Motor Differential CTs 2

[THD]

I need to spec differential CTs for a 3400kW Induction motor. I wish to use 3CTs instead of 6 (by running the phase and neutral through one CT. The FLC is 204A. The CT supplier is asking for a detailed spec before they make the CTs to order. They need knee point voltage as well as volts per turn. Accuracy class is ClassX - or the equivalant. The fault level is 8kA for 3ph and 3kA for 1ph fault.

Kevin Bosch
Rainbow Technologies

 

[ScottyUK]

CT performance requirement is usually dictated by the protection relay you're intending to use. Which brings the next question... what relay?


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If we learn from our mistakes I'm getting a great education!

 

[THD]

Its a Sepam M87 relay (it has current to voltage plug in transducers at the CT input)

Kevin Bosch
Rainbow Technologies

 

[davidbeach]

Since the CTs will never see load current, you can get very good performance from otherwise pretty poor CTs.

So much easier to call out the desired C rating and not worry about all those other parameters that go along with the rating; I'd say that even a C200 would generally do quite well in that application (I don't know that relay though).

 

[fernandoelec]

Do you want protect the motor against internal faul or only measuremet?

If you want the diferential protection, the best is the use of 6 CTs

Fernandoelec

 

[jyj982001]

I was confusedby the frase " using 3CTs instead of 6 " .
How can you get the differential current?

 

[davidbeach]

3 CT motor differential is done by having the terminal end of a phase go through the CT in one direction and the neutral end of the same phase go through the same CT in the opposite direction. Under normal conditions or external fault conditions the two currents are equal and opposite so the CT sees zero amps. Under internal fault conditions there will be a difference between the two currents and a simple overcurrent relay can trip on the difference current.

 

[edison123]

David - Is that 3-CT differential set up being actually used anywhere ? It seems to be a cool concept.

Muthu

http://www.edison.co.in

 

[oldfieldguy]

edison123-

I hear it called "core balance", and I find it to be VERY common here on the US Gulf Coast. You do need to specify a six-lead motor, so that you have access to both the line and neutral ends of each winding. As David said, both ends of a phase's winding go through a single CT.

You can see an example in the manual for the GE Multilin's SR469 manual under "differential current inputs". There's a picture, too.

Another cool part of the concept is that you don't have to worry about two CT's having different saturation characteristics.

old field guy

 

[edison123]

Thanks ofg. I wonder why this is not so popular in my part of the world. As you say, one need not worry about matching CT's and as David says any run-of-the-mill CT would work. Of course, one set of leads have to be flexible and long enough to enter the CT's from the opposite direction.

Muthu

http://www.edison.co.in

 

[davidbeach]

I've never personally seen it in use, and obviously it can also be used for generators. If you can make it work, then it is the best method, but large leads present a problem. I think I've even seen illustrations of windings designed for that use where they wind up right next to each other in opposite directions for the lead connections.

 

[edison123]

The large leads do present a problem. It has to be a donut CT (bar primary CT is out). The ID of donut has to be bigger to accomodate both the phase and neutral leads/busbars. Busbars have to be insulated for the phase to neutral voltage.

And yet this is seems to be the best method. Wish I had thought of this idea.


Muthu

http://www.edison.co.in