[Soumen2011]

In elecrical engineering, the term "Power factor" is defined by mathematical formula, not by any physical concept. Without knowing the physical concept, it is difficult to use this term. How does it affect the energy transmission? Why do we prescribe not to be lower than certain value (say 0.85)?
Cou you please explain this topic?

[jcaron2]

As you probably know, the power factor is the ratio of the real power to the reactive power. So what does that mean? The real power is self-explanatory. That's the power that gets dissipated doing actual work. The electricity used to light the lights, toast your bread, heat the hot water, etc. It's also the power that is measured by your electric meter (and, therefore, the power you end up paying for).

The reactive power is a different story. It's the power that is temporarily drawn or generated as energy gets stored by or returned from a reactive load (i.e. capacitance or inductance). Reactive loads temporarily store energy during one half of the cycle, then return the energy during the other half. This isn't doing any useful work, and it's not causing the electric meter to spin any faster. In an ideal world, this wouldn't matter. The reactive load is just "borrowing" the energy for a half cycle then returning it to the grid for somebody else to use. However, in the real world, the transmission lines and transformers and switches and generators that make up the power grid all have resistive losses. All that extra current flowing in and out of a reactive load means extra energy is wasted in the grid. It also means that the sizes of all the components of the grid need to be increased to handle the extra current. That's why the power company wants power factors as close to 100% as possible.

If you want a physical analogy to this, think about riding a bicycle from point A to point B, where B is uphill from A. It's hard work to pedal from A to B, not only because you're converting kinetic energy to gravitational potential energy, but also because your bicycle and your body and the interface between the tires and the road and the wind blowing in your face all have significant friction and inefficiency.

Now imagine making the same trip from A to B, except that there are a series of very high and very steep hills in between. Granted, all that gravitation potential energy you build up climbing to the top of each intermediate hill is theoretically returned to you in the form of kinetic energy when you coast back down the other side. In a perfect world, once you reached the top of one hill, you could coast down the other side and up the next hill all the way up to the same altitude you started from. However, if you've ever ridden a bike you know that in the real world that never turns out to be the case. You're usually lucky if you can coast halfway up the next hill. Then it's back to pedaling and huffing and puffing to get up the next one. Even though at the end of the day, you've still only gone from point A to point B, you'll end up wasting a whole lot of extra energy if there are lots of hills along the way. And the worse the power factor is, the taller and steeper those hills become.

Does that make it any clearer?

[Soumen2011]

Thanks for the useful explanation. May I ask a question further? In electrical generation plant, the Power factor is restricted not lesser than 85%. Is it only for limiting the energy loss? Or, does it have anything for turbine speed control?

[jcaron2]

Power generation is not my area of expertise in electrical engineering, but I think the PF restriction at the generation plant relates mostly to the reactive load presented by the electrical grid, not by the turbines themselves. All those miles of transmission lines have significant inductance, so the generators see a reactive load, even if all of the customers had perfect 1.00 power factors. So large banks of capacitors and other PF correction equipment are installed at the generation plant to offset that inductance and get the current back in phase with the voltage.

When the generator sees a reactive load, the reactive current flows back and forth at the same frequency as the real power (50Hz or 60Hz), so I don't think it affects the turbine speed or makes it any more difficult to control it. I think it just comes down to energy loss.

Hopefully a power generation expert will read this and elaborate or correct me if I'm wrong.

[sinnadurai]

For generators they say PFCC are not advisable,why? Is it due to possibility resonance?

[jcaron2]

I don't know. Maybe you're right about resonance. Or maybe it's just considered unnecessary since the generator will generally be close to the load.

However, I have another theory.

I think generators themselves tend to have lagging PF's (around 0.8 is pretty typical I think). However, adding external PFCC will not increase the real power that can be delivered by the generator. For example, if a generator is specified to deliver 200 KVA at a PF of 0.8, that equates to 160 KW of real power. The over-current protection within the generator will prevent you from drawing more than 160 KW of power. If you apply PFCC to correct the PF to 1.0, the generator is still limited by the capabilities of its alternator to only producing 160 KW of real power. However, with a higher power factor you could potentially overload the alternator without tripping the over-current protection circuitry.

Hopefully somebody with more knowledge on this subject will jump in and set us straight. :)