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Generators and leading power factor
Leading power factor for backup generators

Hi friends,

At my office we have Diesel generators for back up during power failure. But as i am in IT sector my load is in leading power factor as all the servers are having in built capacitors. So when my DG set is on load my alternator capacity is derated as it is said that the alternator is designed at 0.8 lag so actually what is happening which is leading DG set to trip or derating the capacity.

1 out of 1 members thought this post was helpful...

First of all allow me to correct a mis-conception. When a generator is rated at 0.8pf, and you are opertaing at a higher power factor, eg. 1.0pf, you are not derating the generator, you are simply not utilising the full kVA of the generator. You would still be utilising the full real power the generator is capable of delivering. For example, a 500kVA 0.8pf generator is capable of delivering only 400kW of power. Thus if you operate this generator at 0.8pf, you will have a real load of 400kW and a reactive load of 300kVAR. If the same generator is operated at 1.0pf, it will have a real load of 400kW, and a reactive load of 0kVAR.

Having said all this, I cannot understand why your servers have been set to operate at a leading power factor, with the installation of capacitors. Setting at 1.0pf would have been enough. BUT be carefull, are the capacitors you are seeing installed used as power factor correction capacitors or as harmonics filters, due to the servers' power supplies?

In addition what is the condition that is causing the generator to trip? Most probably, if you have a large number of capacitors installed, your generator is tripping on over-voltage.

Dear Sir

The capacitors installed in the servers are for improving the PF as well as for harmonic filtration also.

But the thing is that practically i am not able to utilise the actual power because my dg set is of 2000kVA 0.8 PF so 1600kW and 2785 Amperes this is written on the name plate of the Alternator. But when ever i try to load the DG set around 2700Amps the generator trips on over load. Discussing with the vendor they reply that due to capacitive load the alternator is not able to deliver actual rated power and current. That is right that even if i switch off my APFC panel kept for correcting the pf to unity but it does go down that 0.95lag.

So actually what is the process taking place which is resulting in derating of the alternator. Also this vendors recommend to switch off the Capacitor bank when the DG is ON LOAD or else the DG will trip. That has also happened so what actually the reaction is taking place leading the alternator to trip.

Allow me to pass a comment.

If you are trying to load the generator with 2700amps at 0.95pf lag, you are actually putting on the generator around 1844kW, which is well above the 1600kW rating of the generator (actually what the generator prime mover is capable of giving). That is why your generator is tripping.

Irrespective of what pf your load is operating at, you are still limited to 1600kW as the maximum real power you can get from your generator set. The change in pf rating of the load will only influence what reactive load can be put on the generator in addition to the real power load.

By Bhagwan H Prajapati on 2 October, 2009 - 6:36 am

Dear Sir
Can we please tell how much reactive power can be loaded on a genset in my condition, your answer has given me clarity but my curiosity has increased to know more about this topic. How can we decide or it is already decided how reactive, active and apparent power can be loaded on a genset used in offices.

If you are running with a real power load of 1600kW, you can go up to 1200kVAr on your genset. For other loads lower than 1600kW, try to find what is called the generator SOAR (safe operating area chart) for your generator (either in your manuals or from manufacturer). This chart will tell you how much reactive power can be loaded on your genset at a given value of real power.

2 out of 3 members thought this post was helpful...

Bhagwan, I concur with Jojo's recommendation that you seek the manufacturer's SOAR... also known as its Capability or Performance chart in some parts of the world.

Your 02-Oct-09 requested permissible reactive kVAr loading with a leading power factor. It can be approximated using representative data for machines of this type:

o The diesel rating is probably around 2,500Hp.

o The generator is a 2,000kVA, salient-pole machine having 10-18 poles.

o Hence its cooling requirement, assuming it is unequipped with a forced-cooling option, severely limits leading PF operation while maintaining a 1,600kW output. My guesstimate is that the limit is about 15 to 20 percent, or about 300-400 leading kVAr.

o The above condition will require limiting output current to about 2,300 A.

o The AVR's minimum setting is probably too high to reduce the field excitation, thus requiring some means of manual control.

In closing, I question why the manufacturer has been unable to provide you with more detail.

Regards, Phil Corso (Cepsicon[at]aol[dot]com

check out here


what happens If i run the alternator of 25MW at 0.95 p.f lag. It is designed for 0.8 p.f.

is there any harmful effect on alternator?

1 out of 2 members thought this post was helpful...


I suggest you post a new thread. Provide detail additional detail like design vs operating permiters. Also include condition of operation... isochronous operation; parallel with another alternator; or in parallel with larger network (grid).

Phil Corso

Generators are typically rated in MVA, not MW. The MVA rating is a total load, real and reactive, not just real power (MW).

