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Can anything help prevent a case ground on a transformer?

Can anything help prevent a case ground on a transformer?

Can anything help prevent a case ground on a transformer?

(OP)
I'm looking at things on our power system that would be a major disaster if they ever occurred - that is shut everything down. One of them is a case ground on the main transformer, which would lockout the main breaker.

I wonder, is there any kind of electronic gizmo or something that could help detect an impending failure due to a case ground?  Is there anything we can get to help pinpoint where the ground is?

When are these most likely to occur? Where are they most likely to occur?

RE: Can anything help prevent a case ground on a transformer?

What do you mean by 'case' ?

If you are talking about the actualy metal structure housing the transformer than you will want to keep that grounded.

RE: Can anything help prevent a case ground on a transformer?

Depends on the system requirement, size, voltage ratings and applicable local codes. Are there Phase to neutral loads?

There could be a few options for system serving phase to phase loads only, no L-N loads. None of them fits "gizmo" category.

Rafiq Bulsara
http://www.srengineersct.com

RE: Can anything help prevent a case ground on a transformer?

(OP)
oops nice point. The transformer is a 3000kVA, Delta - Delta, ungrounded. The diagram I'm looking at shows a wire connected from the case of the transformer to a ground symbol. The CT of the relay is around this wire.

I'm assuming that if a phase conductor grounded to the enclosure, it would make this a corner grounded delta system, that would continue to operate undetected unless this relay is there.   

RE: Can anything help prevent a case ground on a transformer?

Corner grounded system is not an ungrounded system. What do you want it to be, grounded or ungrounded? Ask the engineer who designed it for clarification. Get the facts and objective right before you go off on a tangent.

Rafiq Bulsara
http://www.srengineersct.com

RE: Can anything help prevent a case ground on a transformer?

(OP)
Rbulsara,

This is NOT a grounded system. Its 480V, Delta Delta Ungrounded.

And if a conductor went to ground on this ungrounded system..I say it probably wouldn't do anything: because in affect it just became a corner grounded system...which function like this intentionally perfectly fine.

I appreciate your responses to my somewhat uninformed questions. Its why I ask questions on this site. I don't mean to be off on a tangent.  

RE: Can anything help prevent a case ground on a transformer?

bdn:
Yes, system will continue to operate with the first ground fault. if one of the other phases faults to ground before it is fixed, it would result in a phase to phase fault and the result would be much more catastrophic. There are many ground fault monitoring schemes that would detect the first ground fault, make sure you have one of them installed.

Oh, the first unintentional ground fault is not perfectly fine, usually it quickly results in deterioration of the fault and a total failure. All it does is buys you "some" time. There is a good reason why ungrounded systems are out of favor now.
 

Rafiq Bulsara
http://www.srengineersct.com

RE: Can anything help prevent a case ground on a transformer?

Google "discontinuous ground fault". Recommended reading for anyone considering the reliability of an ungrounded 480V system. Consider the possibility of a ground fault being responsible for the destruction of most of the motors in the plant.
What is the primary voltage? Disconnecting the case ground may lead to lethal voltages being induced on the case by capacitive action. It is also contrary to most electrical codes.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Can anything help prevent a case ground on a transformer?

(OP)
The primary voltage is 15kV and we are not considering removing the case ground. Thanks for the info.

Not that many years ago I worked at a large DuPont site where all 480V systems were corner grounded delta. It was common at many of their sites and likely still is. I know they were upgrading to high impedence grounding...They told me that the use of corner grounded delta came about after the benefits of grounding became known in like the 1950's, but since they had so many delta delta tranformers already in service it would cost a fortune to change them out and that they could get some of the benefits of grounding by going with cgd. That is what I meant by "working perfectly fine..." which they do, and I don't know of any plants burning down because of it. We are not installing cgd at this place.

Places like DuPont, a continuous batch operation, can't just shut down on a dime. The consequences of that can be even more catastrophic in terms of safety and property losses than any potential issue with the ground fault.



 

RE: Can anything help prevent a case ground on a transformer?

bdn:
Thank you for sharing. There is a place for every type of approach and design philosophy, despite their pros and cons, which are only relative to their applications.

A lot depends on how the system is operated, monitored and maintained and quality of the operators.  People have made through their lives safely driving cars built in 1940s, while we regularly see 4-wheel drives with traction controls, anti-lock breaks and all, going off the road and lying on their back like a turtle on a bright sunny summer day.

 

Rafiq Bulsara
http://www.srengineersct.com

RE: Can anything help prevent a case ground on a transformer?

bdn2004, You can have a simple ground monitoring system by using 3 single phase PTs ( or 5 limbeb PTs) on line side with star primary and one of the secondaries in broken delta with a voltage sensing relay connected across the broken terminals.In the event of a line ground 3 times PT secondary phase voltage will be seen at the relay giving alarm.

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