+, - & 0 impedance of Power Cables?

[rail1996]

Please can somebody tell me if there are usually any differences between the positive/negative and zero sequence impendances for HV power cables?

I need to carry out a short circuit analysis for a HV network and the cable data I am provided with are the 'Maximum a.c resistance of conductor ohm/m', 'Maximum d.c. resistance of conductor ohm/m' and 'Equivalent star reactance of 3-phase circuit ohm/m'. Are these sufficient to determine the +ve, -ve and zero sequence impedances of the cables?

Many thanks

 

[jghrist]

Positive- and negative-sequence impedances will be equal. Zero-sequence impedance is different and depends on the cable shield characteristics, shield grounding practice, cable spacing, and the earth resistivity as well as the conductor characteristics.

Impedances may be calculated using the techniques in the Westinghouse T&D Handbook. Fault study software often includes modules for calculating cable impedances.

 

[Bung]

The fun bit is trying to figure out which zero sequence impedance to use - it depends on whether fault current can flow in shields / screens etc or not, and that can depend on the fault type, eg internal or external to the cable run when only one end of the cable screen is earthed.

So if you have an earth fault (eg dig-in) which results in the core and screen being shorted together at the end of a cable which nominally only has its screen earthed at the source end, you will get one value of fault current. If the fault is a few metres further along, beyond the cable termination, it will be a different value as there will no longer be a sheath return path.



Bung
Life is non-linear...

 

[jbartos]

Suggestion: Visit some cable manufacturer sites, e.g.

http://www.kerite.com/catalog/catalogfiles/impedance_data_power.htm

for those impedances