Generator Output vs. Power Factor.
Generator Output vs. Power Factor.
(OP)
I work for a small municipal light Dept. in Massachusetts, USA. We have three 3412 Catarpillar generator sets (325kW each) for peak shaving. The 3 phase generators are rated at: 406kVA, 325kW, @ .8 power facter. The generators are brushless, revolving field, with a solid state exciter. When we run these units the plant power factor jumps from .85 PF to .99PF. I run these units at 325 kW each for a total of 975kW going back on to the grid. The mechanic that we are interviewing to maintain these units says we are overloading each unit by 20%. He claims that if the power factor is above the nameplate 0.8, you have to de-rate the output kW. Now this seems backwards to me, if anything I would think you could run one of these units at 406kW due to the .99PF. He insists that I'm wrong and because this is generation it is different. Also note that, the name plate on each unit specifies that 489amps at 480 volts is the maxumum. When I run them at 325 each, the amperage per phase is approx. 405 via the fluke 43 meter.
Any Thoughts...
Thanks all.
Chris
Any Thoughts...
Thanks all.
Chris






RE: Generator Output vs. Power Factor.
RE: Generator Output vs. Power Factor.
So I would say your mechanic is not wrong with his experience, because the prime movers he dealt did not have overloading capacity.
RE: Generator Output vs. Power Factor.
From your readings, with the generators at 325 kW, 480 V, 405 A are operatiing at a power factor of 0.96 (325 kW/336.7 kVA).
You didn't indicate where the plant power factor is being measured. If the meter is on the HV side of the supply transformer, the PF will include transformer reactive losses. These will be decreased when the plant load is supplied in part by the in-plant generation, hence the increase in PF.
RE: Generator Output vs. Power Factor.
"When we run these units the plant power factor jumps from .85PF to .99PF."
Before I go off on a what may be a tangent, please clarify a few points.
- Is a "small municipal light department" a power generation station? If so, what is the capacity of the primary units (kW and kVA)? Also, are you the sole supplier to your grid or are you interconnected with other stations?
- If you are a generation station, when you say that the "plant" power factor increases to 0.99, are you referring to the combined output of the primary units and the peak shaving units or to just the output of the primary units?
- When you bring the peak units on line and the "plant" PF becomes 0.99, what are the kW and current outputs of the individual primary units? How are these values different from before?
- Are all of the peak shaving units running at the same kW and current or did you just measure one?
Please note that my interest is based on real and reactive power load sharing between interconnected generator sets (or interconnected power generation stations). I am wondering that if you are a generating station then where is the reactive power you were supplying (implied by the 0.85PF) before bringing the peak units on line being supplied from now? Have you shed that load onto another station or is the power factor of the combined output of the primary and peak shaving units of your plant in fact still .85? (If it is and the peak units are at .96PF, you may be exceeding the kVA ratings of the primary units to supply the additional kVARs, ie. primary unit PF went down in the balance...)
RE: Generator Output vs. Power Factor.
1. Please, notice that the generator capability curve also has a field current limit and associated heat with it. This is a portion of the capability curve in its upper capacitive KVAR region.
2. Also, the capability curve has its stability limitation which is in the inductive KVAR region.
3. The above 1. and 2. are in addition to the mentioned stator current or stator heat limit adjacent to the stator end-iron heating limit.
4. Visit
http://www.usbr.gov/power/data/fist/fist1~4/1~4_21.htm
for more info
5. The capability curves are normally supplied with larger generators, however, the smaller generators may have them too, since they are not so difficult to produce. Also, the manufacturer site is available for more info:
http://www.cat.com/products/engines_n_power_systems/spec_sheet_library/EPG/_epg/60.html
RE: Generator Output vs. Power Factor.
For the case in point, I did not imagine that underexcited operation was contemplated, but for the record it is probably not a good idea nor necessary for a plant of this size to operate in the underecited (leading power factor) region to absorb kVAR from the utility. This would imply a high utility system voltage that needs to be reduced by the peak shaving units - not a credible operating scenario.
Given that we were originally talking about operation at a higher than rated power factor, I do not think that field current limitation in the overexcited (lower power factor) region of the curve is a factor in this discussion.
RE: Generator Output vs. Power Factor.
I also disagree with the idea that a generator whose excitation (power factor) differs from that of the bus or utility to which it is connected will absorb or supply (circulating) KVARs as a rule. The load power supplied from interconnected generators will be SUM of that provided by the individual interconnected generators. Any degree of real or reactive load sharing is possible between the generators. Of course, in the extreme it is possible to overexcite the system such that KVARS circulate between generators. It is also possible to overdrive (prime mover throttle) the system such that real power circulates between generators (ie one becomes a motor driving its' prime mover). However, this is not a rule, it is an extreme.
