generator output vs power factor

7/28/2019 Generator Output vs Power Factor

1/16

Guest (Visitor) 13 Feb 0116:07

I work for a small municipal light Dept. in Massachusetts, USA. We have three 3412 Catarpillar generatorsets (325kW each) for peak shaving. The 3 phase generators are rated at: 406kVA, 325kW, @ .8 powerfacter. The generators are brushless, revolving field, with a solid state exciter. When we run these unitsthe 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 sayswe are overloading each unit by 20%. He claims that if the power factor is above the nameplate 0.8, youhave to de-rate the output kW. Now this seems backwards to me, if anything I would think you couldrun one of these units at 406kW due to the .99PF. He insists that I'm wrong and because this isgeneration 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 43meter.

Any Thoughts...

Thanks all.

Chris(2)jbartos(Electrical) 13 Feb 01

19:42

Suggestion: Obtain the "generator capability diagrams (P-Q diagrams)" for your generators from the

manufacturer and try to stay inside the loop curve. This will guarantee that the generator is notoverloaded.

(2) SooryaShrestha(Electrical) 14 Feb 01 7:36

As rightly responded by jbartos the exact condition whether active power can be above the name platerating depends upon the the shape of Capability Curve. I have, however, tried to present here some ofthe pertinent points. In the Capability Curve, the active power at rated power factor is the real rating ofthe generating set. It may be maintained the same or even higher active output at the unity power factordepending upon the type of prime mover, because the controlling factor for the active output above the

rated power factor depends upon the capacity of the prime mover. Generally, the diesel generating setsare not rated above the required output capacity, which is in terms of the active power. So when thegen. set is run above the active power output the prime mover is overloaded or will not be able to supplythe required power. Now if the prime mover has the capacity above the generator rating plus the losses,

then it can be loaded above the name plate active power rating as in the case of the hydrogenerators,which will have the turbines as prime movers in the majority of cases rated higher. This will be very clearwith the capability curve.

So I would say your mechanic is not wrong with his experience, because the prime movers he dealt didnot have overloading capacity.

(5) peterb(Electrical) 14 Feb 01 9:27

I'm not too sure that you will be able to get the capability curve for such a relativley small generator, but

bartos & powereng are both correct. In reality, the generator capability above rated power factor is

limited by stator heating (read limited to rated stator current, or kVA), and also by the prime moveroutput capability. Operating at rated kW and higher power factor (read lower kVAR and also lowerkVA) will not overload the generator. Be guided by the generator metering for kW, PF and A.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 thesupply transformer, the PF will include transformer reactive losses. These will be decreased when theplant load is supplied in part by the in-plant generation, hence the increase in PF.

rhatcher(Electrical) 16 Feb 01
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2/16

13:30

With respect to the Diesel Generator Set load capabilities, several excellent post have addressed that. I

am very interested in the statement:"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 theprimary units (kW and kVA)? Also, are you the sole supplier to your grid or are you interconnected withother stations?- If you are a generation station, when you say that the "plant" power factor increases to 0.99, are youreferring 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 currentoutputs 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 interconnectedgenerator 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) beforebringing the peak units on line being supplied from now? Have you shed that load onto another station oris 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 primaryunits to supply the additional kVARs, ie. primary unit PF went down in the balance...)

jbartos(Electrical) 19 Feb 01 0:58

Suggestion to peterb Feb 14, 2001 posting.1. Please, notice that the generator capability curve also has a field current limit and associated heat withit. 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 thestator end-iron heating limit.

4. Visithttp://www.usbr.gov/power/data/fist/fist1~4/1~4_21.htmfor more info5. 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 formore info:http://www.cat.com/products/engines_n_power_systems/spec_sheet_library/EPG/_epg/60.html

peterb(Electrical) 19 Feb 01 9:33

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.

