What is the zero seq. impedance of a cable that has a ground fault to a a zero impedance grd plane? 1

[relayman1000]

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?

 

[krisys]

Zero sequence impedance of a cable is an absolute value of impedance offered to the flow of zero sequence current.

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.

 

[relayman1000]

I see your point but then how would the positive and zero sequence impedance differ for a ground fault that has no impednace in the ground return path?

 

[magoo2]

The only difference I see is you would take a three phase voltage set and a CT on each phase to apply to a positive sequence test. Get the 3 phase to neutral voltages and the 3 currents. Z1 would be Va/Ia, Vb/Ib and Vc/Ic. You'll probably get some difference between phases. I think they probably average the 3 results to get a Z1 value.

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.

 

[DiscoP]

Many people get told the zero sequence impedance is the return path for a ground fault because it is easy to understand, but it is not really true.

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.

 

[stevenal]

The Cooper Distribution System Protection book has a section on calculating the sequence impedances of UG cables with concentric neutrals.

 

[relayman1000]

magoo2: When you say "add" the three CTs in the zero Sequence tests do you mean parallel the CT outputs or series them? I see that the external circuit of the cable should not be included in the cable Z0 for just the cable only Z0. It looks like the total circuit zero sequence impedance would be Z0 + 3Zn with Zn being the impedance of the ground mat, cable shield, earth, etc. However, the zero sequence of the cable only would just be the same as the cable Z positive but 3 times higher because all three (I0,I1,I2) currents flow in that one cable to ground. In other words cable Z0 = 3Z1.

 

[magoo2]

When I said to add the CTs, I meant to connect a CT around each phase so that the current can be measured. No need to series or parallel the CTs. You just want to take a reading on the secondary of each CT.

 

[relayman1000]

magoo2: "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."

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??

 

[magoo2]

Let me forget the CTs and just think in terms of the currents that flow in a positive sequence test and then a zero sequence test.

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?

 

[relayman1000]

Magoo2, yes that helps. In fact, based on your previous explaination I had come to the same exact conclusion that the difference between the two impedances if you have a 0.0 impedcance return path is the different mutual coupling between the conductors due to Z1 currents being 120 degrees apart and Z0 currents that are in phase. The impedances would be as:

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.

 

[jghrist]

You don't have a zero impedance return path. Even if there is a plant ground with negligible resistance, there is a reactance in the return because of the distance between the current in the phase conductor and the return through the ground.

 

[relayman1000]

jghrist,
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.