The Power Factor of Generator 2

[ciy]

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.

 

[waross]

The power factor is determined by the load.
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

 

[GTstartup]

Sorry but I don't see how a generator power factor changes depending on what prime mover you connect it to. A gas turbine output varies dependant on ambient temperature, humidity, pressure etc. That would mean that the rating would be variable dependant on these conditions also.

My understanding is it is where the field rating and the stator rating intersect. Which would be dependant on the design.

 

[rcw retired EE]

Specifying a generator's power factor rating specifies its reactive power capability. A generator rated 800 kW 0.8 power factor is sized to provide 1000 kVA, 800 kW and 600 kVAr. (sqrt (800^2 + 600^2) = 1000). The same generator rated 0.9 power factor could only supply 435 kVAr at full kVA load.

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.

 

[ciy]

Thanks for these advice!
From manufacture viewpoint, which one is more costly than the other one?

 

[waross]

Talking about ratings and not the actual power factor:
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

 

[Silja]

if you don't know the load details,use 0.8 lagging power factor as in NEMA standard. and I believe all the manufactures will make the genset with 0.8 PF as required by NEMA no matter what cost.

 

[ciy]

Silja, Thanks for your recommendation.

Would you please point out the said NEMA standard and its paragraph directly?

 

[Silja]

I believe in NEMA MG1
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.

 

[ScottyUK]

From the IEC world, here's what IEC 60034-3 says about the subject:

"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."