Power factor correction

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POWER FACTOR IS THE COSINE OF ANGLE BETWEEN V AND I

Power factor is the cosine of angle between V and I

• From power triangle• p f = (VIcosΦ)/VI • = Active power/ Apparent

power

• S2 = P2+Q2

• Apparent power2 =active power 2+ reactive power2

S2 = P2+Q2

• Pf plays an important role in ac ckts since power consumed depends on pf

• P=VLIL cosΦ , IL = P/VL cosΦ , for 1Φ ,

• P= √3 VLIL cosΦ IL = P/ √3 VL cosΦ , for 3Φ

• It is clear that for fixed P and V the load current is inversly proportional to pf

• Therefore for lpf ,higher will be the load current

Disadvantages of lpf

• Large kVA rating of the equipment-kVA=kW/cosΦ .This makes the equipment large

and costly.Greater conductor size-Large Cu lossPoor voltage regulation

Causes of lpf

• If pf is less than 0.8 it is regarded as lpf.• Most of ac and induction type motors have lpfThey work with very small pf during no load

(0.2-0.3) and rises to 0.8 - 0.9 at full load.Arc lamps ,Electric discharge lamps , heating

furnaces etc operate at lpf.Load variations in the power system causes

increase in magnetising current.

Pf improvement

• Cause of lpf is the presence of inductive load.• Therefore some devices taking leading pf can

be connected in parallel to improve pf of the load.

• One such device can be a capacitor.It completely or partially cancells out lagging current.

C draws current Ic that leads supply voltage by 90 degree. Resulting current is

the phasor sum of IR and IL-IC, I1is at Φ1 and I2 is at Φ2. Since Φ2 is less than Φ1 , angle of lag has been reduced and the power factor has been improved

Foll. points may be noted

•The ckt current I2 is less than I1 after pf correction•Active component remains the same before and after pf correction•I1cos Φ1 =I2cos Φ2

•But I2sin Φ2 =I1sin Φ1 - IC

•VI1cos Φ1 =VI2cos Φ2

•Active power remains the same

•VI2sin Φ2 =VI1sin Φ1 -VIC

•Therefore net kVAR after pf correction =lagging kVAR before pfcorrection – leading kVAR of equipment

Pf improvement devices

• Static capacitors• Synchronous condensors• Phase advancers