Power plant earthing design 1

[ELEP]

Hi everyone,
I am designing an earthing system for 240MW power plant, the area is 230x720m, average soil resistivity 14,000ohm-m(Sedimentary rock), fault current 20kA. I used 120sqmm cooper condr with 15x15m spacing, 0.5m depth. With this data I am getting ground grid resistance 9 ohm, GPR more than 7000V and abnormal step & touch potentials.

1. can anyone suggest how to achieve ground resistance up to 5ohm(Tech. spec. requirement)?

2. is it proper to use treated earthing pit(consting, charcoal, bentanovate material, salt etc,) at various locations connected to main grid?

3. is it possible to model treated earthing pit in ETAP?

There is constraint, we can't increase no. of conductor, I mean minimum grid conductor spacing 15m.

thanks!
ELEP

 

[rcw retired EE]

Several options to consider:

Add deep ground rods to reach a layer of lower resistivity soil underneath. Sometimes there is a water table that helps reduce the resistivity.

Bore holes, insert ground rods and backfill with ground resistivity enhancing material like bentonite clay or Erico GEM material. Holes could be 10 to 40 meters deep, 150 mm diameter.

Use Ufer grounding by connecting all the plants’ foundations to the grid. If steel piles are used to support the turbines, connect them to the grid. The mass of concrete, rebar and steel in the earth is a better connection than buried copper. (I assume you can't reduce your 15m x 15m spacing because equipment foudnations are in the way. You should reduce the space to 4-5 meters where ever posssible.)

If a metallic water well casing is present, connect it to the grid.

Review the current split factor to verify that all 20 kA fault current is actually traveling through the ground grid to remote earth. Maybe some of it (25-50%) is returning to the remote substation or fault via the overhead conductors and static wire (overhead ground wire).

Most of these enhancements probably cannot be modeled in Etap. In my experience at high resistivity sites we drilled the holes for the deep rods and tested the resulting grid resistance to find out if we needed more. We kept adding until we achieved the required impedance to remote earth. Not very economical, but it worked.

The piles and well casings can be simulated by a large diameter ground rod in the calculations. A standard ground rod in a bored hole backfilled with an enhancing material can be simulated by increasing the diameter of the rod. (That is essentially what the backfill material does.) It is engineering judgment as to what diameter you use.

If your minimum spacing of 15 meters is dictated by economics, that has to change.

I have not used Etap for several years. The older grounding calculation model could not handle multiple areas with different resistivities.

 

[ELEP]

rcwilson:
Thanks for detail reply.
I am going to consider all your mentioned points while construction. But my client wants precalculation of 5ohm. To satisfy clients requirement, I am planning to break up ground grid area in to no. of parts such as Transformer area, Water treatment area, cooling tower area, control building area, boiler are, switchyard area etc. I shall give more attention to the particular area and make separate calculations for each area where personnel are expected to be working.

are my assumptions right to get overall good earth resistance?

ELEP

 

[jghrist]

How does breaking the grid into different parts help satisfy 5 ohm requirement?

 

[ELEP]

jghrist:
Breaking ground grid area is not going to reduce sufficient overall grounding resistance. But to take care of step, touch potential in areas where personnel are expected to be working. I want reduce spacing, use more ground rods, use some backfilling material etc.