why power factor controlling not required for generators

Yes it does, sort of. It is a sad state of affairs, not to know the answer, But, at the same time it separates people, who operate a power station responsibly from those, who seem to be willing to blow it. It is definitely not a "getting interview right" question. If you are ignorant, get out of there, before something happens and you are criminally responsible.

A generator produces an inductive, neutral or capacitive factor, depending on its phase relative the network. It can do that, but since those factors only help to run the network and do not represent power that can be sold, it is in that sense wasted power.

The Power Factor is the factor of the load - inductive, capacitive or resistive not the gen.set. The gen.set will run at the power factor of the load. The mechanical load on the generator will depend on the powerfactor of the electrical load (kVA x pf = KW)

At 0.8 (~an electric motor) due to the recirculating current you have more amps in the system; so the gen.set engine does not see this. This is why there is a tendency to rate alternators at 0.8pf because it is showing them at there best. Once upon a time they were quoted at 1pf, what they will give on a resistive load bank. This is what the engine is giving less the inefficiencies. But when some manufacturers quoted output at 0.8pf it made their alternator look lighter / more compacts so others followed.

Strip lights etc. tend to have pf>1, this is more problematic for generating sets as the AVR (Automatic Voltage Regulation) can be upset by loads with a large pf. This tends to balance out with a mixed load (especially domestic) so rental gen.set output is often quoted between 0.8pf & 1 pf. We also have problems with early UPS / battery chargers as the spikes produced by thyristors firing upset AVRs added to the power factor.

I do not like the statement a generator (alternator if ac) will give enough volts until its insulation breaks down - are you not confusing output (power, i.e. amps x volts x pf) with insulation class, generally H & F in commercial alternators. Generating sets do not go on giving output until the insulation goes! Protection circuits will stop this before overload. The insulation is for fault / short circuit conditions.

This matter has been explained in detail.Please see the link you will get very good answer.

I only suggest every one Please gothrough all the posts in this forum, you will get the solution, before you post your queries.

http://cr4.globalspec.com/thread/13598#comment136363

To bluntly answer the original question, PF is dependent on load, not generation.

After reviewing the article you linked to here, I find it was informative, but some minor corrections should be noted. The article stated that the reactive power creates heat in the generator. Heating is a function of the resistance, not the reactance, and therefore can only be created by the real power component. Also, it stated that the engine would only see the real power as far as its own power output. This is incorrect, the engine sees the apparent power plus any losses.

One last note is that even though PF is load dependent, generators, as in the case of synchronous machines connected to the grid, can be used for PF correction. Once the synchronous machine is brought on line, as either a motor, generator, or synchronous condenser, the machine can generate either leading or lagging VARs by either under-driving or overdriving the machine's field.

I just voted yours a good answer - then I realised I am not exactly sure I agree with what you are saying in the first paragraph, sorry. Maybe I take it back!! Unless by apparent power you mean the kW.

The circulating current will produce some heat, any flow of current will produce heat [we are not dealing with super conductors], even recirculating current. The alternator will run at a temp that is effected by the current flow i.e. the Amps (a function of the load) however the engine load is the kW (plus inefficiencies of course) The engine only see the sum of the alternator currents (+ inefficiencies)

Hypothetically, ignoring cooling patterns etc: If one lowers the power factor of the electrical load the engine load (power output / throttle - not rpm) will go down - so the engine temp reduces but the alternator would see the same Amps, so will run approx. the same temp. [obviously in practice the engine power reduction would effect cooling within the system]

This is precisely why alternators are quoted at 0.8 p.f. because this puts them at their best i.e. the highest Amps output. The engine sees the total kW load, not the kVA, the VAR component.

A 1kVA alternator will only give 0.8kW at unity, so at full output this is a 0.8kW load (+ the inefficiencies) on the engine. It looks better in kVA so alternators are quoted that way.

For a rule of thumb when sizing an engine to an alternator the engine kW must be equal to or more than the kVA of the alternator, as by chance overall losses & overload allowance can be taken as ~20% [10%+10%] So at full load a 1kW engine driving an 1kVA alternator will give 0.8kW electric; with 10% overload 1 hour in 12.

"Your Strange teacher" notes:

1. Power losses in the generator windings = I *R but I = S/V = Sqrt(P²+Q²/V

and PF = P/S => S = P/PF so: I*R = P/(V*PF) in the single phase relations

Example: Change PF from 1 to 0.8 will increase losses 1.25 times (by 25%) at the same real power P

so Watt Losses are function of PF

2. The Engine /turbine e.i. any mechanical mover will be responsible for "shaft power" = generated real power + electric power losses + mechanical losses (e.g. in bearings). All Q on the electric side of energy transfer are electromagnetic in its nature not mechanical. Of course there is some small influence from PF and hysteresis and curry current losses in the electrical machine/generator.

Now I feel better, and you? Anyway, it is good not to believe even old dogmats

What an error!

