ETAP Cable Zero Sequence Impedance for L-G faults
ETAP Cable Zero Sequence Impedance for L-G faults
(OP)
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.
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.






RE: ETAP Cable Zero Sequence Impedance for L-G faults
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".
RE: ETAP Cable Zero Sequence Impedance for L-G faults
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.
RE: ETAP Cable Zero Sequence Impedance for L-G faults
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.
RE: ETAP Cable Zero Sequence Impedance for L-G faults
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.
RE: ETAP Cable Zero Sequence Impedance for L-G faults
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.
RE: ETAP Cable Zero Sequence Impedance for L-G faults
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.
RE: ETAP Cable Zero Sequence Impedance for L-G faults
RE: ETAP Cable Zero Sequence Impedance for L-G faults
RE: ETAP Cable Zero Sequence Impedance for L-G faults
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.
RE: ETAP Cable Zero Sequence Impedance for L-G faults
RE: ETAP Cable Zero Sequence Impedance for L-G faults
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.
RE: ETAP Cable Zero Sequence Impedance for L-G faults