Finally, there is such a thing as a generator whose only purpose is to supply reactive power. That device would be called a "synchronous condensor".
RE: Generator Output vs. Power Factor.
Let's try to clear up a couple of things here, which are supported by the usbr link that jbartos refers to -
- When a generator is supplying lagging power factor load, it is operating overexcited. The generator appears to the load to have a capacitive reactance for this condition (it is supplying kVARs).
- When the generator is supplying leading power factor load, it is operating underexcited and appears to the load as an inductive reactance (it is absorbing kVARs).
- Operation of the generator in parallel with a utility system has analogies between the exciter (controlling reactive power flow) and the prime mover (controlling real power flow). If the prime mover power is increased, the real power delivered to the utility bus increases. Similarly, if the excitation current is increased the reactive power delivered to the load is increased - this increase in excitation current is in the direction of overexcitation. Conversely, decreasing the prime mover input power decreases the real power delivered to the utility bus and decreasing the excitation current (in the direction of underexcitation) decreases the reactive power delivered.
The generator will not be supplying circulating kVARs - each generator will supply the amount of kVARs determined by its excitation current and system conditions. If the excitaton current on a machine is decreased, there will be a point where the generator is absorbing kVARs from the utility system - this is an underexcited condition and will give a leading power factor measurement at the machine terminals. Reference to any generator capability curve will confirm that the machine has some capability to operate underexcited, but this is limited by stability considerations. Note that on an interconnected system it may be necessary to operate some machines in this part of the curve in order to reduce system voltages under light load conditions.
The synchronous condenser is essentially a synchronous motor that supplies lagging kVAR to the system. It operates on the positive kVAR axis (overexcited), with minimal kW input from the system to run the machine. Note that the term "condenser" is synonymous with "capacitor", and the synch condenser appears as a capacitor to the system - it supplies kVARs, just as a static capacitor does.
RE: Generator Output vs. Power Factor.
Just a few small thoughts here. The pf rises to .99 when the sets are brought on line?. A synchronous or salient pole motor (read alternator not supplying ) operates as a pf correction device when run with excitation > shaft load.
If the pf of the load is climbing to .99 when sets come on line are the cat motors driving above load requirements and working in a leading pf range?
Something doesn't feel right
What have I missed?
RE: Generator Output vs. Power Factor.
RE: Generator Output vs. Power Factor.
To get back on track, the situation described is not right. In an effort to be brief (which I am not good at) I will refer to my post of 2/16 and Don01's post of 2/23. Does anyone else see it?
RE: Generator Output vs. Power Factor.
I would take a look at the vendor documents, and especially a capability curve if available as suggested by others, but I would be VERY suprised if your mechanic wasn't in error about this. The generator curves I have seen decrease MVAR limits as you go away from a unity power factor (either lagging or leading). In other words, a power factor between 0.8 lagging and unity at the nameplate KW rating should NOT be overloading your machines. A power factor LESS than 0.8 at rated MW WOULD be overloading your machines. Verify, but I believe that your thinking is very correct on this Chris.
Now this seems backwards to me, if anything I would think you could run one of these units at 406kW due to the .99PF.
Chris, this appears sound, i.e., from the generator point of view you haven't exceeded its kva and amp rating and a pf approaching unity should let you run higher KW - BUT! This is where you really DO need your vendors gen capability curves, and maybe ratings of your diesels. I would give Cat a call on this one. If I had no supporting information, I would never exceed my generator nameplate KW even above 0.8 lagging pf. And since you haven't been exceeding 325kw per your description, I think you are good to go, but your mechanic might not be....;)
Some excellent info already by many in this forum, including an great description of overexcitation vs. underexcitation by peterb. I recommend the very good online references for voltage regulation/excitation at the Basler web site. Not sure of which document, but one has a good tutorial on generator basics, including capability curves, protection, etc:
http://www.basler.com/html/dwntech.htm
RE: Generator Output vs. Power Factor.
First of all, let me put this way. The active power flows from one node to the other if there is difference in phase, as active power flow is directly related to the Sine of the power angle, which is nothing but the phase angle difference between the voltages of the two nodes. Of course, the active power flow depends upon the voltages of the two nodes also and the impedance between the two nodes.
But the case of the reactive power flow is different. The reactive power flow is equal to the difference of the voltages between the two nodes, the grid voltage and the impedance between the grid and system from which the reactive power is going to flow.
In the question raised by Courville, it seems that there was one unit of 325 kW running at 0.85 pf connected to the grid. This means it is supplying 325 kW and 201 kVAR. This will give fixed terminal voltage independent of the generator and dictated by the grid voltage. The excitation is also fixed, which makes the internal voltage also fixed. Now when another generating unit is brought to synchronism at the same internal voltage with the same terminal voltage (a small change can take place, which will give actual value a little different from calculated value) and the grid voltage.Constant voltage difference will dictate the same reactive power supply. Of course the synchronous impedance of the machines affects little bit to the power supply, the machines being of very small rating.
