How to think about Power Factor when adding new devices to a generator

Question

I've been tasked with checking to see if an existing backup generator will be powerful enough to handle a new building in addition to the buildings it is currently handling.

First, here is the details I got off the nameplate of the generator:

Name: Olympian
Model: GEH275
Rated Power - Prime:
    250.0    kVA
    200.0    kW
    0.80     COS θ
Rated Voltage: 415/240 V
Phase: 3
Rated Frequency: 50 Hz
Rated Current: 348 A
Maximum Ambient Temp: 27 *C (There is a note that it can go up to 50 *C but will affect outputs and that the manufacturer should be contacted)

The new building will have 33 AC units which will be the primary power consumers. Here is the details I found on the AC labels:

Phase: 1θ
Frequency: 50Hz
Voltage: 230V
Cooling Capacity: 6448 W
Normal Input: 2149W
Normal Current: 9.8A
Max Input: 3733 W
Max Current: 20.6A

I checked the generator panel at different times during the day, and this is the highest reading I ever got off of it:

AVG: 415 V L-L 100A 50.0 Hz
L-L: 413V/136A, 418V/72A, 415V/89A
L-N: 239V/136A, 240V/74A, 241V/89A

My understanding of the above is that I am current using:

239*136 + 240*74 + 241*89 = 71.713 kVA

This represents 71.713/250 = 28.7% of the total kVA possible

According to the AC labels they are normally operating at 9.8*230 = 2.254kVA and they peak at 20.6*230 = 4.738kVA. 33 of these units will be running at 67.62 kVA and peak at 142.15 kVA.

If the ACs all turn on at the same time, the generator will need to supply 142.15 + 71.713 = 213.82 kVA which works out to 85.5% total kVA. During normal AC operation the total should be 55.7% total kVA.

I am not sure what I should be doing with the power factor information. Should I be working in kW and using the total of 200.0 kW? I am also wondering how much the ambient temperature can affect things. The ambient temperature where we are operating this generator is regularly above 40*C

Answer 1

You need to look at both kW and kVA. The load must not exceed either. The kW limit is essentially an engine power limit. The kVA limit is essentially a generator current limit. The mismatch between the two assumes that the majority of uses have lower than 1.0 power factor and that 0.8 is about average. That means that the generator set is matched to that average. At that point, neither engine nor generator capacity is wasted. Engine-generator sets are also sold with kVA equal to kW. If the load has a higher power factor, that would be the better choice.

Detailed information about the engine generator set should tell how the ratings are limited by maximum temperature. There may be an altitude limit also. To some extent, operating below the maximum altitude may increase the maximum altitude limit.

Since current is the limiting factor for the generator, the highest current in any phase is the limit. I don't think lower current in the other two phases allows much if any excess current in the highest phase. The engine is limited by maximum total power. Imbalance among the phases probably doesn't matter very much to the engine.

Answer 2

My major worry would be about the ambient heat, 40C is a fair way above 27C and you would be running at least somewhat close to the line if all those AC units kicked in at once.

Way I see it, 33 units, so 11 per phase going the naive route, 20 odd amps per unit, so 220ish amps per phase star connected at all out worst case. Machine is good for 348A and you are reporting 136A on the heaviest phase, so 356A absolute worst case but you can throw two of the units off the heavily loaded phase onto the other two, dropping the load on the heavy one by 40 odd amps worst case and squeaking in under the line for kVA.

On the kW size I make your total ~195kW worst case, painfully close to the prime movers 200kW rating at 27 degrees, talk to the manufacturer and the aircon people about what load they are really expecting. This is in many respects the more critical one as the mass of the alternator will likely take 125% of rated kVA for some time (it has thermal mass), and those compressors should not be full power all the time, the engine on the other hand can only make so much torque at the required speed.

I would probably be adding a second unit, with 40 degree air temp, I would not want my cooling to depend on a single generator being run balls out. Besides, a synced pair will allow you to take one down for maintenance without having to shut down totally (And it is small beer money compared to the cost of the building or even the cost of 30 people being unable to work for a week while you wait for spare parts).