Large single-phase loads on a three-phase generator
Hi,
Almost all generators above, say, 8kVA seem to be three-phase. However, it's very normal for large loads (typically air conditioners) to accept only single phase, especially where the generator is just a backup source for when the mains power is out - the building may have been wired for single-phase grid/mains power in the first place.
In this case, it seems to be normal to wire each large load to one of the three phases, such that when all the loads are switched on, the loading is approximately balanced.
However, it's quite rare to have every single load operating at its capacity at the same time. This means that situations where 80% of the generator rating is being output on phase 1 whilst phase 2 and 3 are (at some instant) completely unloaded.
In this case, I believe that current is sent out on phase 1 and returns on the neutral. From my understanding, this seems to have the effect that the neutral point drifts away from being truly neutral, meaning that the phase-to-neutral voltages across all phases are unequal and can fluctuate by huge amounts. This (or whatever is going on) can trigger normally undervoltage (occasionally overvoltage) protection cutouts on the generator.
I sort-of resolved this by adjusting the undervoltage cutout threshold downwards, as well as adjusting a related time, to try to reduce the frequency of cutouts.
My questions are:
1) Is my understanding of the reasons for the voltage fluctuations correct?
2) Is severely reducing the under voltage cutout threshold likely to be a problem for the generator or for any appliances?
3) Is there a better way to resolve these issues?
Thank you Anton
1 Answer
Hi Anton
Here are a few answers provided by one of our RedR Experts:
1/ As a general rule you can run single phase loads off a 3 phase generator but as noted it is better to try and balance the loads across the phases (it’s the basis of all grid connected systems, just they have lots of different loads , see also 9/ below.
2/ Remember that the power rating for a three phase generator is the total for the three (single) phases. This means that it supplies only 1/3 of that rating for each single phase. So a 15KVA generator can supply only 5 KVA for each of the single phases. You can’t cheat and have say 10KVA on one phase and nothing on the others – you’ll burn out the windings if it doesn’t trip on overload.
3/ As well as generator KVA the actual KW loads that the generator can supply will be 20% less. The 15KVA generator can only supply 12 kW (as a total of three phases) or max of 4 KW on each phase.
4/ The problems being talked about (undervoltage etc) seem to be indicating that the generator is being overloaded on the phase with the air conditioning load/s, not that the neutral point its drifting (it doesn’t.)
5/ Another cause of undervoltage trips could be the very short duration inrush current you get at the instant motors start. So generators have to be sized to be 2 to 3 times larger than the running load to provide enough capacity for this startup.
6/ Changing the undervoltage trip is not going to damage the generator, but the connected load may draw higher current and if the connected load has no overload protection this could cause damage or decrease service life.
7/ Ultimately a generator will stall if the load is to large for the load it is connected to, so if you set the undervoltage to low you may end up with a stalled generator.
8/ The overvoltage trip may just be the “slow” response from the generator regulating voltage device and/or engine governor when the high load turns off . This is more likely if the generator is not large enough.
9/ It’s very hard to distribute loads across three phases on small installations with only a few loads you really need to use a single phase generator, particularly if this is for long term use. They are available up to about 25kVA but may not be universally available.
I hope this helps to answer your question. Please let us know what you do and if you need more information you can post another question here on KnowledgePoint.
Harriette
Comments
Hi Harriette. A while ago that you posted this, but glad I found it. In rural Brazil (a 254 volts 2-phase system) I am figuring out what emergency generator I could use to compensate for the odd power outings we have here. Already dove into the subject of 3-phase generators in combination with a Scott transformer, but that does not seem to fit the electrical system I have installed already (mainly 127 volts phase to neutral with the loads spread over the 2 incoming phase as best as I could). From your explanation I get that I have a more favorable situation then Anton when it comes to generator sizing, since I can use 2 of the outcoming 3 phases, hence, do not need as big as a generator as in the case you would use only 1 of the phase. So no Scott transformer I guess, and just a ...
Johan ( 2019-03-13 13:19:09 )
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