Power losses in the generator windings = I *R < an error! -should be I² * R

but I = S/V = Sqrt(P²+Q²/V and PF = P/S => S = P/PF so: I*R = P/(V*PF) in the single phase relations

Example: Change PF from 1 to 0.8 will increase losses 1.25 times (by 25%) at the same real power P

CORRECTED: Losses = I² * R not I*R so... = P²/ (V * PF ²) and of course

Change PF from 1 to 0.8 will increase losses 1/0.8² times = 1.5625 (by 56%)

Well, when I get it it wrong, do I ever. I apologize to both JohnBob and Southern123. I wish I could remove the second paragraph of my first comment as I completely got things backwards. Perhaps either of you could recommend a good design for a hydraulic foot-from-mouth extractor?

After the delibrations I think you will agree with my posting.

To my mind, this reply obviates the need for such a mechanism!!! Thanks.

Don't despair - if you can get your foot into your mouth you are a very versatile man!

Seriously, that is what discussion pages should be all about, is it not? Get the old brain cells exercised (& the odd foot) as long as we have correct agreement in the end.

Many times I click "send" to see later errors, copied & pasted wrong paragraph etc...

So we should make gentlemen agreement not to be ashame because ERARE HUMANUM EST. In this Latin phrase I probably misspell a word too.

By the way Your adjective "hydraulic" could start another Intelligent Wars

This place (CR4) is so goooooddddd

JOE

P.S. I, personally, admire British Technical Language because they are avoiding any slang, idioms, Eskimo preverbs, etc. Somebody gave me FMI that Basic English 4 Foreigners = 300 sentences + 3K words. I recon (heehaw) this was a reason they had Sun over their heads 24 hours a day.

Mark Twain: Dear Editor use following ---- ____ ....... ,,,,, ::;;;; !!!!!! as many times as you like.

Did we hurt somebody? Patients??? I hope they survive in spite of us!!

Did you mean patience???

The generator is designed to give an out put at certain voltage, frequency and PF.Now the out is obtained at the designed value normally 0.8.Be it any generator large turbo or small generator of a DGset .The power factor is affected by the type of load.Normally Inductive loads which are predominant makes pf lagging and Capacitative far and few makes pf lead.At the load point if PF is low it is corrected .At generation point this is not required as pf is maintained at the designed value.

Sorry, no and no again.

A generator can be driven to any voltage, until its insulation breaks down. It can output higher voltage, than the load - being a generator, or less -being a load. Similarly, it can be driven to be a leading phase or a lagging phase versus the net. With that it appears to be inductive or capacitive in addition to generating power. A generator better be in synch with the net, or there is a major calamity.

The PF of a running generator is set by the types of load. If there is a great deal of inductive load from transformers or motors for example, as is quite often the case, the power factor will get progressively worse.

It is in the interest of the user (not the supplier) to apply some correction (in this case a capacitive correction) to bring the PF as close as possible to unity (1.0) as possible as that way he will only be paying for the actual power used and not for so called "Watt-less power"! He will actually save money.

The Generating people do not really care about the power factor quite so much as they will get paid more for their electricity than is really right!! So they have little or no interest in correcting PF.

A further point is that due to the oscillating currents between the "L" and the "C", they need to be as close together as possible. So correction must only take place near to the cause of the bad PF. Nowhere else!

That is where "L" = Inductance and "C"= Capacitance.

First of all: we are talking about asynchronous generators. Those can generate the voltage of the grid and any load can consume current with leading or lagging phase angle. So generator cannot have any influence on the power factor. If we would talk about asynchronous generators those need ((lagging) exciting current from the grid in order to generate voltage.

Let me try explain in a little different way:

1. Generators are to deliver electric power to the system (grid) - small to small loads, larger to grid (regional, national, almost global).

2. The loads create their combine PF (power factor = real power [kW] / apparent power [kVA])

3. Electrical system to be balanced (and it is the basic natural rule of the grid to work) require PF generator = PF system (grid) so PF generator must fast follow any changes of PF system.

4. If PF gen would be 'regulated/set/forced to the another value' to be different than PF system it will change the value of generated voltage to return to the "current balance or apparent power balance (basic rule of work). Check AC circuits Ohms and Kirchoff's Laws.

5. Loads in the system require only small deviation from their rated (nominal) values (another rule of work for effective load's activity) so any additional voltage drops on the transmission lines must be compensated by adjusting (not setting) generator(s) voltage and it is controlled via exiting generator winding)!

6. Somebody mentioned about 'synchronisation factor or fault'. It is the problem for the generator installed 'far away from the grid' e.i. connected to the grid by a long transmission line (high impedance).

7. Asynchronous machines -SCIM, used as a generators do not have DC magnetic field for exiting so their in natural way keep PF = PF grid. Asynchronous WRIM can act as a synchronous unit. Take/read Electrical Machinery course/book

Any more question Lieutenant Colombo?

Interestingly, and refreshingly all the above replies make some sense, as they illuminate the question from different angles. May I summarize and reduce them to bare bones by saying, that the power supplier can do anything (within reason) with voltages, leading lagging phases as they choose? But, since reactive power cannot be sold, but burdens the generator and transmission system, it is not in the economic interest of the power generators to deal with that?

Who thinks, it is a reasonable encapsulation?