With two units supplying 650 kW and 201.4 kVAR, the overall power factor will be 0.955 and with the three units supplying 975 kW and 201.4 kVAR, the overall power factor will jump to 0.979.
That rhatcher posted as Fact is thus explained above and the raised question of power factor jumping is explained.
The explanation made by davidm will be true only from generator point of view, but not from prime mover point of view. This has been explained in my earlier response.
RE: Generator Output vs. Power Factor.
References:
1. Slemon G. R. "Magnetoelectric Devices Transducers, Transformers and Machines," John Wiley and Sons, Inc., 1966
2. Stevenson Jr. W. D., "Elements of Power System Analysis," Third Edition, McGraw-Hill Book Co., 1975
3. Fitzgerald A.E., Kingsley Jr. C., Umans S. D., "Electric Machinery," Fifth Edition, McGraw-Hill, Inc., 1990
4. IEEE Std 100-1984 "IEEE Standard Dictionary of Electrical and Electronics Terms"
5. Say M. G., "Alternating Current Machines," A Halsted Press Book, John Wiley & Sons, New York, 1976
My posting content is based on Reference 1, Figure 5.44 "Circle Diagram Showing Power Ps and Reactive Power Qs into Synchronous Machine at Various Constant Values of Field Current. Terminal Voltage Es Constant," that is based on complex power entering machine
Us=Ps+jQs=Is x Es*. The left half plane (Ps<0) covers generator and right half plane (Ps>0) covers motor. The upper plane is capacitive Qs and lower plane is inductive Qs.
This is consistent with Reference 2 Figure 2.6 "Capacitor considered (a) as a passive circuit element drawing leading current and (b) as a generator supplying lagging current." Please, notice that the "lagging current does not necessarily imply the inductor when it comes to generator. If the current flows into the capacitor, then it leads voltage and it is related to the passive capacitor element. If the current flows from the capacitive generator, then the current lags voltage. However, the Reference 2 Paragraph 2.5 "Complex Power" defines it as S=P+Q= V x I* which is different from Reference 1. It states that "To obtain the proper sign for Q, it is necessary to calculate S as VxI*, rather than V* x I, which would reverse the sign for Q." The Reference 2 defines the complex power in agreement with ANSI standard. Reference 2 Table 2.1 shows:
Generator action assumed:
If P is +, emf supplies power
If P is -, emf absorbs power
If Q is +, emf supplied reactive power (I lags E)
If Q is -, emf absorbs reactive power (I leads E)
Motor action is assumed:
If P is +, emf absorbs power
If P is -, emf supplies power
If Q is +, emf absorbs reactive power (I lags E)
If Q is -, emf supplies reactive power (I leads E)
Reference 3 Fig 5-15 "Capability curves of an 0.85 power factor, 0.80 short-circuit ratio hydrogen-cooled turbine generator." Shows the Per-Unit Reactive Power (lagging) versus Per-unit Power with field heating limited curves and armature heating limited curves in the first quadrant with per-unit scale positive. This first quadrant generator capability curves correspond to the second quadrant of Reference 1 Fig. 5.44 that incidentally is marked as capacitive Q. This is in agreement with Reference 2 Figure 2.6 (b). Reference 2 shows a construction used for the derivation of a synchronous generator capability curve in Figure 5-16 (in the first quadrant) based on P-jQ=V x I, which is consistent with Reference 2 Table 2.1 "If Q is +, emf supplied reactive power (I lags E)."
Reference 4 defines the phasor power as S=P+jQ=E x I*. IEEE Std 100-2000 (seventh edition, current edition) does not include that definition.
Notice that the capability curve will have the field heating limit in upper plane for S=I x E*=P+jQ power definition (P<0, Q>0 and capacitive), Reference 1, and in the lower plane for S=E x I*=P+jQ power definition (P<0, Q>0 and capacitive), Reference 2.
Reference 5 states in Paragraph "Operating Characterisctics" on page 376 that "For underexcitation the input current may lag for all loads, and the maximum power will obviously be reduced.
Some clarification is needed to the Peterb posting on February 19, 2000 that I am attaching below marked:
peterb (Electrical)
Feb 19, 2001
To correct two inaccuracies in jbartos's post of Feb 19, the field current limit applies to the
overexcited (inductive kVAR?????) portion of the curve and the stability limit applies to the
underexcited (capacitive kVAR????) portion of the curve.
RE: Generator Output vs. Power Factor.
RE: Generator Output vs. Power Factor.
