3 Phase Unbalanced Load Question

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scc851

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Hello All,

I have a 3 phase questions about a system that I was recently working on. I mostly work on the control side of things so my 3 phase Theory is a bit rusty.

A few years ago, this panel was purchased from an OEM. The input power was 600 VAC 3 phase, no neutral. This made sense to me because the load was balanced evenly across all 3 phases. The input power was supplying power to 3 x Transformers and each transformer was getting 600VAC.

After commissioning the manager decided that the system was too large and wanted to reduce power consumption. The OEM ended up retrofitting the cabinet. In this process the load being supplied by one of the transformer were all removed, so one transformer was removed from the enclosure.

If the we remove the 3rd transformer, shouldn't a neutral be brought into the panel to deal with the unbalanced load across the phases. If my calculations are correct. Phase 1 is being used for both transformer but Phase 2 and 3 are only being used for 1 transformer. Therefore a larger amount of current will be going through Phase 1 vs Phase2/3.

The reason why I'm asking is because we've been having issues with the enclosure ever since that work was completed. I couldn't find any issues on the control side of things. So I'm not looking into the distribution side see if something got missed.

Any help would be greatly appreciated. I might be missing something obvious, but as said earlier, my 3 phase knowledge needs a bit of refresher.
 
As long as each load is connected from one hot lead to another, there would be no "neutral' current to worry about.
If any of the loads were wired from hot to neutral, then you could not be using the supply configuration that you describe.

Also, removing one transformer simply converts the closed delta to an open delta, with corresponding degradation of voltage regulation on the "missing" phase. You can still support a limited load on that phase.

It is best if you do not call the three supply conductor phases. If you call the three supply conductors A, B and C, then the three phases are A-B, B-C and C-A (or A-C if you prefer)

The actual supply from POCO could be either wye or delta with no effect on the analysis. But if the POCO supply is truly from a delta transformer bank, the circuit must either be corner grounded or operate ungrounded if the NEC permits for your situation.
 
scc851 said:
Hello All,

I have a 3 phase questions about a system that I was recently working on. I mostly work on the control side of things so my 3 phase Theory is a bit rusty.

A few years ago, this panel was purchased from an OEM. The input power was 600 VAC 3 phase, no neutral. This made sense to me because the load was balanced evenly across all 3 phases. The input power was supplying power to 3 x Transformers and each transformer was getting 600VAC.

After commissioning the manager decided that the system was too large and wanted to reduce power consumption. The OEM ended up retrofitting the cabinet. In this process the load being supplied by one of the transformer were all removed, so one transformer was removed from the enclosure.

If the we remove the 3rd transformer, shouldn't a neutral be brought into the panel to deal with the unbalanced load across the phases. If my calculations are correct. Phase 1 is being used for both transformer but Phase 2 and 3 are only being used for 1 transformer. Therefore a larger amount of current will be going through Phase 1 vs Phase2/3.

The reason why I'm asking is because we've been having issues with the enclosure ever since that work was completed. I couldn't find any issues on the control side of things. So I'm not looking into the distribution side see if something got missed.

Any help would be greatly appreciated. I might be missing something obvious, but as said earlier, my 3 phase knowledge needs a bit of refresher.
Click to expand...
When they removed the transformer what did they do with the delta leg that was connected to it?
Did they connect it to the other two remaining transformers or leave it floating?
 
Ungrounded systems have neutral currents. Since the power system and the grounding are both conductors separated by an insulator, it forms a capacitor. During normal operation this just shows up as a small current, probably 1 A or less in your system. But if you have an arcing fault or switching with capacitive loads it gets pretty nasty because the system capacitance turns into a voltage multiplier.

 
Better understanding of exactly what you have there is critical to answering your questions.

If these "transformers" are a "bank" to derive a three phase output, it is possible to only use two in an open delta configuration, but that still leaves unbalancing on the primary side as opposed to a full delta or wye. Unbalancing doesn't have to involve use of a neutral conuctor. If there are no line to neutral loads there isn't even a need for a neutral conductor, but you can still have more load between two phases than between the other possible phase connections, especially when single phase loads are being supplied.
 
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