Power factor correction √

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 S 2 = P 2+ Q 2 Apparent power 2 =active power 2 + reactive power 2 S 2 = P 2+ Q 2

Pf plays an important role in ac ckts since power consumed depends on pf P=V L I L cosΦ, I L = P/V L cosΦ, for 1Φ, P= √3 V L I L cosΦ I L = P/ √3 V L 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 loss Poor 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 lpf They work with very small pf during no load ( ) and rises to 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 I R and I L -I C, I 1 is at Φ 1 and I 2 is at Φ 2. Since Φ 2 is less than Φ 1, angle of lag has been reduced and the power factor has been improved

Foll. points may be noted The ckt current I 2 is less than I 1 after pf correction Active component remains the same before and after pf correction I 1 cos Φ 1 =I 2 cos Φ 2 But I 2 sin Φ 2 =I 1 sin Φ 1 - I C VI 1 cos Φ 1 =VI 2 cos Φ 2 Active power remains the same

VI 2 sin Φ 2 =VI 1 sin Φ 1 -VI C Therefore net kVAR after pf correction =lagging kVAR before pfcorrection – leading kVAR of equipment

Pf improvement devices Static capacitors Synchronous condensors Phase advancers