I'm Chris Courville, the one who initially started this thread. The responses to this thread have been very helpful, and I personally thank each of you. The mechanic I referenced in the question, as it turns out, was indeed talking about the prime mover being overloaded, not the generator. The prime mover is rated at 483hp and 325kW
(483/1.34(.9))=324.40. I was thinking about the generator only, disregarding the prime mover. The mechanic and I have settled this matter and are in good relations. He was wrong about the gen set being overloaded, but right in saying I could not get more kW out of the set due to the prime mover rating. He just didn't know how to explain it so I would understand.
As far as the power factor goes, I ran some tests with a Fluke 43 and got a .99 pf at three test points: the station xfmr(1.5mVA), the plant bus, and directly at the generators. I got this pf with one, two, and three generators paralleled. Also note that these generators are backfeeding through the station xfmr(1.5mVA 480vac to 13.8kvac) onto our distribution circuit. This is what we would do in a peak shaving situation. When I shut the gens down, the pf goes to .76, .56, .82 for phase A,B, and C respectfully.( These measurements were taken at the station xfmr). After about 5 minutes, the generator cooling tower motors shut down. With this inductive load off line, the pf goes to .94, .63, and .98 for A,B, and C phase respectfully. It seems that the genertors are correcting the pf when they are online.
RE: Generator Output vs. Power Factor.
hi chris,
just looking at your pf(s) when generator is off line is all your lighting on the b phase???
Something is pulling the pf on that phase down hard. I suggets a bit of an investigation of the loads at the distribution boards and tried to balance that pf as well as current load (if not amps are balanced). This imbalance is costing you money when you run the generator and from the supply co. when you don't.
(it may also limit some of your output)
Also I would investigate the sizing of the cooling tower fan motors (and pumps?) they are causing far too much change in the pf. They must be running at a VERY low value.
Any way this thread was just getting fired up!! so why stop the fun.
Best to all Don
RE: Generator Output vs. Power Factor.
1. It is nice to read that the solution was found and was acceptable. As far as the power factor increase of the paralleled generators is concerned, it is the result of overall generator control or regulation concept (or strategy) that tends to deliver as high watt power as practical to the load since nature of the load is sensed by the generator regulators that may interact with the speed governor. One may have a considerable influence over the generator set operations, in case those load sharers are installed. The load sharer may have adjustable "load gain," "adaptation," "droop," and "drive," e.g. Load Sharer T4300 by Selco USA, Inc. Therefore, any significant change, i.e. an increase, in the power factor, while the generators are "on," is not uncommon. However, the previous posting concern, and verification of the control concept for a possible error appear to be justified.
2. PeterB posting Feb 21, 2001 and Mar 5, 2001 somehow omitted the alignment and location of the field heating limit at the generator capability curve. The field heating limit is located in the opposite half plane (e.g. upper half-plane) to the stability limit part of the capability curve, unless the whole curve is upside down, which some manufacturers show.
RE: Generator Output vs. Power Factor.
I have found 12 pages of data in my files for a Caterpillar 3512 engine driving a 1135 KW, 1800 RPM, .8PF. 480 volt, wye connected 689 frame generator. This reactive capability curve for this machine shows the machine output in KW is limited to 80 % of the rated KVA. In other words the portion of the curve which connects the overexcited region to the the underexcited region is a straight line from .8 lagging to .996 leading and it passes through 80 % of rated KVA. This is a clear indication that the engine has less capability than the generator. I also have a note in my files from Cat. stating that the engine is the limiting factor for this unit and has a rating of 1135 KW.
I suspect that your unit has a similar shaped reactive capability curve. Send me your fax number and I will forward this curve. The curve is give in % rated KVA and I believe it will be suitable for your unit.
RE: Generator Output vs. Power Factor.
RE: Generator Output vs. Power Factor.
Furthermore the term 'watt' as applied' to electrical systems is meaningless. If you have a purely reactive load, no power(watts) is produced. However, a generator will still suck away your fuel!
Gen set makers need a rap over the knuckles, for the 'bastardisation' of the term power factor (with no reference to load).
RE: Generator Output vs. Power Factor.
1. Whatever happened to the readings of test meters from which various curves are created?
2. What about witnesses present at the testing, e.g. government witnesses?
3. What about many Customers who come back for the same product and were satisfied by the product including the check of the generator capability curve, and other curves?
4. What about the competition in the generator manufacturing industry?
RE: Generator Output vs. Power Factor.
I haven't purchased too many gen sets in my time (in fact not one) so I can't speak for the voracity of those suppliers but I have had more than one ocasion where even government certs don't seem to match reality. What you say is true about points 1,2,3,4 but hey they aint gurantees.
