Cable Zero Sequence Impedance

[rashid_UK]

Hi all,

It is difficult to find good information on the following -

What I wish to do is perform a SLG Short circuit study (LV & MV).

As an example take the following SLG fault description:
System Type: TN
TX (solidly grounded)
Main earth and neutral are neutralised (connected together) at Main_DB. Example:Link
So for a SLG fault (Phase to frame of DB) the fault circuit is:
TX->Main_DB_Phase_conductor->Sub_DB_Phase_conductor->FAULT->Sub_DB_Earth_conductor->DB_Neautral_conductor->TX

We would say this is the Earth Fault Loop impedance. Performing manual calculations (R+jX) for the entire circuit we might get say 5kA.

Now when utilising commercial and performing a short circuit calculation (IEC60909) on the same circuit the SLG might be say 10 kA. This is done treating the transformer impedance as Z1=Z0.

I am aware that there are some factors such as cable temperature, voltage factor 'c', impedance correction factors etc. These aside, the SLG value is still very high.

This is all due to the default cable Z0 contained within commercial software packages. In these programs for non magnetic circuits R0 = R1 * 1.59, X0 = X1 * 2.59 if my memory serves me correct.

On the following thread Link a board member said Z0 = Z1 + 3ZE where Z1 is phase impedance and ZE is the impedance of the return path. I have manually modified the cable library in a commercial software package to reflect this and the SLG results I obtain are close to manual SLG calculations.

My questions:
How do other users treat SLG calculations and cable Z0?
The factor R0 = R1 * 1.59, X0 = X1 * 2.59 from commercial software, where are these factors pulled from? (I did see some Z0/Z1 ratios in an IEC 60909 guide)
What are your thoughts on modelling cable Z0 as per the linked thread for LV and MV (cables run at ground level)?
I would appreciate users general thoughts and opinions on this topic

Thanks

 

[dpc]

I suspect each program has its own approach. Richard Kaufmann of GE published some papers 50 years ago that provided specific test data for Zo for cables in conduits. EasyPower uses that data to develop Zo values for cables. The paper is "Let's Be More Specific About Equipment Grounding". It used to be available on-line, but my quick Google search did not turn anything up. I have a hard copy.

There are many variables related to Zo in cables. The key factor is having a ground wire within the conduit or cable in question. Other ground paths will be high impedance.

 

[7anoter4]

In my opinion, a good evaluation of cable Zo one could find in this publication also:
Electrical Transmission and Distribution Reference Book Westinghouse Electric Corporation 1964
chapter 4 Electrical Characteristics of Cables sub.4 Zero-Sequence Resistance and Reactance.

https://www.amazon.com/Electrical-Transmission-Distribution-Reference-Book/dp/B000AOMQV2

 

[mgtrp]

Regarding the Kaufman paper, it is available here

 

[rashid_UK]

All, Thank you for your suggestions. I have the Westinghouse book so I will read the recommended chapter.

The paper by Kaufman - I have not seen this before so will check it out.

Anything other suggestions, please do add.

Thanks

 

[jghrist]

So for a SLG fault (Phase to frame of DB) the fault circuit is:
TX->Main_DB_Phase_conductor->Sub_DB_Phase_conductor->FAULT->Sub_DB_Earth_conductor->DB_Neautral_conductor->TX

This does not take into account mutual reactance between phase conductors and earth conductors and the earth return path. The proper equation is If = 3·I0 = 3·E/(Z1+Z2+Z0).