Substation Grounding

[jwparker]

I realize this is a multi-directional complicated topic however I'm looking for a 'next step' in my design.

The design revolves around replacing some oil-filled pole top transformers with a unit substation. The capacity is increasing from 1100kVA to 2500kVA. We are using a wye-grounded to wye-grounded configuration. I'm wondering what, if anything, I should do regarding checking/verifying wheather the existing grounding would be sufficient. The utility tells me that considering Im grounding to their multi-grounded wye network that their impedance is so low that I shouldnt have any issues.

 

[jwparker]

I forgot to mention that the voltages are 12.47kV to 600V 3ph,4w

 

[jwparker]

I've also forgot to mention that this is an INDOOR substation.

 

[alehman]

Yes, this is a very vague question. You might want to have a look at the National Electrical Code article 250, if that applies, and IEEE Green Book. You cannot under any circumstances, rely entirely on the utility service for the ground.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[VTer]

1st thing I would recommend is to test your ground/earth resistance and see what you get. You can either do a fall of potential test or look at fluke 1625 meter for stakeless test if it’s a multi-grounded system. Compare your results with the code requirements and follow the code, whichever one applies where you are for proper grounding practices. For more detailed technical information look at IEEE Green Book. For additional ground/earth resistance testing information refer to Fluke’s website, they have some good information.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[jwparker]

I was never intending to rely on utility's ground. I was more like intending to rely on the existing building ground currently in place.

So one would test the existing ground potential and depending on that ground the substation accordingly.. I realize this IS vague I'm very unsure of correct procedure.

Clearly this project will not be completed my me alone, however I'd like to get the greatest amount of understanding as possible

 

[PHovnanian]

All the following comments assume you are in the USA.

Changing from pole-top to indoor transformers may (will?) result in a change in the governing code.

Pole-top transformer installations, if owned by the utility, are governed by the NESC. Once the function of the transformers is moved to an indoor bank, lots of things will be changing: The governing code will now be the NEC. The premises service voltage will now be >1kV line to ground, with all the code differences that this implies. The secondary of this bank will now be considered a separately derived system and have its own grounding/bonding requirements.

My recommendation would be to start a new grounding design from scratch.

 

[alehman]

I agree with PHovnanian.

Depending on the arrangement it may be possible to use the existing grounding electrode system. There really are no measurements that can tell you if the existing system is adequate. It must be verified to comply with current code for the new installation by inspection or as-built documentation. Depending on the age, etc., you may want to replace it anyway.

The only code-driven test value in North America is that the resistance to earth of the grounding electrode system must be <25 ohms. This may be difficult or impossible to measure accurately in an existing facility. And by itself, is not sufficient to say the system is adequate.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[jwparker]

its actually located in canada.

Im changing from a 5kV pole type transformers which is located indoors in a vault, its being removed and replaced with an enclosed 12kV unit substation.

Im doing my best to comply with section 36 of the CEC however from what I'm reading it doesnt appear to require much more than a standard building grounding system.