I got a stereo rated at 250 watts on a real big label (and australia has the toughest consumer laws going) ~ yeh you guessed it "peak" in little letters
Wasn't there a guy called Nader and a motor car once.
regards Don
RE: Generator Output vs. Power Factor.
1. Statistically, there are isolated events that are not counted.
2. Believe it or not testing reveals a lot, which is incidentally somewhat implied in Don01 May 21, 2001 posting (How else would he know? What did he believe then?).
RE: Generator Output vs. Power Factor.
That under reactive power drawn from a generator will suck fuel, means the fuel has been used for some active power. From no equipment can we draw pure reactive power, because the noload losses are always there.It is immaterial whether the generator is supplying active or reactive power. the windage losses, the magnetizing eddy current and hysteresis losses will always be there.
The term power factor is user friendly version of the relation between instataneous values of the current and voltage. So I do not think it is a bad idea of representing the active power with a factor and the apparent power. Besides, the 2.2 MVA @ o.8 pf has the same current as 1.76 MVA @ unity pf. It is the current limitation of the generator characteristics, that makes them as equal.
So in this case the manufacturer will not be at fault, though I fully agree with iain1 regarding the technicalities that might be used by the manufacturer to shut the consumers' mouth. So why are we power professional here, other than make aware the consumer, of the repercussion of any spec.?
RE: Generator Output vs. Power Factor.
Read BS 5514.
Watts is meaningless for pure electrical systems learn to think for yourself rather than sprout textbook gibberish.
RE: Generator Output vs. Power Factor.
Generator manufacturer's incorrectly 'badge' their units with an incorrect usage of the term power factor. What is the correct usage of the term in this context? A genset that will deliver 1760kVA of power at its terminals at full load and unity power factor is often 'badged' as a 2.2MVA genset (at 0.8pf). This is commercial mischief by gen set makers. The unit will never safely deliver 2.2MVA of power. Which reputable manufacturers are doing this? Is there any evidence of this practice, any documentation?
Furthermore the term 'watt' as applied' to electrical systems is meaningless. If you have a purely reactive load, no power(watts) is produced. However, a generator will still suck away your fuel! Even without any electrical load, the engine-generator set will certainly suck away your fuel just idling along. There are always losses and they are real. There is no such thing as a PURELY reactive load - there is always going to be some resistance involved in the real world.
Gen set makers need a rap over the knuckles, for the 'bastardisation' of the term power factor (with no reference to load). The genset makers that I am familiar with rate their sets in terms of rated kilowatts at rated power factor.
Watts is meaningless for pure electrical systems learn to think for yourself rather than sprout textbook gibberish. I'm afraid that you will have to explain your meaning a bit here - "watts is meaningless"?? "textbook gibberish"??
By the way, Soorya, you are a little off in your calculation - 1760 kVA at unity PF & say 4.16 kV equates to 244.3A, while 2200 kVA has a rated current of 305.3A. What is the same is the real power of 1760 kW in each case (1760 kVA @ unity PF, 2200 kVA @ 0.8 PF)
RE: Generator Output vs. Power Factor.
RE: Generator Output vs. Power Factor.
RE: Generator Output vs. Power Factor.
All world standards ratify this practice (this does not make it correct).
If BS 5514 is not good enough for you, you could try IEC 34.
If the International Electrotechnical Commission is not good enough for you, then I can't help you. Since the IEC reached agreement with CANENA this applies to people living and working in America. Although it was common practice for generator manufacturers to 'make up' the rating of their generators even in North America (where I don't live or work) long before this.
I wanted to 'challenge' this forum with questioning the terms watts and power factor because it is what generator manufacturer's do. Referencing power factor without referencing load is unacceptable. Applying a mechanical 'wattage' to a generator is ok, but it confuses issues such as efficiency and power factor in mechanical-electrical and electrical to mechanical energy conversions.
This is what I was hoping people in this forum would discuss.
RE: Generator Output vs. Power Factor.
I have been taught that kVA (Apparent Power) already has the power factor calculated in. Secondly, kW ( true power) does not reference PF.
2200kVA and 1760kW@.8pf being synonomous.
I do agree with iain1 that Gen mfgs want to make their gen sets look as beefy as possible, but is this just one of the many marketing ploys that we are subjected to each day universely, that challenge us to see the real picture. For example, does anyone really believe that "no one" knew the west coast would have a shortage of power? Why is gasoline priced at $1.9899 instead of $1.99. Does Viagra really "turn back the clock" or are we pumping air into a tire with a hole into it.
I view this just as the many thing we as a race do collectively to catch the consumers eye. Don't forget, thats the first step in marketing. Whether it's the gen mfgers', gas comp., or car dealers. We have to keep our minds sharp to see through the smoke and mirrors.
