May I use the APFC at DG?

How close to full capacity is the power cable between the D and the TPH crusher? If there is capacity in hand on the cable, a PF unit buys nothing.

Some of the heat dissipated in the cable might be reduced. An economic calculation would determine whether PF correction is worth the investment.

Yes and yes....generally speaking, that you understand the reactive power conditions that can exist and your APFC is equipped to deal with reactive power limiting requirement and ensure that under actual operating conditions the kW loading and current loading is not exceeded, which could damage the generator windings....

<...current loading is not exceeded...> That's what fuses and breakers are for!

I would not bother with APFC however. I would just turn on PFC caps with the crusher motor. That's the only significant load that will affect the PF.

But to that point, it's a complete waste of time and effort to judge the PF on an unloaded crusher motor. Without it being loaded with rocks, the motor is just a spinning mass and the PF will be low. That is essentially meaningless though, because why do you care what the PF is when the machine is doing no work? You just shut it off! So start using your crusher and load it to its maximum capacity, THEN measure the PF. If it is still low, then determine the amount of kVARs you need to correct it UNDER LOAD, and have them turn on only when the crusher is running and is getting rock fed into it. The simplest method is to use an aux. contact of the crusher starter, in series with an aux. contact of the infeed conveyor, to control the coil of a suitable contactor that brings in the PFC capacitors. You don't need anything more complicated or expensive.

Please send me your PayPal account information, I'd like to send you my bill for consultancy.

I may be missing something here, but why do you suggest the observed pf is at no load when the OP's statement indicates otherwise?
"We are using 750 kva DG for 200 TPH crusher during full load operation we are observed at DG display pf is 0.58"

Because I am blind as a bat and missed that...

Altering the pf will have little effect on fuel consumption.

If your genset has an input fuel monitor, then you can see for yourself by adjusting the AVR control to shift VAR up and down and observe the change in fuel delivery. The exciter will obviously produce more or less power in response to excitation, but the changes in fuel usage will be minimal.

The greatest benefit in raising the pf closer to unity (0.8 pf is what most gensets are rated at) may be in reducing voltage drop on the connecting cabling and therefore improving the efficiency of the crusher. It might be argued that the crusher will then get more work done for a given amount of fuel, and so you have indirectly reduced consumption.

PFC will save diesel.

Wait till the price of fuel goes up again to get a faster ROI.

I'm paying USD 0.70 for diesel now. Would take a long time to break even through savings with an APFC.

The fixed PFC at the motor idea is a good one.

You don't ever connect this load to the utility supply do you?

The fuel savings is due to lower I@R losses, ie lower current, less loss, less heat.... ergo less fuel for same work performed. Around 1% saving I saw quoted somewhere....not much but it depends what it is 1% of. The way fuel prices are going we'll be getting paid to use it soon (...a man's allowed to dream...)

Read this and look at the "further reading" too...all short, sweet and easy to read.

http://www.reliableplant.com/Read/13339/power-factor-correction

There should be no resonance between generator reactance & capacitors.