Distribution network fault study - DC cable resistance or AC? 2

[Jennett]

I'm modelling an 11kV distribution network in PSCAD. I'm applying a fault at different points on the feeders and measuring the voltages and currents. The underground cables are less than 80km. Based on Glover, Sarma and Overbyes Power System Analysis and Design book (Chapter 5 pg 246 in fourth edition) I'm modelling the underground cables as 'short lines.' The data sheet I've been provided with gives a dc resistance (ohms/km) at 20oC and a resistance R1 at 65oC.

Which resistance do I use in my model - DC resistance or AC resistance and why? As a side note, if I were to model the cables as PI circuits how would I determine the admittance? Its not given in the data sheet. Is this because it is dependent on how the cable is laid?

Any help would be greatly appreciated.

 

[jghrist]

You use ac resistance because the current flowing in the conductor is ac and there will be skin effect. You also need to consider reactance.

You could calculate the admittance from the cable geometry if you want. It won't make much difference for a fault study, but it would for a load flow study. 80 km isn't very short at 11 kV.

 

[MAS2000]

Have you got cable capacitance (charging current) and power loss?

Using these parameters you can calculate the admittance (Y). In PI model you can use 2Y in each leg.

Y=2 * Pi * f * C

M.A.Sh.
Elect. Engr.

 

[Jennett]

Thanks MAS2000. I didn't have capacitance in my original data sheet (from Utility) but I found it in the manufacturers data sheet. PSCAD lets you input line capacitance directly, saves you converting to admittance.