250.21(B) - ground detectors required?

[Isaiah]

What exactly constitutes an ungrounded system and when do ground detectors need to be installed? Lets say I have a 3 phase step-up/step down scenario for voltage drop over a long distance of approximately 2500 feet: the first transformer has a primary that is grounded and secondary that is ungrounded - 208/120V-480V. The opposite end has a step down transformer at 480V-120/208V with primary ungrounded and secondary grounded. Is the 480V side considered to be an 'ungrounded system' and does it require ground detectors per 250.21(B)?

 

[infinity]

The prevalent opinion is yes they're required. Your installation does not fall under 250.20 but it does fall under 250.21(B).

 

[Isaiah]

The prevalent opinion is yes they're required. Your installation does not fall under 250.20 but it does fall under 250.21(B).

Thanks - I was leaning toward this conclusion as well.

 

[LarryFine]

Isaiah, you could use a transformer with a 480/277 high side for the first transformer, and ground that neutral. The neutral, if one is there, for the second transformer 480v should not be connected.

Also, remember that the primary neutral (X 0) of the first transformer should likewise be left floating, i.e, connect the 208v to the primary of the first transformer as if it was a delta.

 

[Isaiah]

Isaiah, you could use a transformer with a 480/277 high side for the first transformer, and ground that neutral. The neutral, if one is there, for the second transformer 480v should not be connected.

Also, remember that the primary neutral (X 0) should be left floating, i.e, connect the 208v to the primary of the first transformer as if it was a delta.

Agreed completely. But what would be the negative impact of solidly grounding the XO on the primary side of first transformer?

 

[LarryFine]

I'm not sure, as I've never read a clear explanation, but it has something to do with "circulating currents" in the primary. That's something that can be searched for or someone else here might explain.

As for the grounding issue, my suggestion would make it easier and cheaper, and maybe safer, than the non-grounded option, unless it's critical that power be maintained during an accidental ground fault.

 

[Isaiah]

I'm not sure, as I've never read a clear explanation, but it has something to do with "circulating currents" in the primary. That's something that can be searched for or someone else here might explain.

As for the grounding issue, my suggestion would make it easier and cheaper, and maybe safer, than the non-grounded option, unless it's critical that power be maintained during an accidental ground fault.

Its for a UPS Unit that interfaces with, among others - the Fire and Gas System - so I would say its critical.

 

[GoldDigger]

I'm not sure, as I've never read a clear explanation, but it has something to do with "circulating currents" in the primary. That's something that can be searched for or someone else here might explain.

As for the grounding issue, my suggestion would make it easier and cheaper, and maybe safer, than the non-grounded option, unless it's critical that power be maintained during an accidental ground fault.

This has been discussed in great detail in earlier searchable threads, but in s nutshell:
The closed phasor diagram of a delta secondary imposes an equation on the three voltage phasors (placed end to end they must sum to zero.)
The incoming wye phasors may not satisfy the same equation. Connecting the wye point of the transformer forces currents, potentially greater than the transformer FLA, to circulate in primary and secondary until IR drop allows the equation to be satisfied.

Sent from my XT1585 using Tapatalk

 

[Isaiah]

This has been discussed in great detail in earlier searchable threads, but in s nutshell:
The closed phasor diagram of a delta secondary imposes an equation on the three voltage phasors (placed end to end they must sum to zero.)
The incoming wye phasors may not satisfy the same equation. Connecting the wye point of the transformer forces currents, potentially greater than the transformer FLA, to circulate in primary and secondary until IR drop allows the equation to be satisfied.

Sent from my XT1585 using Tapatalk

Excellent info...Thanks GoldDigger.

 

[Wire-Smith]

grounding the wye primary opens up the possibility of problems when ground faults on the primary circuit or secondary(if secondary is also grounded) occur. not grounding the wye primary opens the possibility of problems if you loose a primary leg or if you close one leg at a time when energizing, which your probably not doing that at those voltage levels, that issue comes up in old distribution systems, which the solution is an extra cutout to temporarily ground the primary during initial energizing and then open the ground connection after the other legs are closed.

https://apps.geindustrial.com/publibrary/checkout/GET-3388B?TNR=White Papers|GET-3388B|generic

the whys of wyes by GE

 

[Isaiah]

grounding the wye primary opens up the possibility of problems when ground faults on the primary circuit or secondary(if secondary is also grounded) occur. not grounding the wye primary opens the possibility of problems if you loose a primary leg or if you close one leg at a time when energizing, which your probably not doing that at those voltage levels, that issue comes up in old distribution systems, which the solution is an extra cutout to temporarily ground the primary during initial energizing and then open the ground connection after the other legs are closed.

https://apps.geindustrial.com/publibrary/checkout/GET-3388B?TNR=White Papers|GET-3388B|generic

the whys of wyes by GE

Some lovely reading for a Sat afternoon. Thanks.

 

[GoldDigger]

Note however that the ground reference for the ungrounded wye conductors is relatively firmly provided at the source end of the circuit (POCO secondary).
It is true that the voltages applied to the wye windings may be a bit odd (but generally lower rather than higher) when the wye point of the wye primary is left open and one of the three ungrounded conductors is opened.

Sent from my XT1585 using Tapatalk

 

[electrofelon]

Agreed completely. But what would be the negative impact of solidly grounding the XO on the primary side of first transformer?

Just want to clarify a few things as multiple transformer winding configurations and setups have been thrown around and IMO a few things werent necessarily clear:

The best setup is a 208 delta to 277/480 wye for the step up, and a 480 delta to 120/208y for the step down. The high side wye point of the first transformer would be grounded. The second transformer doesnt need a neutral, but does need an EGC.

Secondly, note that the "leaving the xo floating" issue is for a wye-delta (typically resulting when a delta-wye transformer is used in reverse because its what someone had lying around) and does not apply to wye-wye transformers (which are quite rare in the low voltage world anyway).

 

[jumper]

Just want to clarify a few things as multiple transformer winding configurations and setups have been thrown around and IMO a few things werent necessarily clear:

The best setup is a 208 delta to 277/480 wye for the step up, and a 480 delta to 120/208y for the step down. The high side wye point of the first transformer would be grounded. The second transformer doesnt need a neutral, but does need an EGC.

Secondly, note that the "leaving the xo floating" issue is for a wye-delta (typically resulting when a delta-wye transformer is used in reverse because its what someone had lying around) and does not apply to wye-wye transformers (which are quite rare in the low voltage world anyway).

Agree.