For the case in point, I did not imagine that underexcited operation was contemplated, but for the recordit is probably not a good idea nor necessary for a plant of this size to operate in the underecited (leadingpower 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 thinkthat field current limitation in the overexcited (lower power factor) region of the curve is a factor in this

discussion.

rhatcher(Electrical) 20 Feb 0119:43
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3/16

I have to go with jbartos here. I did not visit the provided web links but I support his statements as trueto fact. In my experience I find the terms capacitive and inductive ambiguous when referring togenerator power factor. However, I understand jbartos in the context of his response to mean that anoverexcited generator is capacitive in that it has a leading power factor. His assertion that this isapparent from the generator V-curves is correct. I define, as do I think he does, an overexcited generatoras one whose terminal voltage is fixed by interconnection or synchronization with other generators (basic

premise of V-curves) and whose power contribution to the load has a power factor of greater than 1.0.

I also disagree with the idea that a generator whose excitation (power factor) differs from that of the busor 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 interconnectedgenerators. Any degree of real or reactive load sharing is possible between the generators. Of course, in

the extreme it is possible to overexcite the systemsuch that KVARS circulate between generators. It isalso possible to overdrive (prime mover throttle) the system such that real power circulates betweengenerators (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 devicewould be called a "synchronous condensor".

peterb(Electrical) 21 Feb 0116:07

Responding to rhatcher -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 tothe 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 moverpower is increased, the real power delivered to the utility bus increases. Similarly, if the excitation currentis 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 realpower delivered to the utility bus and decreasing the excitation current (in the direction ofunderexcitation) decreases the reactive power delivered.The generator will not be supplying circulating kVARs - each generator will supply the amount of kVARsdetermined by its excitation current and system conditions. If the excitaton current on a machine isdecreased, there will be a point where the generator is absorbing kVARs from the utility system - this isan underexcited condition and will give a leading power factor measurement at the machineterminals. Reference to any generator capability curve will confirm that the machine has some capabilityto 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 systemvoltages under light load conditions.The synchronous condenser is essentially a synchronous motor that supplies lagging kVAR to thesystem. 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 synchcondenser appears as a capacitor to the system - it supplies kVARs, just as a static capacitor does.

don01(Electrical) 23 Feb 01 8:07

hi guysJust 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 withexcitation > shaft load.
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4/16

If the pf of the load is climbing to .99 when sets come on line are the cat motors driving above loadrequirements and working in a leading pf range?Something doesn't feel right

What have I missed?

pjs(Electrical) 1 Mar 01 9:23

I am agreeing with you, that as the power factor is increased, up to 0.99 lagging, you can draw moreuseful power , i.e. active power in electrical terms. It may be noticed that the current rating of the DGsets are generally rated at the power factor of 0.8, but it does not mean that you can not operate thesystem at higher power factor, till the maximum limits of lagging side. As long as your power factor isnearing 0.99 lag, you can draw the power at maximum efficiency & maximum output torque. If you are

interested in the curve of torque & efficiency against power factor, please ask for the same.

rhatcher(Electrical) 1 Mar 01 21:03

I have been away and need to correct my post of 2/20. With parallel generator sets, increasing theexcitation of an individual set will cause it to assume more of the reactive load and thereforehave a more lagging power factor. That is a fact which I erroneously contradicted. I must have leftmy brain at work that day(haha!). The only thing worse than posting an error is to do so while"correcting" another post. I applaud perterb for catching it right away and setting the record straight (I

had to give a star for that!). I apologize to the group for any confusion which resulted from my post.

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?

davidm(Electrical) 2 Mar 01 23:39

The mechanic that we are interviewing to maintain these units says we are overloading each unit by20%. He claims that if the power factor is above the nameplate 0.8, you have to de-rate the output kW.

I would take a look at the vendor documents, and especially a capability curve if available as suggestedby others, but I would be VERY suprised if your mechanic wasn't in error about this. The generatorcurves I have seen decrease MVAR limits as you go away from a unity power factor (either lagging orleading). 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 beoverloading 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 406kWdue 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 needyour vendors gen capability curves, and maybe ratings of your diesels. I would give Cat a call on thisone. If I had no supporting information, I would never exceed my generator nameplate KW even above0.8 lagging pf. And since you haven't been exceeding 325kw per your description, I think you are goodto 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 voltageregulation/excitation at the Basler web site. Not sure of which document, but one has a good tutorial ongenerator basics, including capability curves, protection, etc:http://www.basler.com/html/dwntech.htm

