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Overhead 115 kV Transmission Line Ground Wire

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kmh1

Electrical
Sep 12, 2003
47
I am attempting to determine the actual 115 kV ground fault current at an industrial substation some 2.5 miles from the main utility 230/115kV transformer station. The fault current has been provided by the utility at the transformer station and the tap line is a wood pole line with two 5/16" steel overhead ground wires. I have a handle on the station ground resistance and am trying to calculate the current division via the overhead ground wires but I can't find any electrical characteristic data for steel wire other than the resistance is somewhere around 7 ohms per mile. Does any one have impedance and GMR information for steel wire?
 
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The SES grounding current split program FCDIST has Rac=6.89 ohm/mile and X=1.04 ohm/mile at 1 ft spacing for 5/16" EHS in its database.

This is a good program to use for current division calculations, which are tricky to do by hand. If you want to do them by hand, I recommend "Fault-Current Analysis for Station Grounding Design", by J. Endrenyi, Ontario Hydro Research Quarterly, second quarter, 1967 as a reference. IEEE Std 80-2000, "IEEE Guide for Safety in AC Substation Grounding", Section 15.9 also has a calculation method. Simple parallel impedance calculations do not work because they do not take mutual coupling into account.

 
Thank you for your response jghrist. I have been looking at purchasing the FCDIST module but for most of the work we do the results obtained using the approximation graphs in Standard 80-2000 suffice so it is hard to justify the cost. We use ETAP software for ground grid modeling.

Regarding the J. Endrenyi reference do you know how I can get a hold of this document? It is specifically referenced in the Standard 80-2000 calculations you mention but it doesn't provide much detail on the methods used by J. Endrenyi. Is it available from IEEE?
 
kmh1,

I'm not sure where you can get the document. It isn't available in IEEE Explore. Maybe from Ontario Hydro.
 
QUESTION: Does any one have impedance and GMR information for steel wire?
ANSWER: For 5/16 Extra-High-Strength Steel - Class C Galvanizing conductor, the range of resistance and reactance are: 6.84 Ohm/mi< R < 6.89 Ohm/mi and 0.85 Ohm/mi < Xa < 1.04 Ohm/mi witch compare well with the data provide in the above post.
The table below contains the 60 Hz inductive reactance at 1 ft spacing factor (Xa) for different current on the shield wire. Those values are related with GMR as follow: Xa = 0.2794.f/60.log(1/GMR).
Steel_Shield_Wire.jpg


QUESTION:….. I am trying to calculate the current division via the overhead ground wires….
ANSWER: I am not sure if the accuracy gaining calculating the current division factor using expensive software can be justified for simple problems like this. The expected results using the graph method described in the IEEE Std 80-2000 appear to me as an acceptable inexpensive alternative for this purpose.
To illustrate the use of the graphical analysis, see the enclose link with an example adapted from the IEEE Std 80.

The information in the next link could be used to determine the current division factor for a single transmission line with or without secondary feeders connected to the substation ground grid.

I hope this input could help you to support the decision regarding what method to use to calculate the current division factor.
 
Thank you for the information Cuky2000.

Typically we use the Std 80-2000 graphical approximation method. In this case the transmission line in question is on 2.4 miles long with the steel skywires. The Std 80 graphs are based on a substatially longer transmission line (23.5 mi) with alumoweld skywire. The caculated step and touch potentials in this particular case are excessive and there is some desire to minimize as many assumptions as possible in the caculations to reduce overdesign.
 
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