CAC
RE: Generator Output vs. Power Factor.
In my experience, the limiting rating factor in all engine generator sets is the output power (kW) of the engine. The rated real power (kW) output divided by the rated power factor will always give the rated apparent power (kVA) of the genset.
The package assembler (who may or may not be the engine manufacturer) will start by selecting an engine from the range available to him. The next step is to purchase a generator (as the engine manufacturerer is not a generator manufacturer). There are some prime considerations in selecting the appropriate generator -
- Ability to accept the engine rated output power (this is rated kW, not kVA)
- Ability to provide the rated power at rated power factor (this is not universally 0.8; could be 0.85 or I am presently working with steam turbogenerators rated for 0.95 and 0.93 PF)
The generator manufacturer builds a machine that is based on a kVA rating. The capability curve (see above discussion, particularly the post from jack6238 on Mar 25) follows from the machine impedances and the excitation, as well as the rated stator and rotor limits.
I don't have either BS5514 or IEC34 available to me. Can you please quote relevant extracts that speak to your point? I am sure that all would be interested.
When you purchase a relatively large genset, it would be prudent to specify a works and/or site load test, with appropriate instrumentation, to verify the contracted rated output, efficiency, temperature rise etc. This should go a long way to alleviating your concerns - there are many ways to keep manufacturers honest and we engineers have a primary duty to ensure that equipment is supplied in accordance with specifications. Performance of this duty may include some or all of the following - preparation of the specification, evaluation of tenders, review of supplier documentation (which should include a generator capability curve and full rating data), inspection during manufacture, witness of works tests, inspection of installation and performance of commissioning tests on site.
RE: Generator Output vs. Power Factor.
This calls into question what 'safe capacity' and 'true output' from a generator means.
Capability curves never reflect what large process loads do (in terms of starting, harmonics etc).
Specifying a site load is half the answer, specifying starting requirements for large loads coming on-line with process running and for 'black start' is just as important, as is the general type of the process load.
RE: Generator Output vs. Power Factor.
If some manufacturer indicates its rating as 2200 kVA and unity pf, which normally nobody does and if somebody does it it must be able to generate 2200 kW. If it can not be generated it means manufacturer's warranty not being fulfilled.
As far as the black start of the generator is concerned, it is the part of the magnetic property of the core. I do not know how it can be read though the capability curve. If some body knows how it can be noted from the capability curve, it will be worth a star. I am myself trying to see documents with such stuffs as the inertial effect of prime mover and the generator rotor and flywheel if any.
RE: Generator Output vs. Power Factor.
How can you get 2200kVA out of a 1760kW machine?
The most you can get is 1760kVA (and this is pushing safe limits - overloadability against efficiency). If you had a load running at 0.8pf the 'absorbed power' is 1408kW, this would then give you around 1250kW of output process power.
Is there a flaw in this argument? :)
btw the 'base' load of the gen set would be 1600kW.
What is the correct 'wattage' of the prime mover?
RE: Generator Output vs. Power Factor.
Rating - VA output at assumed 0.8 pf.
Which lets the cat out of the bag.
The original question is that the 325kW generators are rated for 406kVA at 0.8pf.
However the IEC definition does not (by itself) take account of VA ratings above 0.8 pf.
At a pf of 1 your VA rating could (and normally) is 325kVA.
At a pf of 0.9 your VA rating could (and normally) is inbetween 325kVA and 406kVA. Your VA rating decreases with increasing pf. It is a 'flatline'.
So while your VA rating at 1 pf is 325kVA the name plate rating is allowed to be 406kVA.
Make sense?
Do you agree this is commercial mischief?
RE: Generator Output vs. Power Factor.
I think that there is a basic misunderstanding here. The generators are rated 325 kW, 0.8 power factor, from which the 406 kVA rating follows. There is no question that the real power rating is anything other than 325 kW. This is the rated power output which can be delivered at the terminals of the genset.
The 406 kVA rating is the capacity of the generator, without regard to the prime mover. In other words, if the engine were capable of producing 406 kW, the generator would be rated to supply that power at 1.0 power factor.
The only problem that I can see here is that you are apparently confusing real power (kW) with apparent power (kVA).
Soorya is of course perfectly correct in stating that 1760 kW at 0.8 PF is the same as 2200 kVA. The flat line mentioned is the rating of the prime mover - the actual generator capability curve is an arc of a circle whose radius is the rated kVA of the machine, but the combined characteristic of the generator + prime mover limits this to a straight line. Please refer to the link provided much earlier in the thread by jbartos for info on this topic.
RE: Generator Output vs. Power Factor.