SooryaShrestha(Electrical) 4 Mar 01 2:55

In this thread, I think the question regarding the power factor jumping from 0.85 to 0.99 is stillunanswered. It seems Experts in this forum raised doubts why the power factor jumped from 0.85 to0.99. I am trying to answer the question. I expect comments from our experts.
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5/16

First of all, let me put this way. The active power flows from one node to the other if there is differencein phase, as active power flow is directly related to the Sine of the power angle, which is nothing but thephase angle difference between the voltages of the two nodes. Of course, the active power flow dependsupon 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 ofthe 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 pfconnected to the grid. This means it is supplying 325 kW and 201 kVAR. This will give fixed terminalvoltage 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 atthe 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 willdictate the same reactive power supply. Of course the synchronous impedance of the machines affectslittle 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 thethree 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 isexplained.

The explanation made by davidm will be true only from generator point of view, but not from primemover point of view. This has been explained in my earlier response.

jbartos(Electrical) 5 Mar 01 7:44

Suggestions: There appears to be a lack of References in this thread. Therefore, I am adding some tosupport my postings and their compliance with some References.

References:1. Slemon G. R. "Magnetoelectric Devices Transducers, Transformers and Machines," John Wiley andSons, Inc., 19662. Stevenson Jr. W. D., "Elements of Power System Analysis," Third Edition, McGraw-Hill Book Co., 19753. Fitzgerald A.E., Kingsley Jr. C., Umans S. D., "Electric Machinery," Fifth Edition, McGraw-Hill, Inc., 19904. 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 ReactivePower Qs into Synchronous Machine at Various Constant Values of Field Current. Terminal Voltage EsConstant," that is based on complex power entering machineUs=Ps+jQs=Is x Es*. The left half plane (Ps0) coversmotor. 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 elementdrawing 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 currentflows from the capacitive generator, then the current lags voltage. However, the Reference 2 Paragraph2.5 "Complex Power" defines it as S=P+Q= V x I* which is different from Reference 1. It states that "Toobtain 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 ANSIstandard. Reference 2 Table 2.1 shows:Generator action assumed:If P is +, emf supplies power

If P is -, emf absorbs powerIf 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
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6/16

If P is -, emf supplies powerIf 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-cooledturbine generator." Shows the Per-Unit Reactive Power (lagging) versus Per-unit Power with field heatinglimited 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. 5.44that 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 capabilitycurve 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 powerdefinition (P0 and capacitive), Reference 1, and in the lower plane for S=E x I*=P+jQ power

definition (P0 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 tothe

overexcited (inductive kVAR?????) portion of the curve and the stability limit applies tothe

underexcited (capacitive kVAR????) portion of the curve.

peterb(Electrical) 5 Mar 01 11:15

bartos, please read my Feb 21 post for clarification of the overexcited vs. underexcited condition.

SMLD(Electrical) 7 Mar 01 16:17

Hello everyone,

I'm Chris Courville, the one who initially started this thread. The responses to this thread have been veryhelpful, 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 israted 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 setbeing overloaded, but right in saying I could not get more kW out of the set due to the prime moverrating. 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, andthree generators paralleled. Also note that these generators are backfeeding through the stationxfmr(1.5mVA 480vac to 13.8kvac) onto our distribution circuit. This is what we would do in a peakshaving situation. When I shut the gens down, the pf goes to .76, .56, .82 for phase A,B, and Crespectfully.( These measurements were taken at the station xfmr). After about 5 minutes, the generatorcooling 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.

don01(Electrical) 12 Mar 01 7:17

SMLD
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7/16

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 atthe 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 youdon'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

jbartos(Electrical) 12 Mar 0119:27

Suggestions/Comment:1. It is nice to read that the solution was found and was acceptable. As far as the power factor increaseof 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 issensed by the generator regulators that may interact with the speed governor. One may have aconsiderable 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 T4300by Selco USA, Inc. Therefore, any significant change, i.e. an increase, in the power factor, while thegenerators 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 fieldheating 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 upsidedown, which some manufacturers show.

jack6238(Electrical) 25 Mar 0112:33

For ChrisI 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 machineshows 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 .8lagging to .996 leading and it passes through 80 % of rated KVA. This is a clear indication that theengine has less capability than the generator. I also have a note in my files from Cat. stating that theengine 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 Iwill forward this curve. The curve is give in % rated KVA and I believe it will be suitable for your unit.

