why does the power factor of diesel generator 0.8 lagging

Generally Alternator is used as a generator in DG Set and it is coupled with the four stroke Diesel Engine.

 Nameplate installed on a DG Set gives information about its KVA rating, KW, maximum current and power factor among other parameters.

Most of DG manufacturers mentions 0.8 as rated power factor of DG. But it does not mean that DG must be operated at 0.8 power factor at all the time. It only gives us information about the lowest or minimum power factor of the load on which DG set can be operated without compromising on its Efficiency and performance in long run of its usage.

Also, since DG set cannot absorb reactive power, power factor should stay in the lagging region. Generally it is operated within the range of 0.8 to 1 power factor (lagging).

Now let us understand effect of power factor on the operation of DG set:-

By utilizing the data provided by you, let me give you an example to understand it more clearly.

So we have 750 KVA Generator and 0.8 power factor (These are rated values). So, Active power (rated) which can be delivered = 750*0.8 = 600 KW. Also reactive power (rated value) will be = 450 KVAR.

Now to compare the Efficiency of DG set at 0.8 pf and 1 pf, consider two DG sets - DG1 and DG2 used for supplying same load (i.e same amount of Active power= 600KW).

So DG1 will supply a Active load of 600 KW at 0.8 pf (lag) and DG2 will supply Active load of 600 KW at 1 pf. So the output total (Apparent) power of DG1 will be 600/0.8= 750 KVA and output power of DG2 will be 600/1= 600 KVA.

Now with constant 1-phase load of 230V at its terminals, current flowing through DG1 will be 750000/230=3260.86 Amps and that flowing through DG2 will be 600000/230=2608.69 Amps (rated value or maximum current limit).

It can be seen from above calculation that at high power factor current flowing through DG is lower than that at the lower power factor. Aslo, we know that in Alternator- Copper losses are directly proportional to the square of the current flowing through it. Thus, at lower power factor of operation of DG, current will increase drastically and thus losses taking place inside the DG set will increase much higher. Thus the performance or efficiency of DG set will be degraded, Therefore 0.8 pf is the bare minimum limit with which DG set can be operated.

If operated below 0.8 pf then losses of the Alternator increases and the Diesel Engine have to produce more Active power to overcome it and so it will consume more diesel.

Courtesy:Vatsal