Of course, all other considerations regarding orientation of CTs are still relevant.

EDMS Australia

 

[RRaghunath]

On Delta side, the zero sequence current circulate within Delta and thus don't appear in the lines. If the zero sequence currents are not eliminated on the Star side (by connecting CTs in Delta), there is risk of Relay maloperation.

Rompicherla Raghunath

 

[davidbeach]

Except that was only necessary “back in the day”. Any decent modern transformer differential relay should have all of the CTs connected in wye; the relay can do all of the phase shifting and zero sequence elimination internally. The relay can also provide REF protection for the wye side if, and only if, the CTs are connected in wye.

 

[HamburgerHelper]

It has to do with reversing the phase angle change between the delta and the wye. The old mechanical relays summed up the currents and if the sum exceeded a threshold, the relay operated. New microprocessor relays allow you to compensate for the phase shift between the primary and secondary side so you don't need current transformers connected in delta.

Look at the link below and it becomes obvious why delta connect CTs were needed.

https://encrypted-tbn0.gstatic.com/...io2T2Xh8/s1600/power+system+protection+27.png

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[marks1080]

What you're talking about is only necessary for electro-mechanical set ups. If you're using modern IEDs all your secondary circuits should be WYE.

 

[WHiPCPL]

Thank you all for your inputs, they've been very helpful.

 

[edison123]

marks

Any particular reason why in modern IEDs all your secondary circuits should be WYE?

Muthu

http://www.edison.co.in

 

[davidbeach]

Because if they’re not in wye you’re throwing away valuable data. If you have the data from a wye connected set of CTs you know what was happening. If you have the data from a delta connected set of CTs there’s an infinite set of possible phase currents that could be represented by that event report. Why handicap yourself for no gain?

 

[edison123]

Thanks David. So, it's a better than option rather than a must?

Muthu

http://www.edison.co.in

 

[davidbeach]

I suppose one could say it that way, but I can't imagine a new delta connection. We've got two transformers that have CTs in delta connected to modern relays and they'll get rewired this year or next. Delta CTs can't be used to meet the PRC-002 requirements for phase currents but wye connected can.

 

[marks1080]

Thanks david.

Having all secondary circuits WYE for modern IEDs technically isn't 'necessary,' but it is expected. I would throw away any modern design using delta secondaries, and the IED manufacturers build in a lot of the relays functionality dependent on having a wired neutral. So you could build a modern delta secondary, but you would be throwing away functionality without gaining any type of savings.

 

[edison123]

Thanks, david and marks, for clearing that up for me.

Muthu

http://www.edison.co.in

 

[waross]

I think David's post bears repeating;

If you have the data from a delta connected set of CTs there’s an infinite set of possible phase currents that could be represented by that event report. Why handicap yourself for no gain?

Bill
--------------------
"Why not the best?"
Jimmy Carter