SooryaShrestha(Electrical) 25 Mar 0122:25

I am also interested to see the Cat. capability curve. I am planning to collect such capability curves ofdifferent makes, specially small ones.If possible, please email in my [email protected].

Guest (Visitor) 19 May 01 8:28

Generator manufacturer's incorrectly 'badge' their units with an incorrect usage of the term power factor.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 willnever safely deliver 2.2MVA of power.

Furthermore the term 'watt' as applied' to electrical systems is meaningless. If you have a purely reactive
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8/16

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 noreference to load).

jbartos(Electrical) 20 May 01 1:33

Suggestion to the iain1 May 19, 2001.

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 productincluding the check of the generator capability curve, and other curves?4. What about the competition in the generator manufacturing industry?

don01(Electrical) 21 May 01 8:59

hi jbartosI 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 matchreality. 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

jbartos(Electrical) 21 May 0117:42

Suggestions: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, 2001posting (How else would he know? What did he believe then?).

SooryaShrestha(Electrical) 22 May 01 1:09

I have sympathy to the feelings of iain1 May 19 posting. But we are power professionals and not

philosophers or poets. We firmly believe on facts, figures and data and proper interpretation of them. SoI think if somebody says the term "watt"is meaningless, then we need to think that it must have been

said by non-professional or with genuine reason. I think iain1 must be a power professional. I like tobring this again to the notice of the forum that there are facts in his posting, but some of the unexplained

facts can be explained like this.That under reactive power drawn from a generator will suck fuel, means the fuel has been used for someactive 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 windagelosses, 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 currentand voltage. So I do not think it is a bad idea of representing the active power with a factor and theapparent 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 thetechnicalities that might be used by the manufacturer to shut the consumers' mouth. So why are wepower professional here, other than make aware the consumer, of the repercussion of any spec.?

Guest (Visitor) 22 May 0112:37

Ask any generator manufacturer if they divide the true VA rating by 0.8 and badge it.

Read BS 5514.
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Watts is meaningless for pure electrical systems learn to think for yourself rather than sprout textbookgibberish.

peterb(Electrical) 22 May 0113:46

I have a few problems with the direction being taken here - am I the only one?

Generator manufacturer's incorrectly 'badge' their units with an incorrect usage of the term powerfactor. What is the correct usage of the term in this context?A genset that will deliver 1760kVA of powerat its terminals at full load and unity power factor is often 'badged' as a 2.2MVA genset (at 0.8pf). This iscommercial 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 reactiveload, no power(watts) is produced. However, a generator will still suck away your fuel! Even withoutany electrical load, the engine-generator set will certainly suck away your fuel just idling along. Thereare always losses and they are real. There is no such thing as a PURELY reactive load - there is alwaysgoing 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 noreference 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 textbookgibberish. I'm afraid that you will have to explain your meaning a bit here - "watts ismeaningless"?? "textbook gibberish"??

By the way, Soorya, you are a little off in your calculation - 1760 kVA at unity PF & say 4.16 kV equatesto 244.3A, while 2200 kVA has a rated current of 305.3A. What is the same is the real power of 1760 kWin each case (1760 kVA @ unity PF, 2200 kVA @ 0.8 PF)

electricpete(Electrical) 22 May 0121:58

my head hurts


Thank you peterb for the correction. It is my mistake.


All generator manufacturers divide the true output rating of their sets by 0.8 and badge this as the

'rating' of the generator.

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. Sincethe IEC reached agreement with CANENA this applies to people living and working in America. Although itwas common practice for generator manufacturers to 'make up' the rating of their generators even inNorth 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
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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 powerfactor in mechanical-electrical and electrical to mechanical energy conversions.

This is what I was hoping people in this forum would discuss.

SMLD(Electrical) 23 May 0114:24

I'm confused with iain1's posts....

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 [email protected] being synonomous.

