Plant and generator grounding

[montericci]

Hi all.
I've got a project that seems to be getting the better of me. I am designing an upgraded electrical system for a sewer plant and when it comes to grounding systems, I'm a little lost. The plant consists of several buildings and process areas with the main buildings being the control building, the filter building and the service and pumps building. At each of these building there are existing pad mounted transformers: Filter bld two 750KVA (with tiebreaker switchgear), at the control building two 750KVA (with tiebreaker switchgear) and the service bld two 500KVA (without tiebreaker switchgear..only one useable transformer). The secondary of all the transformers is 480/277V 3 phase 4 wire. In 1972 a ground system was installed throughout the plant but has since been compramised in several locations. My thoughts are to replace the broken wires, CAD Weld the new to the existing that's still in tact and implement several new ground rods. In the big mix of this nightmare I'm to specify two 750KW generators to operate along with the existing 750KW generator and make sure the ground system is correct. I have two PDFs of the plant that are too big to put on this post. One of the one line diagram and one of the grounding system that I've come up with so far. Please let me know if I should be using 3 or 4 pole ATSs with these gensets, and why.
email me for the PDFs or CAD drawings Thanks for your help
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[jghrist]

Some good articles on generator grounding:
http://ecmweb.com/mag/electric_solid_grounding_generator/index.html
http://ecmweb.com/mag/electric_grounding_nonseparately_derived/index.html
http://ecmweb.com/mag/electric_unraveling_confusion_generator_2/index.html

 

[Girl Engineer]

Definately dig into the NEC a little more, I would just scan through the entire Article 250, it seems like you have a lot of different issues that cover that whole article.

Be sure to perform a "Fall-of-Potential" Test when the install is done per IEEE Standard 81 Part 8.2.1.5. and "Point-to-Point" tests. Depending on where you are, you can rent a calibrated meter that will do the fall of potential. That will prove that the grounding electrode and grounding system are good.

 

[__dan]

grounding multiple sources

grounding multiple sources

There are two areas that immediately come to mind that are a likely source of trouble. Not grounding, but the neutral to ground bonding jumper placement per 250-30 (A) for separately derived systems. With multiple dual source switchboards and multiple grounded neutrals at the service transformers, it looks almost unavoidable that any service supplied line to neutral loads will have a parallel path for neutral current over the grounding system. The grounding system should be as robust and substantial as possible.

Since the loads are mostly delta connected pump motors, my first thought would be to go through the place and make sure there are no line to neutral loads at the service switchboards or upstream of the ATS switches. No single phase loads, 277 V lighting could be fed from 480 delta to 480 Y transformers downstream of the ATS switches.

Making all the loads at the dual source switchboards and upstream of the ATS's delta, no neutral, the service transformers and generators could be grounded Y secondary but stop carrying the neutral at the first place code allows and run everything downstream of that point as 3 phase 3 wire no neutral. The ATS's might be 3 pole in that case. For the switchboards where source neutrals are solidly connected, having no neutral connected loads mitigates the problem of the occurrence of neutral and grounding paths in parallel.

The other source of trouble is switching transients generated by switching the large inductive loads. You may be looking at placing TVSS devices and RC snubbers to mitigate these.