Power Factor Correction

Power Factor Correction Power Factor Power factor is the ratio between True Power (kW) and Apparent Power (kVA) it is not expressed in a percentage but in a per unit form. In industry such as in this project electric motors are used to drive fans pumps conveyer and other plant. Generally the electrical drives which are used are Induction motors typically these have a poor power factor which causes inefficiencies in the electrical supply by drawing excess inductive reactive currents increasing loads on cabling and switchgear other such loads are lighting ballasts and welding plants ect. As it causes inefficiencies in the electrical supply the supply authority will penalise the consumer financially for poor power factor. Therefore the better the power factor the more cost efficient. Power Factor = kW

KVAr

KW

KVA

θ θ

kVA Typically power factor for industry is about 0.8 lagging for example a 500 kVA transformer can only supply up to 400 kW this means the consumer is only getting 80% efficiency of there supply. Now we can see the advantages of having good power factor. These excess currents can reduced by means of power factor correction. It is achieved by the delta connecting capacitors in parallel. Methods of Power Factor Correction (P.F.C) There are a couple of methods of (P.F.C) listed below are the most common 1) Individual Load correction using capacitors 2) Group or Bulk using capacitors 3) Automatic switching using capacitors

1) In this method a capacitor is connected to the load commonly we see

this in the case fluorescent light fittings thus improving the power factor at the source. This method is also done with large motors commonly

2) For larger installations the above method may not be so practical. This

is an alternative a large bank of capacitors is connected to the supply at the distribution board they are switched in and out of the supply by means of a timer. At peak time all capacitors would be switched in and would be switched back out at a later time. It is only allowed to be permanently connected in installations up to 25 kVAr

3) Automatic switching of capacitors may be used also. This is the most common method. It is achieved via the monitoring of the reactive power (kVAr) either electronically or by a relay based form when this reaches a certain value then capacitors are stepped in accordingly switched in via contactors.

For the factory I chose the power factor was 0.849 lagging. Considering the cost from the supply company in power factor surcharges I decided it would be more economically viable to correct the power factor to 0.98 recommended by the supply company thus reliving us from the power factor surcharge. Shown here is a table of the installations loads kW Single Phase Power Three Phase Power Totals

kVA

kVAr

P.F

31390

32945.69

10004.4

0.952

138410

168165.1

95510.1

0.823

169800

199913.4

105514.5

0.849

The amount of correction required is given as: KVAr = kW (Tanθ1 – Tanθ2) kW = total installed load in kilowatts Tanθ1 = the original power factor of the installation Tanθ2 = the new required power factor of the installation kW = 169.8 kW Tanθ1 = cos-1 0.849 = 31.89° tan = 0.622 Tanθ2 = cos-1 0.98 = 11.47° tan = 0.203 kVAr required is equal to kW (Tanθ1 – Tanθ2 169.8 (0.622 - 0.203) = 169.8 (0.419) = 71.14 kVAr capacitor

Therefore the value of a capacitor required to correct the power factor from 0.849 lagging to 0.98 lagging is 71.14 kVAr. As capacitors are build to only to standard sizes i.e. 10, 25, 50 and 100 kVAr and so on we cannot get a 71.14 kVAr capacitor but a selection of the above could be used for example a 20, 20, 10, 10, 5, 5 and 1 kVAr capacitors could be used. The automatic switching method will be used therefore if the power factor drops below a certain value certain values of capacitance shall be switched in accordingly Cable required for Capacitor Current in capacitor (I) = 71000 = 102.47Amps √3*400 Also the allowance of 33% for harmonics =136.28 Amps The switch fuse size for the capacitors is 150 Amp The capacitor bank is located 15 meters from the supply the cable size required to supply the capacitor is given as 16000 = 7.82mV/A/m 136.28*15 From the latest ECTI regulations a cable of 35mm2 XLPE would be adequate to supply the capacitor.