I do agree with iain1 that Gen mfgs want to make their gen sets look as beefy as possible, but is this justone 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 ashortage of power? Why is gasoline priced at $1.9899 instead of $1.99. Does Viagra really "turn back theclock" 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


Iain1 -

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 therated apparent power (kVA) of the genset.The package assembler (who may or may not be the engine manufacturer) will start by selecting anengine 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 inselecting 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 Iam 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 impedancesand 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 speakto 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 riseetc. This should go a long way to alleviating your concerns - there are many ways to keep manufacturershonest 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
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specification, evaluation of tenders, review of supplier documentation (which should include a generatorcapability curve and full rating data), inspection during manufacture, witness of works tests, inspection ofinstallation and performance of commissioning tests on site.


If a generator manufacturer deems that his/her machine has a 'mechanical power rating' of (using the

previous example) say 1760kW, the manufacturer is allowed to 'badge' this as a 2200kVA generator (I'lldredge out the clauses, including Amaerican stds, when I'm next in the office). The generator will beviewed to have a 'safe capacity' to deliver around 1760kVA at unity power factor (although it is notadvisable to regulate the load power factor at unity with generator supplies - for risking leading powerfactors). This is overloadabilty traded against efficiency, and would suggest a higher 'true' kW rating of

the actual machine.

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.


I insist that technically, noting 1760 kW at 0.8 pf means same as 2200 kVA. In the case of generatordrawing 1760 kW at 0.8 pf, it actually means it drew 2200 kVA. But it does not mean it drew 2200 kW atunity pf. If the name plate indicates 2200 kVA and the pf is 0.8, it provides active power equal to theproduct. But if it indicates 2200 kW at unity pf, then it precisely meant something different. Thus 1760kW at 0.8 pf and 2200 kW at unity pf, as they both draw the same current, may mean the same thing. Ihave seen generator rated 14200 kVA at 0.86 pf. It not only means providing 12212 kW at 0.86 pf, it alsomeans it will provide 14200 kW at unity pf. We have tested the machine also. This machine is made by

ABB ENERGI, Norway. I have got capability curve also. Generally from the point of the rated pf and ratedactive power the curve will be parallel to reactive power axis, which means the active power does notincrease with the increase of pf. But in the curve which I am referring it is not parallel but it is a circle

with the origin as the center. If somebody is interested to see I can send it.If some manufacturer indicates its rating as 2200 kVA and unity pf, which normally nobody does and ifsomebody does it it must be able to generate 2200 kW. If it can not be generated it meansmanufacturer'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 notedfrom 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.


1760kW is the 'prime mover' mechanical rating at its 'peak'.

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 around1250kW of output process power.

Is there a flaw in this argument? :)

btw the 'base' load of the gen set would be 1600kW.
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What is the correct 'wattage' of the prime mover?

Guest (Visitor) 26 May 01 7:51

(For peterb's information) - the IEC 34 definition is -

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 ratingdecreases 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?


iain1 -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 otherthan 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 apparentpower (kVA).

Soorya is of course perfectly correct in stating that 1760 kW at 0.8 PF is the same as 2200 kVA. The flatline mentioned is the rating of the prime mover - the actual generator capability curve is an arc of a circlewhose radius is the rated kVA of the machine, but the combined characteristic of the generator + primemover limits this to a straight line. Please refer to the link provided much earlier in the thread by jbartosfor info on this topic.

rhatcher(Electrical) 26 May 0114:07

iain1, I do not agree. This standard rating is based on the realistic premise that the load served by thegenerator will have a power factor of less than 1.0. I understand that you are advocating that the primemover output should be matched to the generator output so that full kva rating can be achieved at pf =1.0. For example, a 406kw prime mover powering a 406kva generator so that the output can be rated at406kva at pf=1.0. (note that I am assuming perfect efficiency to keep thing simple)

This would be good if you are powering a purely resistive load as the prime mover and generator run atfull load and highest efficiency. Realistically though, you will be supplying power to a load with mixed realand reactive power requirements. Using the above generator with a 0.8 pf load will obviously allow an
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output of 365kw and 244 kvars for a total of 406kva. Here is the problem with that. You are using a406kw 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 tothat of a prime mover. That, coupled with the fact that pf < 1.0 is the rule, not the exception, leads tothe 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 makessuch good sense that, as you pointed out, the IEC has defined the rating of a generation unit as KVA atpf = 0.8. Generators are, as pointed out already in this thread, rated a pf's other than 0.8, but the idea isthe 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 mustrefer to the generator cabability curves to see specifically how the various limitations of the system affectoutput for a given pf.

