Grounding systems

[MikeKL]

I am involved in designing a grounding system for a 5mw generating plant using 5 each 1MVA gas gensets in parallel on a common bus. Neutrals are floated, Generation is @ 4160 vac. Main purpose is to provide power to the grid. Tie breaker connects common buss to grid via step up transformer (4.16 KV delta / 13.2 Kv grounded wye) and 13.2 Kv CB. I am typically excessive with the design of a ground grid for the facility. Approx. 40 ground rods bonded to approx. 1000' of #4/0 bare copper conductor grid system, 6 to 24 inches below grade. My construction spec calls for each rod to be tested prior to connection to grid and must meet 25 Ohms or less. If 25 Ohms is not measured, than additional rods or soil ammendment is allowed using rock salt or copper sulfate around rods. In this particular facility, additional rods are not an option ( too many rocks) The 40 rods were installed in trenches 2' deep & 8' long. Soil ammendment (Lots of rock salt trenched in around rod followed by water saturation only gets my readings to 30 ohmes +/-,) Soil is mostly river rock and hard pan.

1. What's my solution?
2. What's do I really need for ground in the future, (is a single electrode measuring < 25 ohmes sufficient?

 

[jbartos]

Suggestion: A compliance or meeting the intent of industry standards (e.g. ANSI/IEEE Std 80) is a good idea. One can potentially search for a better ground in the wider area of the genset location where the rock is deeper. Also, the high resistance grounded neutral system (at one genset and the second one on grounded neutral standby aligned with a neutral ground bus) is considered safer/better than the floating neutrals. See Reference:
Beeman D. &quot;Industrial Power Systems Handbook,&quot; McGraw-Hill Book Co., 1955, Chapter 6 &quot;System Grounding,&quot; and Chapter 7 &quot;Equipment Grounding&quot;

 

[peterb]

Try the following for some info that may help:

http://www.erico.com

- look under Electrical Protection/Grounding &amp; Bonding/Chemeical Electrodes.