How to increase the power factor?

That is an erroneous approach. You cannot "improve the power factor of a transformer". You can improve the power factor of the LOAD on the transformer, which will in turn reduce the current through the transformer and reduce the IR losses in it. This will not really save much money in energy costs however, f that is your goal. But f you are being penalized by the utility for poor power factor, or you need to increase the capacity of your transformer, then improvng the pf of the load will help.

So the first step is to evaluate the loads on the transformer and wherever you have an inductive load, calculate the capacitance needed to improve the of of that load to .95. Most capacitor manufacturers will have formulae for doing that task. The alternative is to have a condenser or some other sort of automatic VAR compensator on the system, such as a large synchronous motor, that can add VARs to your overall distribution system as necessary. The risk of trying to do t in bulk with capacitors s that f some of your inductive loads drop off, you can end up over compensating and causing a host of other problems.

Dear Sir,

It is dependent on the load on the transformer (ie feeders) not on the Power Rating of the transformer.

Best Regards,

N.Suresh