I hope this clears things up a little.

jbartos(Electrical) 27 May 01 5:02

Suggestion/Answer to iain1 (Visitor) May 25, 20011760kW is the 'prime mover' mechanical rating at its 'peak'.

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 around1250kW 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?


What happens when you are working in countries where you are 'penalised' for having power factors less
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than say, 0.95 or 0.98, for supplies from public utility networks?

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 therating 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?


Just a couple of points for you, iain1 -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 theplant (say 325 kW at 480V) and has a rated PF of 0.8. This means that the generator is rated for realpower = 325 kW, reactive power = 243 kVAR.

Now, you lose the public supply and switch on your standby generator. When you have switched on allthe plant loads, 325 kW at 0.95PF, the generator will be operating at real power = 325 kW, apparentpower = (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 factoractually 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 thismode of operation, the power factor at which the generator operates is fixed not by the plant load, butby the excitation level of the machine - the bus voltage is largely fixed by the incoming public supply andvariations 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 poleslipping and out of step operation following system disturbances - for sure, you need to avoid a leadingPF 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 supplytheir 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!

bar01(Electrical) 29 May 0116:29

I think it is important to focus on the PF's for your facility. It depends on what kind of facilities you aredesigning. I have designed many high tech facilities. The power factor for these types of facilities is moretypically in the .9 to .95 pf range. With energy efficient motors, and electronic lighting, computers, etc..

PF's are approaching unity power factor. This will only increase in the future.
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For me the .8 pf rating on generators is outdated, unless I have an old site with a lot of old motors. Ihave been caught with someone reading the full load amps off a generator nameplate - and thinking thisis 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 KWout of it - after all we need to deal with total amps on the line. Let the manufacterer pick a prime moverto get you the amps you need at the highest PF you expect.

jbartos(Electrical) 29 May 0117:38

Suggestion: One can look at the generator capability curve as a set of points that uniquely determine theallowed generator loading. E.g. under certain angle fi corresponding to the power factor=cos(fi), there

will be a unique kVA value and the associated unique kW value from relationship PF=cos(fi)=kW/kVA.

Dan76(Electrical) 12 Jun 0119:35

I think the confusion here lies in the difference between Real and Reactive power.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 PowerThis is why a Generator puts out more amps than a simple P=IE equation would suggestOf course At 0 pf (impossible) a generator could theoretically deliver infinite KVA! If only we couldeliminate 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 thatboth capacitance and inductance are very real and can shock the heck out of you!

Also note: at very poor (ie


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facter means it will give you 406kVA only at 0.8pf. At unity power factor it will give you 325kVA. Notethat 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! Andyou 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 makeit very costly to have poor power factors - public supply utilities do not want the current ratings of theirnetworks increased by poor installation practices.

How can you people support/accept this practice?

stevenal(Electrical) 21 Jun 01

15:07

Consider the the kW and kVA ratings to be maximums. 0.8 pf just happens to be the point where both

values can be at their maximum. Below 0.8, kVA sets the limit. Above, kW does.

peterb(Electrical) 21 Jun 0117:12

Iain1 -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 thegenerator 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 or488 A, but the engine output, which determines the rated 325 kW. The generator capacity is higher thanthe engine output at power factors greater than rated, as the limiting factor in this portion of thecapability 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 factorslower than rated.If you bought the above genset, then you must be aware that you will never get more than 325 kW outof 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 becompletely correct.I realise that this may fail to convince you, but I sincerely urge that you do some further thinking andresearch on the matter. The website posted earlier in this thread by jbartos should help somewhat in

understanding the subject.

jbartos(Electrical) 24 Jun 01 0:26

Suggestions: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 thegenerator 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."
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generator output vs power factor

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