Category: Power Capacitors Power Factor Correction
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Power Capacitors Power Factor Correction
General
Under normal operating conditions certain electrical loads draw not
only active power from the supply (kilowatts kW) but also reactive power
(reactive kVA, kVAr). This reactive power has no useful function, but is
necessary for the equipment to operate correctly. Loads such as
induction motors, welding equipment, arc furnaces and fluorescent
lighting would fall into this category.
Correction
Opposing reactive power resulting from the connection of a correctly
sized capacitor can compensate for the reactive power required by the
load. This ensures that only a small amount of reactive power is drawn
from the supply.
Purpose
An apparent reduction in the total current drawn from the supply can
be achieved as a result of connecting a capacitor to an inductive load.
Definition
The Power Factor of a load is defined as being the ratio of active
power to total demand, that is to say kW + kVA. The uncorrected power
factor of a load is cosØ1 (where Ø1 is the phase angle between the
uncorrected load and unity), and the corrected power factor is cosØ2
(where Ø2 is the phase angle between the corrected load and unity). The
nearer the ratio kW+kVA=cos Ø is to unity, the less reactive power is
drawn from the supply.
Benefit
The connection of a capacitor capable of "correcting" half of the
reactive power of a load leads to a reduction in the demand on the
supply ofapproximately 15%. This results in the following:
a) The load on the cables and switches is reduced
b) The supply is now able to support additional load
c) The charges made by the electricity supply company are likely to be
reduced.
By reducing the load on cables and switches, power loss is reduced and
life is extended. The facility to connect additional load is always
useful to an expanding company.
An example: A fully loaded 1000kVA transformer supplying a load with a
power factor of 0.80 can only supply 800kW of "useful" load. By
correcting the power factor to 0.95, an additional 150kW of load may be
connected, increasing the "useful" load capacity to 950kW.
Methods of Power Factor Improvement
1. Individual Power Factor Correction To individually correct the
power factor of a multiple load, a suitably sized capacitor is connected
to each element of the total load.
2. Group Power Factor Correction Group power factor correction is
achieved by connecting one capacitor to a number of different loads
usually sharing the same duty cycle. A shared duty cycle prevents the
use of too much capacitance and avoids over-correction of the power
factor.
3. Centralised Automatic Power Factor Correction This involves the
connection of a number of capacitors, usually to the supply distribution
point. The capacitors are controlled by a microprocessor based relay,
which monitors the reactive power demand on the supply. The relay
connects and/or disconnects the capacitors to automatically compensate
for the reactive power on the system.
Harmonic Distortion & Filtering
In recent years the development of modern thyristor technology has
led to constant increases in the number of converter-fed loads.
Unfortunately converters have undesirable effects on the incoming AC
supply system since they often draw heavy inductive reactive power
and/or a non-sine-wave current. The supply system should be kept free
from distortion in order to prevent equipment malfunction. The converter
current is composed of a mixture of sine-wave currents, a fundamental
component at the supply frequency and a number of harmonics whose
frequencies are integer multiples of the line frequency. When harmonic
currents are superimposed on the AC supply, corresponding voltage
distortion occurs which could lead to system disturbances and failures
within the network. When the harmonic component is low, supply systems
should be equipped with control units with reactor-connected capacitors
in order to prevent resonance phenomena. Where the harmonic component is
high, it may be necessary to systematically suppress the harmonics with
a tuned filter circuit presenting a very low impedance to the individual
harmonic currents. Reactive power compensation can be properly planned
with the aid of the decision diagram shown opposite
Capacitor Sizing
To obtain capacitor sizes (in kVA) for a given power factor
correction multiply the kW load by the number shown at the axis between
existing power factor and required power factor.

Power Capacitors Limited is able to recommend the most appropriate
solution for each requirement.
Reactive Power Compensation

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