Alternator PF

Bulk power factor correction system has multiple capacitors and the PF controller switches these capacitors after calculating the current PF. If PF is > 1 the capacitor becomes a LOAD which dissipates energy from system. If PF is < 1 then the capacitors + generator supplies the reactive current to load. So there should be a balance. Now we set the "Pre-Set Value" 0.95 because after this value, if PF switches another capacitor, there may be a chance that PF would become > 1. But if there are smaller rated capacitors in bank, one can set the Pre-Set value at 0.96-0.97.

Hope this will clear you.

Just a small correction if you don'd mind my saying so ...

PF is never greater than 1. It is either lagging or leading. When it is lagging, capacitors are switched into the system to compensate and bring it to unity. If some inductive load is switched off at this time, the pf goes leading, which is also not good. Then the automatic pf controller switches off the required number of capacitors to bring it back to unity. As you mention, pf controllers can be set to 0.95 lag to 0.95 lead.

Dear kvsridhar

Thanks for correction. Now I read and thought what I have written.

You gave a good explanation except that pF must never lead. ie never greater than 1. Ours is set elctronically to 0.98 lagging, and i believe most will be.

can you check up previous threads?

This has been several times discussed.

You do not try to compensate the load pf to near unity, since the unity or the leading pf is likely to create the instability in the alternator.

Just think about the magnetic circuit power angle in case of leading pf and the concept will be clear.

if your generating plant is running in islande mode, you can do nothing with your PF, the generator PF is the same as the load PF, little enhanced by the capacitive impedance of the distribution line...

if your plant running parallel with a grid, you can adjust your PF by adjusting the excitation current, an over excitation generates more KVAr and your PF decrease, with under-excitation it generates less KVAr and PF goes closer to unity,

Dear friend,

Mr. LE NOBEL is given wright answer. if we operate the generater with Leading power factor the excitation current will be very less and due to induction the roator temperatur will be go high. and also the alternator will be instabiltily.

AS said already, if the generator is running on its own (not parallelled), the pf is dictated(usually) by the load. The Aoto-Voltage-regulator (AVR) does the job to keep the excitation within the required value to maintain the voltage, and this will be enough. If your load has no inductive or capacitive element, then your pf will be unity.

Down-stream, if you have inductive loads (most current situation), the pf will be lagging and you can have a bank of pf correction capacitors to bring it back (downstream) to near unity .

ALSO, the generator Prime-Mover (Diesel engine...) is rated to give active power (kW) at 0.8 pf. If your load is made of only resistive load (no coils or capacitors), you should be carefull not to overload the engine. The pf being =1, the kW drawn can exceed the rating! (calculate...).

PARALLEL operation: The pf can be controlled to match the requirement of the grid. Power Sharing between different GenSets connected to the same grid take advantage of this. therefore, you can run one genset with a leading pf to supply the grid with kVARs and thus work as a pf correction machine. This used to be done quite often. Mind you, this topic might be beyond your present level of knowledge and you might research or study.

The 0.8pf is, as said before, mentioned in the specs of a genset to inform of the maximum active power available at o.8 pf load characteristics. You should not load the Genset more at 0.8pf. if the load has a pf = 1, then watch out not to exceed the Engines Capacity. keep the kW lower than the rated kVA x 0.8

I hope this clarifies somehow.

The load current of the generator has de-magnetizing effect on rotor excitation of the load power factor is lagging, it has cross-magnetizing effect if power factor is unity and lastly it is magnetizing if the power factor is leading. In case of leading power factor operation, as stated above, effect will be magnetizing and therefore field current requirement of rotor will get reduced. As stated in many contributions, this reduced excitation would affect the stability of the operation system. There is another limitation (not applicable in case of small gen sets but very important in case of large Power Generators) what is known as the "End Zone Heating" of the stator winding overhang support structure, Stator casing, Stator core end plate etc. I am trying to give below a typical Generator Capability diagram where operation limits are controlled by End zone heating on leading power factor side. In some designs which I have come across, even at unity power factor, generator cannot supply rated mega watt / rated MVA. I thought this aspect, though related to Large generators, may be of interest to the members.

I don't believe any of these posts answer the question of why industry use near unity PF rather than what would seemingly be the more efficient PF=1, I assume it's to allow for the non-linear effect of the capacitor in the circuit which is also subject to other less ideal conditions resulting in the need for allowances to be made.

I don't believe any of these posts answer the question of why industry use near unity PF rather than what would seemingly be the more efficient PF=1, I assume it's to allow for the non-linear effect of the capacitor in te circuit which is also subject to other less ideal conditions resulting in the need for allowances to be made.