negative and zero sequence resistance

[sabrepoint]

Hi All
On HV overhead line and underground cabling systems, how do the negative and zero sequence RESISTANCES differ from the positive. Can they be assumed to be similar? I don't have a problem with the reactances ... just need to know more re the resistances.
Any help anyone?
Thanks
Graeme

 

[jbartos]

Suggestion: Actually, the negative sequence impedances, positive sequence impedances and zero sequence impedances are supposed to be cited. If you are missing the resistance portion of the positive, negative or zero impedance, chances are that it has been neglected.

 

[jghrist]

The negative-sequence resistance of lines is the same as the positive-sequence. The zero-sequence resistance includes the resistance of earth and ground wire return paths.

Are you sure you know about zero-sequence reactance? I can't imagine someone knowing all about zero-sequence reactance without knowing that the zero-sequence resistance differs from the positive-sequence resistance. Anyway, the zero-sequence reactance also includes the effect of the earth and ground wire return paths.

Carson's equations, as given in the Westinghouse T&D Reference Manual are used to calculate the resistance and reactance values.

 

[sabrepoint]

Well, I was only trying to ensure the responses related to resistance; not reactance. But in any case your second comment "zero-sequence resistance includes earth/ground" confuses me a bit as I take the total line-earth fault impedance to be Z1+Z2+Z0+3Zf+3Zn, where Zn is the impedance of the wire return path you refer to - not in Z0. Can you comment further? Thanks.

 

[jghrist]

The impedance of the wire return path is part of Z0, so there should be no 3Zn in the total line-earth fault impedance. From Westinghouse T&D Manual:

Z0=Z0(a) - Z0²(ag)/Z0(g)

where Z0(a) = zero-seq self impedance of the 3ø circuit.
Z0(g) = zero-seq self impedance of the ground wires.
Z0(ag) = zero-seq mutual impedance between the 3ø circuit as one group of conductors and the ground wire(s) as the other conductor group.

Z0(g) and Z0(ag) include the effect of earth return.

 

[jbartos]

Suggestion: The medium voltage and up power distribution systems are often considered with reactances only. The resistances are neglected. This approach includes the symmetrical components resistances.

 

[sabrepoint]

Thanks for all the help. Yes, jbartos, most texts I've recently seen talk in terms of impedances seemingly being equiv to reactances. I take your point but just wanted to get more of a handle on the ZPS resistance. Thanks again.

 

[jbartos]

Suggestion: Since the resistance is a part of impedance, i.e. Z=R+jX, it assumes the same treatment as Z or X, no matter in which form it is.