I would add from my experience with supplying mobile gen.sets (up to 200kVA) to Electricity Boards, in the early days we did have power factor control for generating sets which they might use to 'prop up the voltage'. This is when a spur line is short of power, locally a gen.set is synced in to provide this extra power. This extra piece of kit for p.f. control was £1,000 or more; so a relevant percentage of the overall gen.set cost.

We found through experience that if the gen.set was only to be synced for short periods with an operator present the p.f. control was not necessary. A p.f. meter was fitted and using alternator output voltage adjustment p.f. would be brought to 0, neither lagging or advanced. If lagging the gen.set is being driven by the mains so some of the potential output is wasted, if in advance re-circulating current is being wasted. The mains voltage & p.f. are quite stable so the operator would watch the meter and adjust every few minutes, - it may stay stable for 5-10min.

Latterly we used auto-sync but early manual procedure was to set the gen.set frequency marginally higher than the mains [~0.2Hz] adjust the volts to mains voltage. A bulb would be connected between the mains bus and the gen.set output. It would glow as the differential rose and go out as the two were in sync. When one could predict the next time the bulb would go out the knife switch was closed! This is obviously dangerous and not for the faint hearted as a fumble could put one out of phase, this would result in a large flash with engine rev. change as the gen.set was pulled into sync. with the mains,! Obviously you cannot be many degrees out of sync. or the voltage differential would be too high - BANG!

Slightly off topic the other consideration is power control as the gen.set sees the mains as an infinite load, therefore a special 'output control' unit piggy backs the AVR to restrict the gen.set from overloading. This would be adjustable so we would reduce to 10% output before we disengaged from the mains to save contactor life.

So it depends on the application, with a stand alone gen.set power factor control is not needed, the expense & complexity of the control unit is more than the small inefficiencies, e.g. the alternator running slightly hotter due to re-circulating current or minutely higher fuel consumption.

When synchronising gen.sets together with each other or with the mains some form of control is needed to take into account p.f., together with load control, load share & synchronising.

You have not understood the problem correctly.

A Power supplier must supply voltage withing a relatively small range.

The frequency must be exact when measured over a 24 hour period, usually to within a single cycle.

The PF is not and should not be within his control, it is the problem of the USER only!!! If he has a load with a bad PF, then he pays more for his electricity than he should, his problem!!!

The Power Supplier cannot in all safety correct the PF in any way that helps him or the user, if he did, there would be massive circulating currents developed between say the inductance of the customer and the capacitance of the supplier, all along the supply cables.....

Also, what happens if suddenly the Inductive customer goes off load and a capacitive customer goes on load?????

Forget PF correction at the generator side!!!!

"The PF is not and should not be within his control, it is the problem of the USER only!!! If he has a load with a bad PF, then he pays more for his electricity than he should, his problem!!!"

Partially correct. Electric meters, the older ones at least, do not measure VAR. They only measure apparant power. Hence the user pays only for the power he/she uses, yet the POCO (power company) has to supply the actual (apparant plus reactive) power and is only getting paid for the apparant power being used. There are some hefty charges from the POCO for commercial/industrial users that have a low power factor for this exact reason. That is why it is up to the user to remedy a low power factor, it is in their financial interests to do so.

I mean to say: Hence the user pays only for the apparent power he/she uses,

It depends the way one looks at it - you can alter the generator to take into account the power factor of the load, in the generating set world this is called 'power factor control'. Many modules available commercially, but they are about looking after the alternator - not altering the load. I am talking about gen.sets <1MW; not stand alone but when synchronized with the mains or each other.

A used / electricity customer can buy equipment to correct the power factor of their load, this is a balance of the cost of the equipment and their electric bill. As you say suppliers have to be informed of unusual (often very low) p.f. loads.

Also you are correct about the frequency, it may vary slightly during the day but must be correct during a 24 hour period; the suppliers 'add' a few hertz during the night to cover any drops during the day. This is necessary as many appliances run on synchronous motors [frequency dependant] such as clocks, the time would be wrong if the frequency (count) was not correct!

The European Union are striving to achieve a common mains standard: after 1 January 2004 the UK mains supply should be 230V (-6%, +10%), 50 Hz (±1%). ie a range of 216.2 - 253V This replaces the UK's former 240V ±6% (ie a range of 225.6 - 254.4V) so we now cover 220V to 230V, the mainland European range, rather than 240-250V.

The downfall is you needing larger cables to carry the same power on the lower voltage, truth is the British just like being different (or is that difficult? ;-)

The person who asked the original question may find the following useful to read to help understand the discussion & p.f.:
www.froment.co.uk/Data/TheRoleofLoadBanksinGen-setTesting.pdf

Andy and John. It appears to me that my summary what CAN be done with a generator is essentially accepted. However, you two stress, that what SHOULD be done on the producer's and user's side is a quite different agreement. I agree wholeheratedly.

It was a pleasure.

And...CR4 awards go to....

Let's return to the First Question. And several answers... Did they help him? or will help in future interviews? Maybe, but according to my "theory of conspiracy" the job will get brother -in-law of the Interviewer or niece of his boss.

Those type of questions are for the reason of finding "elimination cause". Remember the question:

Your honor, have you finally stopped to beat your spouse? Yes or No!