Power Factor of Diesel Generator

Your gens capacity is 250 kva when the pf = 0.80. At this pf the max kw = 200. If the pf = 0.50 then the kw will have to be reduced. If you improve the pf using capacitors to say 0.90, then the kw allowed would be 225 kw. The KVA is fixed at 250 kva.

I would suggest that you check with the mfg.

The max. KW may be 200, regardless of the PF. OP has not given us all the nameplate info.

Actually OP is confusing the ratings with the operating points. The Generator Capability Curve gives the locus of points that represent the boundaries of operation independent of the system to which the generator is attached:

The blue sloping lines intersect at the rated power factor point, the curved between those points represents the rated MVA, notice that the reactive capability curves (upper and lower) truncate the rated MVA curve. Operation is generally permitted anywhere inside these curves, but the system and the prime over may impose additional operating restrictions.

Thanks PowerSolutionsFBD.

I would like to understand more about KVAr:

If your kVAR (inductive) exceeds beyond 150kVAR, will overload the alternator and its excitation system.

If kVAR (capacitive) exceeds will result in loss of magnetic coupling, hunting of rotor and burning of field winding due to overheating in pole shoes as a result if Eddy currents induced.

How can I decide what should be the maximum level of KVAr(Capacitive)? If my PF is 0.5 and I wish to make it to 1, then I need to supply Capacitive KVAr 125? will it cause any problem?

Manish, You did not mention value of total kVA or kW corresponding to 0.5.

Based on total kVA or kW load, Calculate kVAR and select Capacitors based on calculated kVAR and type of load.

If you are in India then I can supply suitable system.

Dear Mr.mjain.

Definitely it can develop more power and You can over load your DG set for a short duration with in 5 to 7% of the capacity. Generally the Prime Mover will have more capacity.

Your 2nd question,

The answer - Yes, You can add capacitor and improve the P.F. Less P.F. means more Line Current and the winding will get heated more, as the winding temp. is decided by the current and not the PF. At lesser PF the excitation system will be strained.

DHAYANANDHAN.S

In addition to all the excellent posts I would suggest that you determine the service (duty-cycle) factor of the DG and make sure it is used in your calculations.

A low duty-cycle machine requires even further reduction of loading in order to avoid overheating and damage to the machine.

usually motor is designed to handle load at 80 to 90% of its full load capacity which gives max efficiency at which pf will be around 0.85

so we have make sure that every equipment or motor which is loaded on dg should be operated at 80-90% of its full load capacity which gives 0.8 PF

adding of cap bank leads to nothing but fuel consumption

as the loading is not for along time it does not any mayor effect on eddy or hysteresis losses

:-) /////

Effect of Low Power Factor on the Genset

• The Alternator converts the Mechanical Energy, which it gets from the engine, to the Electrical Energy.

• The Alternator is designed to deliver the output in kVA and assuming the Power Factor of the load is 0.8 lag the kW is calculated.

• If the Power Factor of the load is lesser than the minimum required Power Factor i.e. 0.8 lag, the excitation system of the alternator gets overloaded.

• The low Power Factor of the load produces the DEMAGNETISING effect in the magnetic field of the alternator, and to keep the terminal voltage constant more power is required by the excitation system of the alternator. Hence getting overloaded.

• The excitation system of the alternator comprises of the Exciter Stator, Exciter Rotor, Rotating Rectifier Assembly and the Main Rotor.

• Due to this overloading of the excitation system the failure may occur in the either of the components of the excitation system of the alternator due to overheating.

Recommendation of using the Capacitor Banks with the Genset

• The Capacitor banks can be used for improving the Power Factor of the system powered by the Genset.

• The care should be taken that at no time the Power Factor of the system should be more than 0.90 lag. At no time only capacitors are connected across the output terminals of the alternator. If this happens, the Power Factor goes beyond 0.90 lag and terminal voltage starts shooting up and ultimately lead to the failure of the alternator.

• If the genset is operated beyond the 0.9 lag Power Factor, the PF may go beyond Unity i.e. on the leading side during switching 'On' & 'Off' the load.

• If genset is operated on the Unity or Leading PF the alternator undergoes the MAGNETISING effect. Because of this effect the terminal voltage of the alternator will shoot up beyond control and this high voltage may damage the main windings and the AVR of the alternator.

· As far as Engine is concerned, normally the engines are also rated for 0.8 lag.

If the genset is operated above 0.8 lag. (By improving the Power Factor using capacitor bank), the engine gets overloaded. Thus the kW load on the engine is the limiting / deciding factor.

· Your can only improve the Power Factor to the extent, the load does not exceed the rated kW of the engine during operation.

Thanks..

<...Capacitors...> are sometimes provided to better match the load characteristics to the <...Generator...> to reduce heat losses in the cables feeding the load. An economic case can be used to justify their addition.

Economics usually vary from place to place and as such fall outside the domain of an international engineering forum; they are commercially sensitive and have no international value.