Power Factor CorrectionPower FactorPower 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 industrysuch as in this project electric motors are used to drive fans pumps conveyerand other plant. Generally the electrical drives which are used are Inductionmotors typically these have a poor power factor which causes inefficienciesin the electrical supply by drawing excess inductive reactive currentsincreasing loads on cabling and switchgear other such loads are lightingballasts and welding plants ect. As it causes inefficiencies in the electricalsupply the supply authority will penalise the consumer financially for poorpower factor. Therefore the better the power factor the more cost efficient. Power Factor = kW KVAr KW KVA θ θ kVATypically power factor for industry is about 0.8 lagging for example a 500kVA transformer can only supply up to 400 kW this means the consumer isonly getting 80% efficiency of there supply.Now we can see the advantages of having good power factor. These excesscurrents can reduced by means of power factor correction. It is achieved bythe 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 commonly2) 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 kVAr3) 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 thecost from the supply company in power factor surcharges I decided it wouldbe more economically viable to correct the power factor to 0.98recommended by the supply company thus reliving us from the power factorsurcharge.Shown here is a table of the installations loads kW kVA kVAr P.FSingle Phase Power 31390 32945.69 10004.4 0.952Three Phase Power 138410 168165.1 95510.1 0.823Totals 169800 199913.4 105514.5 0.849The amount of correction required is given as:KVAr = kW (Tanθ1 – Tanθ2)kW = total installed load in kilowattsTanθ1 = the original power factor of the installationTanθ2 = the new required power factor of the installationkW = 169.8 kWTanθ1 = cos-1 0.849 = 31.89° tan = 0.622Tanθ2 = cos-1 0.98 = 11.47° tan = 0.203kVAr 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 from0.849 lagging to 0.98 lagging is 71.14 kVAr. As capacitors are build to onlyto standard sizes i.e. 10, 25, 50 and 100 kVAr and so on we cannot get a71.14 kVAr capacitor but a selection of the above could be used for examplea 20, 20, 10, 10, 5, 5 and 1 kVAr capacitors could be used. The automaticswitching method will be used therefore if the power factor drops below acertain value certain values of capacitance shall be switched in accordingly Cable required for CapacitorCurrent in capacitor (I) = 71000 = 102.47Amps √3*400Also the allowance of 33% for harmonics =136.28 AmpsThe switch fuse size for the capacitors is 150 AmpThe capacitor bank is located 15 meters from the supply the cable sizerequired to supply the capacitor is given as 16000 = 7.82mV/A/m136.28*15From the latest ECTI regulations a cable of 35mm2 XLPE would be adequateto supply the capacitor.