Relationship between Generator specified power factor and effeciency 2

[waross]

Almost all generators above about 10 KVA will be rated at 0.8pF.
So many loads are inductive that the overall power factor will be less than unity. It is common practice to size the prime mover in kW at 80% of the KVA rating of the generator. So if your alternator is rated for 100 KVA the prime mover will be rated at 80 mechanical kW, (plus a little for losses).
The load determines the power factor. It is what it is. Are you concerned with alternator efficiency, engine efficiency, overall efficiency of converting fuel into Watt-hours, or some other figure?
Loading has a lot of effect on efficiency, power factor little if any, depending on the conditions.
Consider a generator supplying a single family dwelling. During the evening when there is no load but the generator must run regardless, the overall efficiency and the electrical efficiency will be zero. Fuel in, no energy out.
With a small load the electrical efficiency will improve and may be quite good depending on the excitation demands relative to the load but the fuel to energy efficiency (which determines the operating cost) may still be very low. It could be 20%, 10% or less depending on the load.
So, what do you want to know?


Bill
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"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Not much. But (for rated load) efficiency will be somewhat better since current is lower at .9 than at .8 power factor.

The generator does not have a power factor in itself. It is the load's power factor that decides.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

It would depend largely on whether the 0.9pf was leading or lagging, but assuming a lagging 0.9 instead of a lagging 0.8, the efficiency would improve for a given kW output due to reduced I²R losses in the stator and also lower I²R losses in the field.


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If we learn from our mistakes I'm getting a great education!

 

[defrost]

Thank you Waross

 

[cuky2000]

Enclosed is a typical efficiency vs. PF curve from one of the gen-set manufacturer.

 

[waross]

Thanks cuky.
Yours Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mgtrp]

"It would depend largely on whether the 0.9pf was leading or lagging, but assuming a lagging 0.9 instead of a lagging 0.8, the efficiency would improve for a given kW output due to reduced I²R losses in the stator and also lower I²R losses in the field."

Reduced losses in the rotor I can understand, but lower I2R losses in the rotor is not intuitive to me. Why does this happen?

 

[waross]

The leading power factor will cause excitation and tend to drive the voltage up. As a result less rotor current will be required to maintain the voltage, the AVR will reduce its output and the I²R loss in the rotor will be less.
Conversely, a lagging power factor requires greater excitation, the AVR increases its output and the rotor losses increase.
Don't forget, however, that the efficiency of the diesel engine in a diesel set will change at different loadings and the overall efficiency change may, at times, overshadow the electrical efficiency changes.
Hang around generators for awhile and this may become intuitive also.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mgtrp]

Apologies - "...but lower I2R losses in the rotor is not intuitive to me." - should have read "stator" not "rotor". Rotor losses being different for leading/lagging power factor makes perfect sense to me, it's the stator part that doesn't.

I'm also wondering whether we're all starting to talk about leading vs lagging power factor from different perspectives? Operating a generator at a leading power factor implies reduced excitation, and will tend to depress the system voltage, not drive it up.

Or maybe I just haven't had enough sleep this week!

 

[waross]

Stator: Load circuit: The same KW load at a higher PF will result in lower current and less I²R losses in the stator.
The same KVA load at a higher PF will make little difference to the stator losses as the current and the I²R losses will remain the same.
Rotor: Field: Anything which requires more field current will cause more I²R losses in the rotor.
Power factor, importing and exporting VARs. In an islanded set, the load sets the power factor.
In a networked set, there may be several reasons to run a set at other than the load PF. VARs may be exported to increase and control the voltage with a relatively small station at the end of a long transmission line. There are occasions when the utility will buy VARHrs.
But any condition that causes a reduction in excitation current in the rotor will result in reduced I²R losses in the rotor and improved efficiency.
In regards to the efficiency graph posted by Cuky, this graph may or may not include the exciting losses. At those efficiencies it may not include excitation. I may be wrong.
And, the result of an increase of PF from 0.8 to unity improves the efficiency by 1%.
To put it into perspective, while we are discussing 1% improvement in the stator and possible much less in the rotor, winter comes and we switch to winter diesel fuel. An instant 10% drop in overall efficiency. (KWHrs per Gallon of fuel.)

Bill
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"Why not the best?"
Jimmy Carter

 

[defrost]

Thanks to all who responded. I have been enlightend. I wondered why generators are usually specified by apparent power (MVA) and power factor. I wondered if the real power it could produce would be be limited to the MVA*powerfactor by something other than the power capability of the prime mover. Would a 10MVA set specified with a power factor of 0.8 be allowed to operate at 10MW at unity power factor if the prime mover was capable of providing 10MW plus or are there other restrictions?

 

[waross]

I think that we may be looking at the wrong end of the horse. The prime mover is generally sized to the load, or in the case of hydro power, the amount of water available. The alternator is then sized at 125% so that the prime mover capacity is not limited by a poor power factor.
I have seen exceptions with diesel generators in the under 1 Mega-Watt range.
These were prime power sets going in to remote areas. The engine was oversized by 25% so that rebuild intervals could be extended. The theory was that with extra power available, the engine could continue in service with valve seals leaking and blow-by past the rings for a year or so more before a rebuild and still have enough power to supply full power to the alternator. When these sets were new they could supply full rated KVA at unity power factor.
I worked on one install and in the course of that saw the specs for several others. I wasn't around long enough to find out if the theory played out in practice.

Bill
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"Why not the best?"
Jimmy Carter