This would be good if you are powering a purely resistive load as the prime mover and generator run at full load and highest efficiency. Realistically though, you will be supplying power to a load with mixed real and reactive power requirements. Using the above generator with a 0.8 pf load will obviously allow an output of 365kw and 244 kvars for a total of 406kva. Here is the problem with that. You are using a 406kw prime mover to provide 365kw of power to the load. This is pretty inefficient.
The fact is that the efficiency of a generator is pretty high over wide range of load when compared to that of a prime mover. That, coupled with the fact that pf < 1.0 is the rule, not the exception, leads to the standard practice of sizing the prime mover for desired kw and then oversizing the generator to allow for the inevitable reactive power requirements of the load. In fact, this practice is so prevalent and makes such good sense that, as you pointed out, the IEC has defined the rating of a generation unit as KVA at pf = 0.8. Generators are, as pointed out already in this thread, rated a pf's other than 0.8, but the idea is the same and the following guidelines still apply.
The following guidelines apply when looking at generator ratings. For example: the rating 406kva at 0.8 pf provides enough infomation to know the following:
- will produce 365kw and 244 kvars at 0.8 pf
- might not produce more than 406 kva at any pf (armature current limitation)
- might not produce more than 365 kw at any pf (prime mover output limitation)
- might not produce more than 244 kvar at any pf (excitation limitation).
Now, when I say "might not" above, I mean that the basic rating doesn't give you enough info to know so you must assume that the rated outputs (365kw and 244kvars) are limiting. To know more, you must refer to the generator cabability curves to see specifically how the various limitations of the system affect output for a given pf.
I hope this clears things up a little.
RE: Generator Output vs. Power Factor.
1760kW is the 'prime mover' mechanical rating at its 'peak'.
<<O.K.>>
How can you get 2200kVA out of a 1760kW machine?
<<Simply, mathematically by
1760kW/2200kVA=0.8 = power factor
Physically, by connecting 1760kW prime mover to the genset rated 2200kVA, and 0.8 PF and neglecting smaller kW losses. This does not mean that the prime mover cannot be more powerful to satisfy the maximum generator kW resulting from the genset capability curve, e.g. 1850kW or so. It is necessary to remember that the kVARs in kVA**2=kW**2+kVAR**2 are inactive or reactive power, which may be hard to understand without adequate amount of education in electrical engineering.>>
The most you can get is 1760kVA (and this is pushing safe limits - overloadability against efficiency).
<<Please, could you prove it or provide some references?>>
If you had a load running at 0.8pf the 'absorbed power' is 1408kW, this would then give you around 1250kW of output process power.
<<Please, could you prove it or provide some references?>>
Is there a flaw in this argument? :)
<<Yes, missing proofs or references.>>
btw the 'base' load of the gen set would be 1600kW.
<<Please, could you prove it or provide some references?>>
What is the correct 'wattage' of the prime mover?
<<See above.>>
RE: Generator Output vs. Power Factor.
Your electrical system is then 'geared' around these power factors, protection system, cable ratings etc.
If you switch to a generator supply you would want it for those power factors.
To say a power factor of 0.8 is common for a process plant is true. But to say this acceptable for the rating of your generator I don't believe is a good thing to be advocating.
Most generator capabilities hit the rated VA curve only for an extremely limited power factor region. Around 0.75 to 0.8. After that you are guessing without the capability curve.
Generators should provide the VA rating written on the nameplate for at least 0.8-1 pf.
This would be based on a more 'realistic premise' for where 1 pf is the rule not the exception.
Agree?
RE: Generator Output vs. Power Factor.
1. Assume your plant is being supplied from the public supply. You have improved power factor to say 0.95 - very admirable. Further assume that your generator is rated to carry the real power load of the plant (say 325 kW at 480V) and has a rated PF of 0.8. This means that the generator is rated for real power = 325 kW, reactive power = 243 kVAR.
Now, you lose the public supply and switch on your standby generator. When you have switched on all the plant loads, 325 kW at 0.95PF, the generator will be operating at real power = 325 kW, apparent power = (325/0.95) = 342 kVA, reactive power = 105 kVAR.
As you can see, the load is well within the rated capacity of the generator. Increasing the power factor actually decreases the load on the circuit. When supplying an isolated load, the generator PF is set by the load PF.
2. The generators in question here (originally) are operated in parallel with the public supply. In this mode of operation, the power factor at which the generator operates is fixed not by the plant load, but by the excitation level of the machine - the bus voltage is largely fixed by the incoming public supply and variations in machine excitation will determine the reactive power supplied by the machine.
For parallel operation, you need to operate at a lagging power factor to avoid the possibility of pole slipping and out of step operation following system disturbances - for sure, you need to avoid a leading PF or one close to 1.0. This is a general statement that will apply to all "small" generators operated in parallel with the grid.
3. As stated above in several posts, generators willsupply their rated kW at rated PF. They will supply their rated kVA at power factors higher than rated, which actually results in reduced generator loading.
