Zero sequence CT
Zero sequence CT
(OP)
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
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






RE: Zero sequence CT
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.
RE: Zero sequence CT
thread237-211758: earth fault protection
It's same, what Dave said , but with attached pict by 7Another4.
Regards.
Slava
RE: Zero sequence CT
RE: Zero sequence CT
Regards
Marmite
RE: Zero sequence CT
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