Yup, another 3-phase transformer question

[TwoBlocked]

First some background. A site had 480V 3-phase service. One of the feeders went to a transfer switch to provide back-up power from a 480V 3-phase generator. The loads were from some 3-phase air compressors, a large 480V-120/240 split phase transformer feeding an MCC, and a very small 480V-120/240 split phase transformer feeding a panelboard with hardly any load.

One fine day during a planned outage the genny and transfer switch did their job, then the genny tripped out a minute later. Upon investigation it was noted that it tripped on current imbalance, which was set at 25%. (Somehow this had gotten changed from the original 75%). The only 3-phase demand was the 3 air compressors which were programmed to wait a few minutes after a power restoration and then come on one at a time as needed. No surprise it tripped with the low imbalance trip and nearly no load on one of the legs.

My suggestion was to install a 277V-120/240 split phase transformer and feed both the small panel board mentioned and a larger one fed from the MCC. This would remove some of the load from the 2 legs feeding the large transformer and move it to the remaining single leg going to the additional transformer. I doubt they did so and probably just left things like they were. (Been job hopping lately...)

But I got to thinking about this and have a question. Let's say the change was accomplished and the large transformer drew 50 amps on phases A to B while the small transformer drew 50 amps on phase C to neutral. I am used to thinking that if all the legs draw the same amps, there is no neutral current at the source. Or is that only the case with split phase?

 

[winnie]

I am used to thinking that if all the legs draw the same amps, there is no neutral current at the source.

The above statement is a true approximation. The exact statement is 'the neutral current is equal to the vector sum of the phase currents.'

In a normal 3 phase situation, you would have 3 similar 50 amp currents, with 120 degree phase difference between them. In this symmetric case the vector sum of 50A @ 0 degrees, 50A @ 120 degrees and 50A @240 degrees is 0 on the neutral.

But in the hypothetical situation you posted, the phase angles would be quite different from the normal 120 degree offset.

The L-L single phase load would give 50A at 0 degrees on one leg, and 50A at 180 degrees on the other leg. The L-N load would give 50A at 90 degrees. The vector sum of these 3 values is 50A.

Jon

 

[Strathead]

My first thought, before I got through this was, "Why would they feed 120/240 single phase for a large load from 3 phase." I would want to fix that.
Change the large transformer and panel to three phase and redistribute the loads. Is my thought. Anything else is lipstick on a pig. But your thinking is correct, for the feeders to come from the generator and the main service. It would be a juggling act to balance the system, in the way you suggest.

 

[Strathead]

One more thing, You didn't mention lighting. The other load that would unbalance the system, would be lighting. Is is 120 of 277? If 277 you should be able to move it to the third phase and offset all of the single phase 120 loads.

 

[TwoBlocked]

The above statement is a true approximation. The exact statement is 'the neutral current is equal to the vector sum of the phase currents.'

In a normal 3 phase situation, you would have 3 similar 50 amp currents, with 120 degree phase difference between them. In this symmetric case the vector sum of 50A @ 0 degrees, 50A @ 120 degrees and 50A @240 degrees is 0 on the neutral.

But in the hypothetical situation you posted, the phase angles would be quite different from the normal 120 degree offset.

The L-L single phase load would give 50A at 0 degrees on one leg, and 50A at 180 degrees on the other leg. The L-N load would give 50A at 90 degrees. The vector sum of these 3 values is 50A.

Jon

Woser! Ok, more to consider here. THANKS!

 

[TwoBlocked]

My first thought, before I got through this was, "Why would they feed 120/240 single phase for a large load from 3 phase." I would want to fix that.
Change the large transformer and panel to three phase and redistribute the loads. Is my thought. Anything else is lipstick on a pig. But your thinking is correct, for the feeders to come from the generator and the main service. It would be a juggling act to balance the system, in the way you suggest.

One more thing, You didn't mention lighting. The other load that would unbalance the system, would be lighting. Is is 120 of 277? If 277 you should be able to move it to the third phase and offset all of the single phase 120 loads.

I believe the site was originally 120/240 split phase including air compressors. Then it grew, and grew, and grew. The air compressors were replaced with 480V wye/delta start ones and then that part of the site was given a 3-phase genny. Another part still has a single phase genny. Then there is the part fed with an ungrounded 480/240 delta/delta. They didn't know that until I pointed it out... No 277 loads anywhere at all. Some lighting is 120, some 240. Other strange but true set ups at this site and other sites with the same outfit.

My latest job (starting Monday) may send be back there so I'll just stop here.