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Emerald51 (Mechanical) (OP)
17 Nov 11 17:39
I'm new here but read through thread238-35570: Genset fuel consumption and/or additional load capacity with input from bigamp and others. The thread was about improving power factor in diesel generator-powered elec systems and how an improved PF does not significantly change fuel consumption, if KW output is constant. I'd like to get a publication or textbook reference on this topic that I can cite. Can anyone help me out? Thks.
waross (Electrical)
17 Nov 11 19:36
Go to manufactures websites. Cat (Olympian, Marathon) Onan, FG Wilson, John Deere, etc. They publish fuel consumption at various loadings. Beware, in the fine print will be the SG of the test fuel. Some have tested with heavy fuel to make the consumption look better.
Improved Power factor reduces the current and so reduces the I2R losses slightly. A little less KW but not much less.
An improved Power Factor reduces the excitation current for less I2R in the field circuit. A little less KW but not much less.
You may have a challenge finding this information distilled in this form in a text book.

"Why not the best?"
Jimmy Carter

Emerald51 (Mechanical) (OP)
17 Nov 11 20:39
waross, thanks. I have looked at a bunch of fuel/load curves but none of the mfrs has more than one curve at the standard 0.8 PF. I'd like to get a performance graph with a curve for, say, 0.6 PF, 0.7 PF, as well as for 0.8, so I can look at the change in fuel with PF, even if small. I am in a difficult discussion about this and "official" data from a mfr, textbk or tech paper would help a lot. Thanks again.
itsmoked (Electrical)
17 Nov 11 21:24
It's not going to change much - if any.  The PF aspect just makes all the magnetics and wiring requirements go up - not the fuel consumption.

Keith Cress
kcress -

cuky2000 (Electrical)
18 Nov 11 15:17
I concur with Waros and Itsmoked that the power factor has very impact on the fuel consumption.
Although current increase for load power factor with the same kW at the load, The generated kvar do not require real power at the driver shaft. Therefore, not significant fuel is required other than the ancillary services and possible extra heat generated in the genset.
rmw (Mechanical)
20 Nov 11 8:11
The words used in this thread are "significant", "not much", "little less", "not much less", etc.  In fact it is something - and you never get something for nothing.  You don't generate all that heat in the windings and core and transmission lines for free.  It takes fuel to do it, fuel that is not producing real power but heat in all the components from the I2R losses waross mentions.

If it weren't a real problem, there would be no need for sync condensers and capicator banks located out on the distribution system.  The power companies would just generate all the VARs needed at the central plants and send them across the lines to the users.  The reason that they don't and that they locate devices to take care of it near the loads is that it is expensive to ship VARs.  They can't afford the fuel costs.

I don't know if it is in a textbook or not, but I remember being taught this in basic Thermo class in college.

Now, the question is... can you measure it in your situation?  My view is that the answer to that question is "doubtfully."

itsmoked (Electrical)
20 Nov 11 18:29
Yes but the question was about the generator and about published results with respect to the generator.   With respect to the generator the difference with PF is probably less than the energy content in different season fuels.  So none of the gen set makers are likely to bother with publishing the sought after curves.

It's the classic data that 'no one cares about it' except the occasional user.  

Keith Cress
kcress -

waross (Electrical)
20 Nov 11 19:10
You can probably extrapolate the information and get an estimate that, while still an estimate, is an order of magnitude closer to the real figure.
Use the electrical efficiency of the gen set to find the losses at full load in KW. Using I2R work backwards to find a resistance value. This will be a representation of the combined resistances of both the main windings and the excitation system and the field. Now use this R value to find the I2R losses at different current values. That is, the current at 80% PF and the current at the same KW load at 60% or 79% PF. I have ignored windage. This may be the largest loss after I2R losses at higher loading. If someone can suggest a method to estimate and remove windage and hysteresis losses from the total losses the accuracy of the estimation will improve.


"Why not the best?"
Jimmy Carter

Emerald51 (Mechanical) (OP)
28 Nov 11 19:32

Thanks for your posts about this issue.

Paraphrasing itsmoked,regarding the lack of ratings data from manufacturers, "It's the classic data that 'no one cares about it' except [those who do care about it]"

cuky2000 (Electrical)
1 Dec 11 1:55
Enclosed is a typical 500 kVA performance curve as follow:
   1) Efficiency vs. generator loading at different power factor
   2) Generator losses vs. power factor.
I hope this help to clarify the issues regarding the losses (fuel consumption) at different power factors.

