Zero sequence CT 1

[JLSN]

Hi,

We had a trip on a motor due to 50G when we had a fault in other part of the system that has nothing to do with the motor. The system has a low resistance ground system. The zero sequence to the 50G function is provided by a toroidal CT where you run the 3 power cables thru.

Now, looking at the zero sequence CT in the books, I have found two different versioin in how to connect it.:

1.- IEEE-242 recommends when you have shielded cables, the grond shield must go inside the toroidal CT.

2.- In the Power System Relaying book by S. Horowitz & A. Phadke says the the ground to the shiel must go outside the toroidal.

The question to the forum is which one is right?

Thanks!

Jorge

 

[dpc]

They both can be correct. The devil is in the details.

What you want is for the current in the shield to NOT be included in the flux summation. So normally, when the CT is placed over insulated shielded cable, the shield current is cancelled out by running the drain wire back through the window of the CT in the opposite direction. This effectively eliminates the contribution of any shield current to the flux summation.

If the jacket is removed and shield is pulled back before the CT, then the shield conductors and/or drain wires should not pass through the CT opening.

 

[slavag]

Please seee attached.
thread237-211758
It's same, what Dave said , but with attached pict by 7Another4.
Regards.
Slava

 

[davidbeach]

What dpc said. You need to make sure you are measuring exactly what you need to be measuring. Any ground current that goes in through the CT needs to come back out through the CT. Often it is easiest, when using shielded cable, to run the whole cable through the CT then break out the shield in the termination kit and run the shield back through the CT next to the cable. Then connect the shield to ground. When all is said and done you are only measuring the current on the conductor and not the sum of the conductor and the shield.

 

[Marmite]

Another thing to note is that if the cable gland is metallic it has to be insulated from the cable box. I would call it an island gland, I'm not sure what the terminology would be in the US. If the insulation breaks down or is compromised this can affect the correct function of the CT.
Regards
Marmite

 

[oldfieldguy]

If you grab a manual for a GE/Multilin protection relay, they have an excellent picture in there for showing HOW the shield ground lead should be routed. That shows HOW to do it.

This being a community of engineers, we should be able to figure out WHY it needs to be done that way.

I've explained it with simple pictures to a bunch of people. It's a delightful feeling the see the little light bulbs over their heads light up when they understand the simple truth.

old field guy