Generators and leading power factor

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
 
Z

Zafar Mahmood

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.

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
 
Z

Zafar Mahmood

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.

Thanks...
MNM
 
K

krishan Bhardwaj

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.
 
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)control.com 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 control.com 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.
 
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.
 
C

Chris Dulnik

In a leading PF load situation it is the load that reverse excites the engine generator causing the voltage to rise.

The voltage will continue to rise very quickly, limited only by the impedance of the wiring and the alternator. As the voltage increases, so do the amps to the load, tripping the output c b. Adding inductive reactors is the only way to compensate the Xc reactance.

>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.
 
very well said. to overcome with this problem of underutilized the DG alternator capacity, can you please tell how we can restrict our load on DG at 0.8PF?
 
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