Zero Sequence Impedance Calculation 6

[jwilson3]

Does anyone know of a tech literature source that describes a way to calculate the zero sequence impedance of three phase circuits in grounded steel conduit? The process for overhead circuits is straight forward, following the methods in "Symmetrical Components" by Wagner&Evans, however it doesn't cover what I'm looking for.

 

[alehman]

It is difficult to calculate accurately due to the unpredictable way cables will lay in a conduit. Tables of measured values have been published however.

see thread238-99311

 

[jghrist]

As alehman says, it is very difficult to calculate because of the many variables. The Westinghouse T&D Handbook gives some guidance and tables.

 

[dpc]

There is an old paper by R. H. Kaufmann of GE giving actual test data of zero sequence impedances for a variety of conduit and raceway systems. The results of this testing caused a movement away from reliance on using the conduit system as a ground and towards the use of a separate ground conductor inside the conduit.

Paper is "Let's be more specific about equipment grounding" from Proceedings of the American Power Conference, 1962, by R.H. Kaufmann. I would highly recommend trying to get a copy.

 

[wareagle]

J
Caculations of fault current is very complex due to the fact that the affect of the steel on the impedance of the cable varies with the magnitude of the fault. This site

http://www.atcelectrical.com/links.php

is the site of Allied
Conduit. They along with Ga. Tech University developed a program that will caculate fault currents within steel conduit. I hope this will be of some help to you.

 

[TSJ]

Do you have any idea about the technology Power Planner?

 

[cuky2000]

Do to the complexity of the zero sequence impedance calculation for UG cable one practical option is to use a simplified approximation as suggested below:

1- Determine the Positive/Negative cable impedance using the conventional resistance and reactance equations with actual electrical and physical cable parameters provided by cable manufacturer.
As alternative to calculate positive reactance, use the enclose nomogram:

http://www.okonite.com/engineering/images/conductor-reactance.gif

2- For magnetic conduit, a simple approximation may be estimated as follow:
Z_o= R_o+j.X_o =[π].(R₊)+ 2.5j(X₊)

Other simplified approach is to obtain directly the impedance from table. For example consider the enclose information for 3-conductor zero sequence impedance with galvanized steel interlocked armor. This may be similar than the 3-single conductor cables in steel conduit but add additional allowance (ex. 20% to 25%) to compensate for cable random lay and transposition.

Please see the enclose impedance data and assumptions how this information was developed.

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