 RMW, synchronous condensers, capacitor banks and other reactive compensation equipment are used no exclusively for I2R losses reduction. In many cases those devices are installed to improve the operation providing voltage support and maximizing the generation of MW in the generating plants.

Generation of reactive power require real power (fuel) only for the losses associated with frictions, windage, core losses, hysteresis, ancillaries load, etc. For instance, a synchronous condenser approximately consumes real power fewer than 3% of the rated MVAR capacity.
tlrols (Electrical)
2 Dec 11 14:39
A clarification is needed here.  It is NOT fuel costs that make it "expensive" to ship VArs...that statement is simply incorrect.  VArs do not ship well because the overwhelming characteristic of a transmission line or distribution line is inductively reactive...therefore it "gobbles up" any VArs (i.e. capacitive) being shipped across it.  THIS is whay caps are located close to the place that needs them.  This of course presumes the line is loaded at, or above the surge impedance loaing point...   
cuky2000 (Electrical)
2 Dec 11 21:55

I am not sure if you should agree with the statement that synchronous machines can absorb or delivery VAr (indirectly related with PF) if operated under or over excited in similar manners as capacitors. However, if the generator produce VAr the impact on the fuel consumption will not be significant.

The discussion above is focus in the following:
•    The original post is about "Diesel Gen Fuel Consumption vs. kW/KVA..../ PF."
•    You post is related with VAr/PF/Fuel Consumption in interconnected T&D grid.

We also need some clarification on your post:

1.    ".....characteristic of a transmission line or distribution line is inductively reactive..." This is only true for OH bare T&D lines. For insulate lines (usually UG) is the opposite.
2.    ".....This of course presumes the line is loaded at, or above the surge impedance loading point..." Capacitors in T&D transmission systems are used for multiple purposes not exclusively went the system is operated at or above SIL. If the system is operated at SIL, the voltage profile is constant. Above SIL, the system requires capacitance compensation to boost the voltage within acceptable operating limits.


Emerald51 (Mechanical) (OP)
5 Dec 11 14:19
Thanks for the performance curves. Is there a citation for them?
cuky2000 (Electrical)
6 Dec 11 1:23
Below is a link with the perfoamce characteristics of a typical diesel generator. The IEEE Std 112 provide the bases how to test a generator.

 For specific rating of a particular unit, refer to the manufacturer of the genset.

waross (Electrical)
6 Dec 11 5:52
Thanks Cuky.
As I read the curves from both your posts, efficiency drops about 1% when power factor drops to 0.8 at full load. Am I reading that correctly?
That will also equate to 1% difference in KWHrs per gallon of fuel.
To put that in perspective, there may be a 10% difference in KWHrs per gallon between different grades of fuel.  

"Why not the best?"
Jimmy Carter

majesus (Electrical)
7 Dec 11 0:34
Agreed Waross, I've been through this many times, an improved PF adjustment has marginal effects on the fuel consumption. If you want to optimize fuel consumption look at ways to make more efficient use of the load. For example we just finished a job up north at a remote, off the grid site being powered by genset units.
We used LED technology for the outdoor lighting. Even thou they were approx 2.5 times the capital over outdoor HPS lamps, we calculated that at $0.32/kwh, the LED technology paid back themselves after three years. That was also neglecting the luminaire maintenance for HPS as well. Which would make it more favorable.

Anyways just an example...
cuky2000 (Electrical)
8 Dec 11 10:37
Hi Bill,

Regarding your questions:
a).... efficiency drops about 1% when power factor drops to 0.8 at full load. Am I reading that correctly? .Yes, this is correct, there is a drop of 1% in efficiency from 100% PF to 80%PF. For additional details see the enclosed figure.

b) That will also equate to 1% difference in KWHrs per gallon of fuel. This is also correct. Ancillary load and additional heat require more fuel (real power)approx. 1%.

c)To put that in perspective, there may be a 10% difference in KWHrs per gallon between different grades of fuel. In addition to the fuel grade, the genset overall efficiency may also count.  They may be differences between manufacturer and equipment design factors that impact efficiency. (i.e. fan, pump, exciter, etc.
cuky2000 (Electrical)
8 Dec 11 10:41

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