New Transformer Grounding

[sobeys81]

Hi All,

I am seeking a logical explanation for a new transformer grounding. It is Pig/Hog processing facility and the owner is planning an expansion. I am new to this world of industrial installations and I am sure you guys have seen and done it all.

They plan to install one big 66 kV/13.8kV 10MVA transformer.
and then four small progressive addition of 2 MVA transformers being fed from big one. I am sure the local utility will advise the transformer configuration.

In a typical world, what you guys would see: Here is what I think:

1. 66 kV/13.8 kV Delta/Star Low Res. grounded

and then those 2 MVA 13.8kV/600 V shall be Delta/Y solidly grounded or low res. grounded.

Could you guys kindly advise, what am I missing here or
this does not makes sense to you. I know all types of grounding it is just that I have not seen to much in these few years of experience so a suggestion would certainly help.

 

[slavag]

Hi Sobeys81.
It seems OK, .
For the more information, please see greenbook.

http://standards.ieee.org/colorbooks/sampler/Greenbook.pdf

Best Regards.
Slava

 

[sobeys81]

Thanks Slavag. I have read Green book and that is where I understood the idea of grounding.
My question is more related to the tradition in industrial and processing facilities.

Recently, I was working on an arc flash study of a casion and I noticed that they have 2 MVA 13.8kV/600 V transformer and the ground fault current on the secondary of the transformers is in 20 kA. So I was wondering, why dont we make these sec. high or low res. grounded rather leaving them solidly grounded.

I am just looking for a basic explanation that what could be a possible reason for leaving them solidly grounded.
I would appreciate a word.

Thanks

 

[dpc]

Not sure where you are located, but low resistance grounding at 480 V or 600 V is not even legal in the US per NEC. In the US, a 600 V installation would either be solidly-grounded or high-resistance grounded. Never low-resistance ground - too many problems at low voltage.

High-resistance grounding at 600 V or 480 V has some advantages in terms of safety and reliability, but it does impose a requirement that maintenance electricians be well-trained and diligent in tracing single ground faults. I would not make that assumption at a slaughterhouse and would lean towards the brute force approach of solid grounding.

If you want to educate yourself on grounding, I highly recommend this book:

"Industrial Power System Grounding Design Handbook" by Dunki Jacobs, Frank Shields, and Conrad St. Pierre.

Regards,

Dave

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[sobeys81]

Thanks dpc for the comment.

I recently bought this book. I even have personally spoken to Conrad St. pierre in NYork a while ago regarding a substation grounding issue.
I am in Canada actually.

I had never heard of the disasterous effect of low vol. grnd. on 600V or 480 V being illegal in US.
Could you kindly drop a word, what makes it so bad.

Thanks

 

[dpc]

With low resistance grounding, the goal is to selectively coordinate overcurrent devices in the event of a ground fault. This means every feeder must be equipped with low level ground fault detection so that it may trip selectively. And circuit protected only by fuses, will not trip at all, or must trip the main breaker.

I don't believe it is allowed by the NEC below 1000 V.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[rbulsara]

Most 15kV and below services fed by utility sources are solidly grounded in the USA. Standby sources, 600V and above for critical applications do find resistance grounding.

The benefit of detection and clearing any faults as quickly as possible when on a utility source and the fact that higher fault currents allows more room for coordination favors solidly grounded systems.