What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
(OP)
What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
In other words, if a cable is in a plant with a very good ground mat so that a ground fault has a zero impedance return path what is the value of a the cable zero sequence impedance? Is it the same as the positive sequence impedance?
In other words, if a cable is in a plant with a very good ground mat so that a ground fault has a zero impedance return path what is the value of a the cable zero sequence impedance? Is it the same as the positive sequence impedance?






RE: What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
is not clear.
Please note that irrespective of whether the ground grid is good or bad, the zero sequence impedance of the cable would remain same.
RE: What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
RE: What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
For zero sequence, you apply a single-phase source to all the conductors at one end, add the 3 CTs and jumper the other ends together with a low impedance connection back to the source and measure V/I again.
With the zero sequence test, the currents and magnetic fields are all in phase. With the positive sequence test, they are not.
RE: What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
Most of my time at work is setting and testing protection relays, so thinking of zero sequence as the return path helps keep my thoughts simple when performing these roles.
As sad as this sounds, sometimes the truth just adds complications you don't need!
As magoo2 said, it is more to do with the magnetic interaction between phases.
RE: What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
RE: What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
RE: What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
RE: What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
Still trying to understand:
Are the "conductors" the actaul cable conductors or the CT leads? Are the "ends" the cable's or the CT's "ends"??
So you have 3 in phase souces connected to the 3 conductors, the other ends of the 3 conductors are connected together. Then you measure the output of the CT's, add the currents, multiply by their ratio and divide the total current into the voltage put in by the sources??
RE: What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
I made a sketch to illustrate.
With a positive sequence test, apply a three phase test set to one end of the cable conductors (a,b,c in left side in my sketch). I showed the neutral as a single conductor grounded at the source end even though it may be 3 concentric neutrals or tape shields, etc. Put a three phase fault connection at the load end and tie it to the neutral. Because voltages are 120 degrees apart, there is no return current. All you get is an impedance from the three V/Z measurements based on the phase impedances.
Now consider a zero sequence test. You only need a single phase voltage source to do this, so I showed it as just phase a. So you get three currents that are in-phase with each other. At the load end where they are all tied together, you connect the common point to the neutral of the cables. So each phase wire sees 1 pu current and the neutral sees 3 pu current. That 3 pu current will actually return between the neutral and the earth.
If your zero impedance ground plane means that you can neglect the neutral impedance, then the zero sequence impedance will be different than the positive sequence impedance strickly because the currents are in-phase where the positive sequence current are 120 degrees apart.
Does this help?
RE: What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
Z0 per phase = 3 x V at 0 degrees/3I0 = V at 0 degrees/I0
Z1 per phase = V at 0 degrees/I = V at 0 degrees/I1
The only difference being that I0 and I1 are not the same because the mutual coupling impedance is not the same value. Thus, the difference in angles of the currents due to mutual coupling differences is the only difference between the Z0 and Z1 values.
Thanks you for your help.
RE: What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
RE: What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane?
Thank you. Yes, as I see it if you assume that there is a return path of 0.0 ohms the only difference between the positice and zero sequence impedances is because of the mutual coupling between 3 phase currents that are 120 degrees out of phase and the different mutual coupling due to the 3 currents being in phase.