34.5kV grounded system WITH NO GROUND?

mshields

Senior Member
Location
Boston, MA
I've encountered a service to a college campus which is a 34.5kV overhead service derived from a utility owned delta wye transformer but from which neither a neutral nor a ground has been run. They simply come into the campus with 3 cables. Is this common? Is this safe? In the event that there is a ground fault, how is the fault cleared? Does the high voltage allow for sufficient current to flow through the earth back to it's source? How does one deal with such a system from an overcurrent protection and surge arresters standpoint?
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
I've encountered a service to a college campus which is a 34.5kV overhead service derived from a utility owned delta wye transformer but from which neither a neutral nor a ground has been run. They simply come into the campus with 3 cables. Is this common? Is this safe? In the event that there is a ground fault, how is the fault cleared? Does the high voltage allow for sufficient current to flow through the earth back to it's source? How does one deal with such a system from an overcurrent protection and surge arresters standpoint?
Sounds like a uni grounded wye system, done every day by many utilities in both old and new installs. Maybe not as safe as running a ground but its allowed by the NESC. In this case all loads need to be delta and reclosers and breakers need sensitive earth fault detection for ground faults. Usually something like 5 amps is ok. In this system the earth is the ground wire. Any fault goes through it then through the substation ground mesh than the XO.
 

GoldDigger

Moderator
Staff member
I've encountered a service to a college campus which is a 34.5kV overhead service derived from a utility owned delta wye transformer but from which neither a neutral nor a ground has been run. They simply come into the campus with 3 cables. Is this common? Is this safe? In the event that there is a ground fault, how is the fault cleared? Does the high voltage allow for sufficient current to flow through the earth back to it's source? How does one deal with such a system from an overcurrent protection and surge arresters standpoint?
Yes.
When dealing with a voltage like 34.5kV, the earth path has a low enough resistance to be an effective (although not particularly pleasant in its side effects) fault clearing path. That is, when you calculate the resistance of the ground electrode attached to the wye point of the POCO transformer and the resistance of your GES anywhere at the load end, they will allow enough current to flow to trip some sort of OCPD within the utility. Until that trip happens, though, the voltage on the ground system relative to remote earth can get pretty high.
Your thread title says that it is a grounded system. You know this, or assume it to be true based on the presence of a wye transformer with a visible ground electrode or primary neutral connection?

As long as all of the actual loads on the campus are derived from transformers with ground electrode and in most or all cases neutral connections, the nature of the 34.5kV service should not affect you.
As far as surge protection and OCPD on the 34.5kV system itself, I know so little about those voltages that I should not make any statements at all. That much I do know. :)

Last comment: On the primary side of a three phase transformer, with delta input, it would actually be harmful to run and utilize a neutral even if the primary supply was in fact a grounded wye.
 

mbrooke

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Location
United States
Occupation
*
Oh, just forgot to add, lighting arresters are based on the earths resistance. If the earth tends to have a higher resistance than arresters are rated phase to phase instead of phase to neutral since a faulted phase will cause a voltage rise before it is cleared. Usually its best practice to have them set up this way. But in any case if the run is short run an EGC since the cost is minimal, perhaps even setting off the price in the fully rated surge arrestors. Now if this is a long run involving miles and miles of line then the savings are huge but I doubt that's the case on a college campus.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
Yes.
When dealing with a voltage like 34.5kV, the earth path has a low enough resistance to be an effective (although not particularly pleasant in its side effects) fault clearing path. That is, when you calculate the resistance of the ground electrode attached to the wye point of the POCO transformer and the resistance of your GES anywhere at the load end, they will allow enough current to flow to trip some sort of OCPD within the utility. Until that trip happens, though, the voltage on the ground system relative to remote earth can get pretty high.
Your thread title says that it is a grounded system. You know this, or assume it to be true based on the presence of a wye transformer with a visible ground electrode or primary neutral connection?

As long as all of the actual loads on the campus are derived from transformers with ground electrode and in most or all cases neutral connections, the nature of the 34.5kV service should not affect you.
As far as surge protection and OCPD on the 34.5kV system itself, I know so little about those voltages that I should not make any statements at all. That much I do know. :)

Last comment: On the primary side of a three phase transformer, with delta input, it would actually be harmful to run and utilize a neutral even if the primary supply was in fact a grounded wye.
It depends. When using such a system it is highly recommended a recloser or breaker be equipped with sensitive earth fault logic. And since there are no line to neutral loads that will be easy. Using standard ground tripping where the ground trip value is set say 1/3 of the phase value is a really bad idea. As are fused cutouts for protection of laterals. I have seen 34.5kv systems where a phase came down on concrete or asphalt and nothing tripped because it was a muli grounded neutral system and the ground trip had to be set high enough to allow for normal wye load imbalances. Yes at 34.5kv the 19900 voltage will push more current and can increase the chance of a device opening but that device does not always open up when ground trips are set high. Its usually in the 115kv range and above where high and medium resistance ground faults are of less worry.

As for the transformers not sure which ones your referring to but if its on the primary fed by the utility its ok. However, after that transformer, any campus equipment on the load side of the uni grounded wye distribution must have a delta or ungrounded wye primary. The NESC does not allow the earth to be the only neutral for the load.
 

gar

Senior Member
131117-1528 EST

I read mshields' first post as saying he had a secondary Y, but that it had neither a neutral or ground connection. Thus, this is roughly equivalent to an ungrounded delta.

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