Residual GF Protection, Single Phase Load

[mjr5097]

We have a 3p 3w solidly grounded 480V system where this is a need to connect a relatively large single phase load. The proposed solution is a 480V - 120/240V single phase transformer to serve a 100kVA (208A at 480V) transformer. My concern is that this 1500 kVA system only has about 75A of 3p load normally and the 3w ground fault protection on the proposed feeder is set to 100A. 75A 0 Deg + 275A 120 Deg + 275A -120 Deg = 200A 180 Deg.

I found this Schneider Electric FAQ that says the trip unit is "not looking for an imbalance" but instead a non-zero vector sum. What am missing here? How could an imbalance result in a zero vector sum? Unless the current phase angles change drastically to compensate, I'm not sure how this is possible.
https://www.schneider-electric.us/en/faqs/FA167448/

Assuming we proceed with this installation (not likely), could this L-L single phase load cause a misoperation of a 3w residual ground fault relay? I know there are better ways to do this but I'm more interested in learning about how residual ground fault would work in this situation than I am in suggestions for how to serve these loads.

 

[wvengineer]

Stupid question, but how are you going to feed a single phase transformer from a 3W feed?

 

[wvengineer]

Stupid question, but how are you going to feed a single phase transformer from a 3W feed?

Nevermind, it was a stupid question. Thinking about it like a core balance CT as long as whatever goes out one phase comes back on the other phases wouldn't it sum to zero?

 

[kwired]

Clamp an ammeter around all three supply lines. Even with unequal load on each of the lines your meter still reads zero (or very near zero) as long as no current is seeking an alternate path (like would happen during a ground fault). Your GFP measures current in similar way and only trips when that current seeking an alternate path exceeds the trip point.

 

[LMAO]

We have a 3p 3w solidly grounded 480V system where this is a need to connect a relatively large single phase load. The proposed solution is a 480V - 120/240V single phase transformer to serve a 100kVA (208A at 480V) transformer. My concern is that this 1500 kVA system only has about 75A of 3p load normally and the 3w ground fault protection on the proposed feeder is set to 100A. 75A 0 Deg + 275A 120 Deg + 275A -120 Deg = 200A 180 Deg.

I found this Schneider Electric FAQ that says the trip unit is "not looking for an imbalance" but instead a non-zero vector sum. What am missing here? How could an imbalance result in a zero vector sum? Unless the current phase angles change drastically to compensate, I'm not sure how this is possible.
https://www.schneider-electric.us/en/faqs/FA167448/

Assuming we proceed with this installation (not likely), could this L-L single phase load cause a misoperation of a 3w residual ground fault relay? I know there are better ways to do this but I'm more interested in learning about how residual ground fault would work in this situation than I am in suggestions for how to serve these loads.

according to the article you cited having a 2 phase transformer should not cause any problems. It is not written articulately but from what I gathered the trip unit simply sums up all three phases so having an unbalanced load will not make any difference as the the 2 unbalanced phases will cancel each other out. It looks for imbalance in all 3 phases collectively.

 

[kwired]

according to the article you cited having a 2 phase transformer should not cause any problems. It is not written articulately but from what I gathered the trip unit simply sums up all three phases so having an unbalanced load will not make any difference as the the 2 unbalanced phases will cancel each other out. It looks for imbalance in all 3 phases collectively.

Correct, all current that goes through the CT in one direction comes back in the other direction leaving a net zero on the CT output. What is unbalanced is the source itself.

 

[mjr5097]

The proposal was to use a 2 pole 480V breaker from an MDP for the single phase transformer. This MDP is fed from a 3p 3w breaker with ground fault. I was concerned with that upstream protection picking up.

 

[LMAO]

The proposal was to use a 2 pole 480V breaker from an MDP for the single phase transformer. This MDP is fed from a 3p 3w breaker with ground fault. I was concerned with that upstream protection picking up.

it won't pick up