The Power Factor of Generator
The Power Factor of Generator
(OP)
There is a concept need to clarify, that is
How to determine the power factor when I select a generator set?
I find some cases about the power factor, it is always 0.8 or 0.85.
What is the difference between them?
What is the key point to select the power factor?
Thanks for your advice in advance.
How to determine the power factor when I select a generator set?
I find some cases about the power factor, it is always 0.8 or 0.85.
What is the difference between them?
What is the key point to select the power factor?
Thanks for your advice in advance.






RE: The Power Factor of Generator
The power factor rating of a generator set is the ratio of the kW rating of the prime mover to the KVA rating of the generator end.
Example:
Two identical generator ends are rated for 100 KVA.
Set #1 has a prime mover able to supply 85 kW plus losses and reserve. This set is rated at a power factor of 85%.
Set #2 has a prime mover able to supply 80 kW plus losses and reserve. This set is rated at a power factor of 80%.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: The Power Factor of Generator
My understanding is it is where the field rating and the stator rating intersect. Which would be dependant on the design.
RE: The Power Factor of Generator
Typical industrial loads have an aggregate power factor of about 0.85. If the system has a lot of transformers or long distribution lines, the kVAr demand and the power facotr will be lower. The 08,0 pf may be needed in that case.
For generating plants, the power purchase agreement may specify a required minimum power factor at full load; usually 0.90. Since a generator step up transformer requires reactive power to operate, the generator may need to be specified at 0.85 or even 0.80 to be able to supply all in-plant reactive power needs, the GSU reactive power and still deliver 0.90 at the interconnect point.
RE: The Power Factor of Generator
From manufacture viewpoint, which one is more costly than the other one?
RE: The Power Factor of Generator
Generator ends are rated in KVA.
Prime movers are rated in kW.
I wouldn't specify a power factor, I would look at what is available. In the sizes I see, 0.8 is the most common.
Which is the most expensive? It depends.
Example #1> 1000 KVA,
At 80% this will have a 1000 KVA generator end and a prime mover capable of supplying 800 kW.
At 85% this will have a 1000 KVA generator end and a prime mover capable of supplying 850 kW.
An 850 kW prime mover is often more expensive than a 800 kW prime mover so the 85% set will cost more.
Example #2> 1000 kW,
At 80% this will have a 1250 KVA generator end and a prime mover capable of supplying 1000 kW.
At 85% this will have a 1176 KVA generator end and a prime mover capable of supplying 1000 kW.
A 1250 KVA generator end is often more expensive than an 1176 KVA generator end so the 80% set will cost more.
In the first instance a generator end rating is the product of the rated voltage and the rated current. The result is expressed in VA or scaled to KVA or MVA.
The kW rating of a complete set is based on the minimum power of the prime mover but may be less.
It is common to oversize the prime mover 10% on prime power sets to allow for overloads. I have seen prime movers over rated 25% to allow for extended time between overhauls.
And another example of how ratings may change;
If a set rated at 75 KVA, three phase is reconnected for single phase, the new KVA rating is 2/3of 75 KVA or 50 KVA. This still has the same prime mover. The prime mover is still capable of producing 60 kW (75 KVA x .8 PF). This may now be considered as a power factor of 100% and an overload capability of 20%.
This may be limited by the size of the breaker which will be the same but 2 poles will be used instead of 3 poles. In any event I would be concerned with any 20% overload lasting for more than about 2 1/2 minutes.
As for the KVAR capacity of the set, I have seen diesel sets in the low MW range run at mostly reactive power to boost the voltage at the end of a long transmission line. Only enough Watts were produced to lessen the effects of wet stacking. These sets were probably running at a power factor of 15% or 20% or less but the MVA rating was not exceeded.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: The Power Factor of Generator
RE: The Power Factor of Generator
Would you please point out the said NEMA standard and its paragraph directly?
RE: The Power Factor of Generator
32.1 BASIS OF RATING
"Synchronous generators shall be rated on a continuous duty basis, and the rating shall be expressed in
kilovoltamperes available at the terminals at 0.8-power-factor lagging (overexcited)."
And
"33.2 RATINGS
33.2.1 Power Factor
The preferred value of power factor is 0.8 lagging. "
I knew it from the manufacturers and did not looking on NEMA text before.hope it will help.
RE: The Power Factor of Generator
"The power factor shall be agreed upon between the purchaser and manufacturer.
Standardised rated power factors at the generator terminals are 0,8, 0,85 and 0,9 overexcited."