Standby generator sizing for loads containing VFDs

[Mila2015]

I was told by VFD vendors that we need to double the size of generator supplied VFD loads. I also read many articles online saying the same thing, like the one at ABB site:

https://library.e.abb.com/public/0007878791e17d0985257cca005bb048/LVD-EOTN64U-EN_REVA.pdf

On the other hand, my generator vendor (Caterpillar) said VFDs will not affect their generator sizing, and that their voltage controller is designed to handle harmonic issues.

The question is, who is correct? Is it different for generator vendors? If that's the case, how does generators deal with large VFD loads (in our case it's a 550KW generator, and (3) 125HP VFDs in addition to smaller linear loads).

 

[waross]

If you have problems with VFD loads on your present generator, it may be more cost effective to upgrade the Automatic Voltage Regulator than to upsize the generator.
It may be prudent to size the generator for an extra 25% of the largest motor.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TugboatEng]

I have just been down this road. Our vessels were supposed with 150 kW prime rated generator sets for a 15kW base load with an intermittent 75 horsepower winch motor. I believe the sizing was done for a direct on line start of that motor which is why the generator was double capacity. The engines ended up being major oil burners consuming a gallon of lube every 24 hours. Our motor is on a VFD so there is full control of current at all times. There is no need to oversize the generator as a result. We dropped to a 99kW unit and went with a better AVR and PMG excitation. The new smaller sets are more stable and provide better power quality than the large sets ever did.

 

[waross]

Thanks for sharing Tug.
By the way, were the 150 kW sets new or old and worn?
I have had new sets pump oil to the point of a low oil shutdown in a few hours due to light loading.
I load banked them for a couple of days to seat in the rings and had no oil loss.
They often ran at 10% load or less, but all capacity was needed when the A/C started to cut in.
Load banking won't help an old, worn set but it will work wonders for a new oil pumper.

Mila2015
Are you looking at prime ratings or standby ratings?
Prime rating plus 10% gives you the standby rating.
A standby set rated at 110 KVA will be rated at 100 KVA prime power.
That said, I trust Caterpillar. They know what they are doing and know what their sets are capable of.
If you have given Caterpillar complete and accurate information, they will give you a suitable set.
Anecdote warning:
I designed and installed the distribution system for a diving club that was a number of cabins spaced along a hillside.
The cabins each had a few lights and a few outlets for electric razors or computers. Possibly a hair drier.
The cabins each had an on-demand butane fired water heater for showers.
I emphasized and the owner agreed that the system would not support electric water heaters. Never. Don't even think about it.
No problem We will never use and electric water heater.
A few months later the water heater in the manager's cabin failed and guess what.
An electric, on-demand water heater drawing 45 or 50 Amps on a 15 Amp circuit.
I don't want to talk about it any more.
Be sure that you give Caterpillar complete information including plane for futire additions of equipment.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TugboatEng]

We have 8 sets consisting of a John Deere 6081 (6 cyl 8.1L) attached to a Marathon 150KW generatore with a SE350 voltage regulator (the weakest and most basic one available). We ran these sets from 0-35,000 hours. They just never broke in, the oil consumption was constant through the entire life. We replaced them with a John Deere 4045 engine (4 cyl 4.5L) and a (I believe) Marathon DVR2000 voltage regulator with PMG. Our first sets have hit 1000 hours.

 

[waross]

I installed a 250 KVA set that maxed out starting one large motor.
The motor was essential, but only ran for a couple of hours several times a week.
The rest of the time the load varied between 10% and 25%.
The vendor insisted that we load bank it for five days as a condition of warranty.
We boiled a lot of water, but the set never used oil.
I had a couple of smaller sets that were bad oil pumpers, brand new. Load banking fixed them.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!