If I understand the post correctly, you are asking if you can run a generator at a power factor that is "greater" than the rating.

A power factor of 0.8 would mean higher VArs than a power factor of 0.95. A power factor of 1.0 is zero VArs.

Operating a generator at a "higher" power factor (that is, a power factor closer to 1.0) is actually better than operating it at a power factor with a numerically lower value. Power factor is akin to an efficiency rating, so a number closer to 1.0 is better than a number less than 1.0 (whether or not we're talking about leading or lagging power factors).

Every generator is provided with something called a reactive capability curve, or something like that title. That graphically depicts the limits of operation of the generator. Find the one provided with the generator at your site to see the limits of operation for the generator at your site.

By M M Ahsan on 8 May, 2010 - 6:25 am


It's better to run your generator at 0.95 PF lag than to run at 0.80 PF lag. The higher the power factor value (below 1.0) you run your generator at, is the better for your generator. May it be leading or lagging. The best is to run it at 1.0 PF with 0 VAr.

Too high lagging VAr (lower PF value) generates too much heat in the generator which has detrimental effects on the generator. And too high leading Var (lower PF value) can damage your generator seriously. Hope this helps.

- M M Ahsan

Dear sir,

if we have 2000KVA generator with 0.8PF on its name plate that doesn't mean we can take only 1600kw power with unity power factor. also, with unity power factor we can go up to 2000kw. also, means in any condition we can go up to 2785A. waht ever the power factor would be, the 0.8 pf notation is only indicating that for good life of generator we should not go for load having PF below 0.8.

1 out of 1 members thought this post was helpful...

Ronak... I disagree with your position that the generator's kW output can be (arbitrarily) extended to its nameplate kVA rating @ unity power-factor!

The generator's output is restricted by the prime-movers capacity!

Regards, Phil Corso

i agree with phil there, ronak. 0.8pf is not merely a guideline, its the pf of the alternator

By Wayne Bromfield on 26 April, 2011 - 4:50 am

Hi All

Please note, that unless your power factor that you have installed, is capable of switching in real-time, ie, within less than 1 network cycle, it will not be able to handle the switching from the utility to the DG.

If you want to run at unity Power Factor, the only system worldwide capable of achieving these results is the Elspec Equalizer.

Download a technical brochure from their website on :, or contact me on for more assistance.


Dear Sir

I found your comment very useful and helpful.

Would you please clarify a little more. May be I am asking the same question.

If a DG customer requests the test results of a DG at PF=1, what should be the expected value. E.g. for a DG of 250 KVA Standby Power (227 KVA Prime Power), what should be the 100% load output power (KW) at PF=0.8 and at PF=1. The customer insists that at PF=1, the full load KW should be equal to KVA.

I would be really obliged if you could kindly help and clarify.

>Ronak... I disagree with your position
>that the generator's kW output can be
>(arbitrarily) extended to its nameplate
>kVA rating @ unity power-factor!
>The generator's output is restricted by
>the prime-movers capacity!
>Regards, Phil Corso

By Zafar Mahmood on 29 June, 2011 - 9:08 am

Dear Sir

I found your explanation very useful. My question if related to the last line of your reply 1st para "if the same generator is operated at 1.0 PF, it will have a real load of 400kW and a reactive load of 0kVAR."

Is it not a mistake of "real load of 500kW and a reactive load of 0kVAR".

At PF=1, the real power will be equal to full KVA.

Would you kindly clarify. If possible, pls also send me an email at

Your reply will be very valuable for me please.
Best regards
Zafar Mahmood

1 out of 2 members thought this post was helpful...

Zafar Mahmood,

If you wish to communicate off-line with Phil Corso (, you need to send him an email with some information about yourself (company/school affiliation, primarily) and then he will decide if he will correspond with you (or not).

As Mr. Corso said (unfortunately), most generators are usually rated slightly higher than the prime movers driving them. This is so that if, under some conditions, the output of the prime mover is higher than rated that it will not be "over-driving" the generator. I have seen some generators that were not rated as high as the prime mover, but in those cases (when not over-ridden by site personnel) there were limiters to prevent the prime mover output from exceeding the generator rating.

Best of luck.

Dear Sir

Thanks a lot for your kind comments.
I sent an email to Phil Corso with introduction and a request for guidance. The email bounced with an error. I understand that email address is all correct. Any alternate email please.

Thanks and Regards
Zafar Mahmood

1 out of 1 members thought this post was helpful...

Zafar... I will be out of my office for a few days. Your e-mail was probably interpreted as "Spam", and sent to my "Your Attention Required" file. I will return in a few days, so hang on!

Regards, Phil Corso

1 out of 1 members thought this post was helpful...

