DG Set Power Factor

Read the manufacturer's documentation. Generally, if you increase the power factor, you must derate the rated power output.

Most power factor improvement is on the load side and not on the generator side. The DG generate the power to your load such as the power distribution board and from your power distribution board you can improved the power factor at there.

The power factor is translatable to the KVA rating through the KW of the DG set.

As mentioned (and in a lot of previous posts too) the pf has nothing to do with the generator, it is what the load demands and the generator has to supply it, whether it likes it or not

If you improve the pf, you still get the same KW, but the reduction in KVA (and hence the current) will improve a tiney bit in the internal losses and hence the heating of the generator coils.

sir thanks for your information. what it means DG set has a pf 0.8

The DG set does not have a power factor. It can drive a load say 500KW with a power factor of 0.8 (ie in other words the DG set is of 625 KVA)

So in another way to look at it, the load that you connect can not cross the KW as well as KVA ratings.

Sir, very nice and clear information.This is very much useful for all electrical engineers

Sir i know a low pf will decrease the loading capacity of Transformers and Generators that's why we measure their capacity in KVA or MVA. If we look on a transformer name plate there is no pf mentioned, but in case of a DG set its pf mentioned as 0.8. That make me confused, and if the transformer is loaded with a lagging load we use power capacitors to improve the pf. I think now you understand my problem, Sir I hope you will help me.

Regards

JK

Don't confuse yourself with transformer and DG set. Transformer does not generate electricity, it only step down from high voltage to low voltage or step up from low voltage to high voltage. That why the transformer does not have the PF in its name plate.

The reason why we use capacitor to improved the power factor is because our induction load such as motors that causes the low power factor. Low power factors contribute a lot of factors to the cable, transformer, power wasted and e.t.c. By improving the power factor it help to saves cost. Higher power factor reduce the current and hence the electricity bill also reduce.

Transformer is Current (or Voltage) in and out

It only changes the ratio and is least bothered about the power factor and hence power. In transformer equations

n₁I₁ = n₂I₂ or V₁/n₁ = V₂/n₂

you may see that the power does not come in picture.

Where as the generator (or a motor) is the one which is in active power region ie it transduces power not transforms its characteristics.

So for generator the basic thing is the power (KW) whereas for transformer it is KVA.

However here the second factor comes in- the generator current of course is decided by the power factor, thus a generator say 7.5KW/415V will be supplying 10.43A to a load with unity power factor,

but the same load if it has 0.8pf the current will be 13A and for 0.5pf it will be 20.9A.

Now you can see that the generator conductors are getting more and more stressed, and hence though the energy conversion it can handle, the conductors will not handle the current demand.

So we put capacitors that will handle the non-active (or reactive current) through the energy exchange and only the active one will be by the generator.

Hence the pf that is mentioned is only giving an idea of the current carrying capacity of the conductors used in the design (and also the excitation system that has to compensate for the power factor)

Hope it makes clear.

Sir you mean a generator having pf 0.5 have conductors bigger in dia than a generator having pf 0.8 and a unity pf DG have conductors smaller than both of these. For example 3 DG set having capacities 500kva/pf 1, 500kva / pf 0.8, 500kva / pf 0.5 and having same voltage and frequency, Can we connect these DG with motors having capacity 500kw/pf 1, 500 kW/pf 0.8, 500 kW/0.5 respectively. Sir Please don't feel bad with my questions

500KVA pf 1 = 500KW (active power),

500KVA pf 0.8 = 400 KW active power

500KVA pf 0.5 = 250 KW active power.

However usually since generators are connected to its mechanical drive (engine, turbine etc) its rating is in KWe (Electrical Kilowatts - based on the mechanical power - losses) that is the active power.

This will be able to drive a motor with similar active power (provided you just use a bit smaller motor with the required losses taken into account).

The problems are now not the power - but the current.

The generators not only have to supply the power but the current too (and due to power factor, the actual current may be quite a bit higher than the active portion of this current)

It is a bit complex, but could have been easier if your back-ground is known.

The total current

I = I_a + jI_r

or

I = √(I_a² +I_r²)

The Ia part is tha active and the Ir (reactive) part is not useful for any energy output but still has to be supplied by some body.

That somebody may be

a) Capacitor- compensating

b) generator itself : When the current lags (the voltage) at the generator- you may try to visualise that the stator field goes back from the rotor field (say for rotor excited ie the poles on rotor type) by the same amount. It has to rotate at the same speed of course. To stop it from escaping, it has to be pulled up, and for that purpose the rotor magnets have to be strengthened. And that can be done by increasing the excitation current that creates these two poles.

Thus you see that it is not only the stator conductors that has to carry more current, but we have to strengthen the rotor fields too to supply the useless demand of the load. Obviously there is a limit on these both aspects and you cross them and the generator trips (provided you have the protections in place)

There is a thread on- on this aspect a single generator is supplying a single motor, and the generator trips on motor starts-

the only reason is that during the start the excess current the generator is unable to supply (you must be aware that the starting current of a motor is about 3-4 times the FLC and that too with a high reactive component)

Sir is it correct a generator having pf 0.5 have conductors bigger in dia than a generator having pf 0.8 and a unity pf DG have conductors smaller than both of these,stator or rotor or both conductor? it was just an example that 500kva DG.

You must also ensure that the AVR is suitable for connecting to a load with pf correction. When switching over a load with pf correction from mains supply to generator I normally disconnect the pf equipment.