i have a little confusion about power factor. can anybody help me?
with this example.

suppose we are generating 120MW and are connected with the grid. reactive power is 60MVAR and power factor is 0.90. does it mean 90% of 120MW is being used for useful work and the 10% is wasted of our real power?

 

It means 90% of the total volt-amperes is being used for useful work.
Power factor is the cosine of the arctan of MVAR/MW.

 

saeed,

While the power/reactive power numbers might not add up to a 0.9 pf, a 0.9 pf does, indeed, mean that only 90% of the energy being produced is going into "real", useful work (watts, KW, MW) and 10% is going into reactive power.

Now, reactive power is necessary, but unless you are getting paid for the reactive power you are producing (via a VAr-hour meter, or by some contractual arrangement), producing power at power factors less than 1.0 means you are not being very "efficient" with the power being generated and you could be getting a little more revenue by generating as close to unity as possible.

I'm presuming you are a "merchant" power plant connected to a large grid, getting paid for power (watt-hours). Some purchased power agreements require generators (power plants) to produce power at some power factor at certain times of the day or year. If your power plant is "captive" or "island" then you are pretty much forced to generate the VArs required by the load.

Hope this helps!