Zafar... please repeat your request to:


Regards, Phil Corso

I found your explanation very useful. My question if related to the last line of your reply 1st para "if the same generator is operated at 1.0 PF, it will have a real load of 400kW and a reactive load of 0kVAR."

At PF=1, the real power should be equal to full KVA.

How do we justify that at PF=1.0, the real load is still 400KW. Why it is not equal to rated KVA of 500 at unity power factor.


Zafar Mahmood

2 out of 2 members thought this post was helpful...

Zafar... the comment >> if the same generator is operated at 1.0 PF, it will have a real load of 400 kW and a reactive load of 0 kVAr << was in Jojo's 13-Sep-09 (12:11) post!

Regards, Phil

Dear Sir

Thank you very much for your quick reply here. This is highly useful for me in handling my current problem with a customer.

And I am also extremely thankful for your detailed technical reply on my personal email after my direct contact with you as directed by this forum. You have been so nice to respond to my request with your valuable technical opinion.

Best regards
Zafar Mahmood

> Zafar... the comment

>> if the same generator is operated at 1.0 PF, it will have a real load of 400 kW and a reactive load of 0 kVAr <<

> was in Jojo's 13-Sep-09 (12:11) post!

>Regards, Phil

For understanding the operation of a Generator at a power factor other than its rated value (0.8 lag), please remember the following:

For PFs between 0.8(lag) to unity, the prime-movers gets over-loaded;
For PFs below 0.8(Lag) to zpf (lag), the prime-mover gets under-loaded, but the field and excitation system will get over-loaded.

Please check the suitability of prime-mover and excitation systems for PFs other than rated value. Of course, it is assumed that kVA rating remains constant at the rated value.


my dear gentlemen,

may i ask question, what makes prime mover burden itself? is it current, power factor, voltage, or kw? am sure kw is the product of voltage X current X power factor.

Our dear CAM,

Electric motors are driven by electric generators.

Electric motors are devices for converting amperes into torque.

Electric generators are devices for converting torque into amperes.

If one wants to "load" (increase the burden of) an electric generator one needs to increase the energy flow-rate into the prime mover of the generator.

A generator is really a dumb device. It can only convert torque into amperes. Apply more torque to the generator and it increases the amperes it is producing. Decrease the torque being applied to the generator and it will decrease the amperes it is producing.

You are correct: Power (watts) is the product of multiplying the voltage by the current by the power factor the generator is being operated at. Most generators produce power at a relatively constant voltage (usually within a very small range of the generator's nameplate voltage rating--approximately +/-5% of the nameplate's voltage rating). So, changing the voltage will have little effect on the total power output.

The power factor of a synchronous electric generator is varied by changing the voltage being applied to the generator's rotating magnetic field.

Hope this helps!

Our dear CAM,

Somehow I forgot to include that electricity is one method of transferring torque from one location to another. A large prime mover (turbine; reciprocating engine; etc.) can produce a lot of torque to drive one generator, which can be connected to many electric motors (and other "loads" including the 'virtual torque' required by computers and computer monitors) via wires to produce torque in many far-flung locations. That's what electricity is for: transmitting torque from a place where it's plentiful to places where it's needed or can be useful.

So, to increase the load (burden) of a generator one needs to increase the torque provided to the generator.

This all applies to a generator that is connected to a grid with other generators. For a generator that is operating independently of other generators (in Isochronous governor mode) as more motors and lights and computers and computer monitors are started and turned on the load (burden) on the generator will automatically increase to maintain rated frequency (speed).

There has been quite a lot written on (speed) about Droop- and Isochronous Speed Control (governor modes). There is a really nifty 'Search' field cleverly hidden at the far right edge of the Menu bar of every page. It's recommended to use the 'Search' Help feature as it's not exactly intuitive (it's actually kind of retro). Please have a read of one of the dozens of threads related to speed control if you're interested before asking questions about it (Droop and Isochronous Speed Control, that is).

Hope this helps! AC power generation is very simple, but it has many facets--speed control (which is directly related to frequency control) is one of the most important.

By krishan Bhardwaj on 11 July, 2012 - 5:26 pm

I am Krishan bhardwaj doing job in IT industry also. First of all i tell you D.G never trip in this type of case. only D.G incomer ACB will trip because reactive power will decreases, it may go in minus also. The main reason of leading power factor is a lot of UPS are used in IT industry which has capacitance load that's it.

hello sir

is this possible that power factor value is equal to 1 practically?


> is this possible that power factor value is equal to 1 practically?

we can attain average power factor of 1. We can come to near to unity by adopting Thyristor switched APFC panel instead of Conventional Contactor switching and with proper selection of capacitor ratings.