Power Factor and Generator Stability

hi!

absolutely true. PF affects the performance of generator. PF near 0.99 lag is very near to leading condition of power factor. due to variation in loads, instanrtenously, pf may went to leadiing side, and at that time, generator will also act in a reverse mode i.e. like a motor. then trippings due to reverse power or uinder power or negative sequence current & voltage components, trippings may be there.

normally, after studying your load conditions / fluctuations, you may safely operate your system with 0.92 pf, with my experience. But all generators are rated at pf 0.8 . So, you may go as low as up to 0.8 pf, with sacrifice in efficiency.

What do you mean by "PF near 0.99 lag is very near to leading condition of power factor. due to variation in loads, instanrtenously, pf may went to leadiing side, and at that time, generator will also act in a reverse mode i.e. like a motor." ?

When applying power to a load, its power-flow direction is the same (generator -to-load) at all power factors. Even by going back-and-forth between inductive (lagging) - ohmic (unity) - leading (capacitive) power factor load, the machine does not change from generator to motor state.

The only case in which a machine goes from generator to motor situation is (in case of a machine connected to an electrical grid) if its input mechanical power is less than the sum of all losses (mechanical & electrical) in the machine.

manoj pati,

Harry has laid it out for you. Attempting to maintain .99 PF simply leads to down time due to the fact that your loads are not static. Any start or stop sequence also has dangerous resonant frequency crossings that can do great damage.

Which saves more for you;

less down time at a more realistic PF and efficiency ,

or more down time and the cost of the extra correction apparatus that cannot realistically maintain .99 PF anyway?

Remember, the difference between generating and motoring is a microscopic difference in timing between E and I, and the MRG2 relay is doing a good job of protecting your expensive generator.

Trying to walk the tight-rope of .99 PF is going to cost you more, with no real benefit to counter-balance your trouble.

Regards, CJM

Absolutely right. This is not practical to run a genset near unity PF, 0.90 is a good and safe choice. but I am also facing a problem. I recently installed a generator GAs ENGINES of 350kVa at 6600volts. it runs independently. I mean isolateed without synch with any other genset. Its currenet rating in HT side is 27 Amp Maximum. It runs very efficiently at about 150kW but as u cross this 150kw limit, its amperes suddenly jumps from 15 to 29 and it trippes.As it was a used genset, some parameters and monitoring system is not working as PF. I have asked my client to install power factor controller at LT side(load side). Do you think that the problem is correctly diagnosed by me or there is any other reason and remedy.Please reply

Guest, what is the nature of the load applied to go beyond 150kW? Is it having to do with starting a motor?

Ya, load is inductive like motors which are of the rating frm 15 to 30kW approx.But it does not mean that the load before ie150kw was resistive.All load is inductive but why behaviour changes after 150kW?

According to the instruction manual for that relay, adding a load of 15-20% of the generator rating may trip the vector surge protection of the relay.

The vector parameters are quite different for a setting of 1 ph and 3 ph also. The parameters may need adjustment for the load you are adding.

As for the PF leading issue, Auto Voltage Control is triggered to reduce exciter power, and if sufficiently reduced, excitation loss can even lead to slipping a pole -not good. The relay vector change protection is set to tight parameters to avoid all such damage. CJM

If the generator is rated 350 kVA at 6600 V (line-to-line) the measured current (15 A) leads to an (apparent) power level of 171 kVA, which is about 50% of the rated value (350 kVA).

The indicated limit of 27 A represents 308 kVA (88% of the 350 kVA stated)

If the real power level has been determined as 150 kW, the resulting power factor would be 150/171 = 0.877.

The jump in current to 29 A, would represent 331 kVA, also under the 350 kVA rating, but above the limiting 308 kVA ( But where comes that limiting 27 A ??)

What is the rating of the driving motor, because its rating (HP/kW) limits the amount of real power available (as input mechanical power to the generator) at that level.