I hope this helps to bring us a little closer to closure on this item!
RE: Generator Output vs. Power Factor.
For me the .8 pf rating on generators is outdated, unless I have an old site with a lot of old motors. I have been caught with someone reading the full load amps off a generator nameplate - and thinking this is the amount of amps he could get out of the gen set - but not thinking about the prime mover!
My generator spec says that the prime mover is to be rated (oversized) to handle the generator KVA rating I specifiy at .9 pf load. As electricals I think it is best to deal with KVA's and PF's - and leave KW out of it - after all we need to deal with total amps on the line. Let the manufacterer pick a prime mover to get you the amps you need at the highest PF you expect.
RE: Generator Output vs. Power Factor.
RE: Generator Output vs. Power Factor.
Real Power: Does work or Makes Heat (kW)
Reactive Power: Energy stored in Electric and Magnetic Fields (KVAR) +/- 90* from Real Power
Apparent Power: Vector Sum of Real+Reactive Power
This is why a Generator puts out more amps than a simple P=IE equation would suggest
Of course At 0 pf (impossible) a generator could theoretically deliver infinite KVA! If only we could eliminate resistance completely! :)
Then again to avoid consuming fuel the prime mover would have to be absolutely frictionless.
Apologies in advance for quoting textbook gibberish above, but I can confirm by personal experience that both capacitance and inductance are very real and can shock the heck out of you!
Also note: at very poor (ie <.8) power factors KVA can increase (as above), at least to the current limits of your cables, and/or windings. Output tends to drop to 0 when these are exceeded too much or too long, and the Fire department has to be called. So the capability curve is a composite of the electrical and mechanical capacities of the machine.
I really enjoyed this post, very lively, -Dan76
RE: Generator Output vs. Power Factor.
kVA = kVA x PF + j kVA x sin(fi) = kW + jkVAR = kVA x cos(90°) + j kVA x sin(90°) = kVA x 0 + j kVAR = 0 + jkVAR, in kVARs
Therefore, the kVA can be infinite for kVAR equal to infinity, i.e. for kVA equal to infinity.
Remarks, any harmonics are not considered for simplicity.
RE: Generator Output vs. Power Factor.
your message
"3. As stated above in several posts, generators will supply their rated kW at rated PF. They will supply their rated kVA at power factors higher than rated, which actually results in reduced generator loading."
-This must be a typo or I am reading it wrong - I think we have made the point already that generators are rated at an 'assumed' power factor of 0.8. The name plate rating of 406kVA, 325kW, @ .8 power facter means it will give you 406kVA only at 0.8pf. At unity power factor it will give you 325kVA. Note that the 'rated' kVA (406kVA)is not supplied at power factors above 0.8.pf!
Generator manufacturer's don't even have to put 325kW, 0.8pf on their units - 406kVA will suffice! And you have to 'be in the know' that this really means at 0.8pf only. This is commercial mischief.
As we have already made the point 0.8pf does not reflect power factor in countries where the laws make it very costly to have poor power factors - public supply utilities do not want the current ratings of their networks increased by poor installation practices.
How can you people support/accept this practice?
RE: Generator Output vs. Power Factor.
RE: Generator Output vs. Power Factor.
Let's try this one more time, then I promise I'm done with it.
Taking your example, and assuming 480V rated voltage:
406 kVA @ 480V gives rated current of (406/(sqrt(3)*0.48) = 488A. This is the rated current of the generator and this WILL NOT CHANGE with increased power factor.
What limits the generator output at 1.0 PF is not the generator capacity, which remains at 406 kVA or 488 A, but the engine output, which determines the rated 325 kW. The generator capacity is higher than the engine output at power factors greater than rated, as the limiting factor in this portion of the capability curve is the stator current heating (rated 488 A in this example). Below rated power factor, the limiting factor is the rotor current heating, which results in reduced KVA capacity at power factors lower than rated.
If you bought the above genset, then you must be aware that you will never get more than 325 kW out of it, no matter what power factor it runs at. This is a function of the engine output only, and there is nothing at all misleading if the [b[generator nameplate reads 406 kVA, 0.8 PF, 325 kW as this would be completely correct.
I realise that this may fail to convince you, but I sincerely urge that you do some further thinking and research on the matter. The website posted earlier in this thread by jbartos should help somewhat in understanding the subject.
RE: Generator Output vs. Power Factor.
1. Check with Manufacturer how the prime mover and generator have been married powerwise
2. Theoretically, the prime mover can have smaller, equal or higher shaft kW power output then the generator rated kW output at PF=1.0.
3. There are no rules how Manufacturer shall marry the genset. Things may be fairly free, and ultimately "money talks."