1. 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
2. 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.
3. 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 kVA kVAr P.F
Single Phase Power 31390 32945.69 10004.4 0.952
Three Phase Power 138410 168165.1 95510.1 0.823
Totals 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
4. 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.
