ETAP Cable Zero Sequence Impedance for L-G faults

[bdfig]

When attempting to validate IEC 60909 L-G fault calculations from ETAP, I find that the fault levels are higher than I calculated. It appears that ETAP is doing

Sqrt(3) c U / |2*Z1 + Z0|,

with Z0 taken directly from the R0 and X0 in the cable library entry, rather than using Z0 = Z0 phase cable + 3*Z0 neutral cable. Is this how ETAP calculates the zero sequence impedance? This made me unsure whether the zero sequence impedance information included in the cable datasheet I have includes the neutral conductor or not. Any help is appreciated.

 

[healyx]

I don't have ETAP, but I am not sure what you describe there makes sense.

Zero sequence for cables is usually a single figure for the combination (Phase and Neutral). You don't really add it up the way you have described. The equation you show would have been used to determine the library figure for Ro. For Xo it is a bit more complex than just arithmetic addition. Zo for cables may also include the return path through "dirt".

 

[healyx]

Sorry, to clarify, the Z0 = Z0 phase cable + 3*Z0 equation is just not right. You don't ever get total Z0 by adding up component Z0 items (which don't exist in practice) like you have.

I don't have the standard on me know, but I think you mean R0 = Rphase + 3*Rneutral...there may be a divide it all be 3 in there too

As I said X0 is harder to calculate.

 

[bdfig]

My understanding was that for a single line to ground fault

If = 3*V / (Z1 + Z2 + Z0 + 3*Zn)

With V the phase line to neutral voltage, Z1, Z2, Z0 the sequence impedances of a phase line, and Zn the zero sequence impedance of the neutral conductor. I guess what I am after is how does the zero sequence resistance and reactance from a cable data sheet relate to the zero sequence impedance in the L-G fault equation.

 

[redfurry]

That equation looks correct for a ph-grnd fault.

As mentioned by others, Z1, Z2 and Z0 are the sequence impedances for the complete conductor group (i.e. phase conductors and any neutral conductor present).

Zn is the impedance of the fault itself (i.e. 0 for a bolted fault). It has nothing to do with the conductors.

 

[dpc]

The zero sequence impedance of cables is not straightforward. In addition to the neutral conductor, if there is one, there is also possibly the cable sheath and another parallel path through the conduit, earth, etc.

The old Westinghouse T&D book has a pretty good section on this. ETAP should be able to explain their methodology and what their Z0 represents.

 

[bdfig]

dpc, when you say ETAP should be able to explain their methodology and what their Z0 represents where do you think I can get this information? I sent an email to ETAP's help desk, but they have not responded.

Also redfurry, I believe Zn includes the zero sequence impedance of the return path, which includes the fault impedance, and the neutral/protective earth line.

 

[jghrist]

redfurry is correct. Z0 includes the impedance of the return path. Zn (actually it would be more correct to use the symbol Zf) is the impedance of the fault.

 

[bdfig]

However, doesn't the 3*Zn come about, because the zero sequence currents are in phase, and all travelling on the return path (so the algebraic trick is to change 3*In*Zn to In*(3*Zn))?

 

[davidbeach]

No, Zn disappears into the equivalent circuit Z0. Zf, as pointed out by jghrist, is the impedance of the fault. Zn, back when it had a separate existence, was in parallel with the cable Z0 and so the presence of a neutral Zn would reduce the overall Z0.

It's been standard procedure for a very long time to evaluate an actual physical circuit, including all quantifiable ground paths, and create an equivalent that can be represented by, and is mathematically interchangeable with, a three conductor equivalent. That equivalent is probably what ETAP (which I've never used) is looking for and thinks it is using.

 

[dpc]

You could call them. I'm sure someone at ETAP can explain their methodology. I don't use ETAP, so it is pointless for me to speculate. You could try changing some of the circuit parameters (neutral conductor, shielding, ground wire, etc) and see how this changes the Z0 that the program comes up with.

 

[veritas]

tmorstyn - by definition Z0 = Z1 + 3ZE where ZE is the impedance of the return path which as mentioned before could be a single conductor of various parallel paths.Zn is not the impedance of the neutral conductor. The latter is contained within Z0 as by the definition above.

Others have correctly pointed out that "Zn" represents the fault impedance and should more appropriately be labelled something like ZF.

Also, and most important Z0 for a cable is quite variable depending on the cable itself as well as the method of installation. Even something simple like earthing the sheath one end or at both ends will affect the value of Z0. I do not often find manufacturers specifying Z0 and my guess is for the reason mentioned above. If they do, they should hopefully clearly stipulate the method of installation.

and yes, calculating Z0 is not easy. For this reason it is preferable to standardise on method of installation resulting in standard Z0 values.

 

[uconalpukat]

I am new user in ETAP software. I hope I can learn any information about ETAP in this forum.