Grounding Grid Calculations in ETAP

[bim802000]

Hi,

This is the first time that i'll be doing a calculation on the ground grid study of a plant. During my simulation of the ground grid study, i noticed that the maximum grid area that ETAP can calculate is 10,000 sqm.

The plant which I'm doing the study on is definitely bigger than that. May questions are:

1. How to perform this calculations on a plant that is bigger than the area set by ETAP?

2. Assuming the I divide the plant area in smaller areas (e.g. area 1, area 2, etc.), Does the indivdual step and touch voltages that i will be getting on each of those areas can be considered a representation of the overall touch and step voltage levels of the entire plant? (e.g. getting the average of the voltage levels)

3. And what about the GPR? (ground potential rise). What will be the relation of the individual GPR's to the total GPR of the plant? And how can I calculate it?

Thanks!

 

[nollij]

I believe the purpose of this software is to model outdoor utility substations, not to model an entire industrial plant (if it is anything like SKMs version).

If this plant has more than one substation, you could always do an analysis of the ground grid for each substation.

I could imagine that the computer processing power needed to do a large plant might start pushing into "not feasible."

 

[rcwilson]

10,000 m2= an area about 325 x 325 feet. That's not a very big plant, no wonder you are having a problem.

Dviding the plant into smaller areas will create modeling issues at boundaries between areas where the software assumes the grid stops. You might be able to get some good approximations by overlapping the modeled areas to compensate. Results for the middle of each area would be usable, after some scaling.

But you can't combine the results by paralleling all of the grid resistances to calculate the total grid resistance to remote earth or average the GPR. The values don't combine that way.

One method is to inject a 10,000 amp current into the model grids and calculate/analyze the GPR and the step and touch potentials. The ratio of the step and touch potentials to the GPR will be relatively accurate for most of the grid area, except around the edges. Later, when you find out the real GPR you can scale the step and touch values accordingly.

Use the equations from IEEE 80 to estimate the grid impedance for the total site. Multiply by the fault current flowing into the grid to get the estimated GPR.
[GPR= If x Rgrid] Use that GPR to ratio the step and touch values from the models.

A basic first guess at the resistance of your grid is R = (rho/4r) +(rho/L). Where rho is soil resistivity in ohm-meters, r = radius of a circle in meters whose area is equal to the area of your total grid (r= sqrt[Area/pi]), and L = total length of ground wires. Calculate "L" for the first pass by drawing a grid on your site with 10 or 20 meter spacing.

With large sites, unless it's a generating station or has a 230 kV transmission line coming in,
the grid resistance will be low enough to keep the GPR low. Low GPR = low step and touch.

If the results show step and touch values close to the danger values (>75%), a better analysis is needed. But this method will get you in the ball park.

 

[bim802000]

Rcwilson,

I agree with you on the divide-and-conquer approach.

In IEEE 80-2000, page 65, Schwarz?s equation uses the constants k1 and k2 and refers you to graph?s with curves of different slopes. However, there?s also the condition that the depth of the area should be 1/10 x √area or 1/6 x √area.

Does this mean that I have to size (divide) the areas I?m working on to approximate those depths?

 

[kingpb]

Hi,

i noticed that the maximum grid area that ETAP can calculate is 10,000 sqm.

huh? Not sure if it changed from version to version but 7.0 goes up to 5000ft (1524m) for length, which for a square area is roughly 0.9 sqmiles (2.32sqkm)

That is a pretty sizable area. I checked to make sure it is the same for IEEE method and finite element, and it is.

 

[bim802000]

hmm, I'm not aware of that feature in ETAP 7.0. What I have is ETAP 6.0.. but thanks guys for your suggestions and comments.

cheers!