Page 59 - Energize September 2021 HR

TECHNICAL

result if drives and capacitors are installed on the same system.
When power factor correction is required in the presence
of nonlinear loads, or the amount of harmonic distortion must
be reduced to solve power quality problems or avoid penalties,
the most reliable, lowest cost solution is often realised with the
use of harmonic filters. In a shunt harmonic filter (see Figure 3),
a power factor correction capacitor is combined with a series
iron-core reactor. This filter provides power factor correction at
the fundamental frequency (50 Hz) and becomes an inductance at
frequencies higher than its resonant.
The reactors used to provide harmonic control are referred to as
detuned reactors.

Switched PFC devices
Figure 2: PF correction capacitor connection (Eaton)
Switched PFC devices provide a flexible range of reactive
compensation by switching inductance or capacitance in or out
PFC capacitors are rated in kVAr or VAr, the rating indicating of circuit to either increase or decrease reactance. Mechanical
how much reactive power the capacitor will supply. Since this switching devices were previously used but have largely been
kind of reactive power is caused by inductance, each kVAr of replaced by thyristor switches in equipment available today,
capacitance decreases the net reactive power demand by the although mechanical switching is still used where rapid operation
same amount. PFC capacitors are also rated in terms of the is not required. Switched devices comprise thyristor-controlled
voltage withstand and current carrying capacity. PFC capacitors reactors (TCR) and thyristor-switched capacitors (TSC).
are installed with fuses or circuit breakers to protect them from
possible internal short circuits. PFC capacitors often contain Effect of switching on PFC devices.
internal fuses. Transients can occur both when connecting and disconnecting
capacitors, due to the voltage difference between the system
Inductances voltage and the capacitor voltage. Capacitors can only be on or off,
In a similar manner to capacitors, inductive reactors are used to and firing angle control cannot be used with switched capacitors.
correct leading PF, usually associated with distribution lines.

Effects of harmonics
The presence of harmonic distortion due to non-linear loads within
a network or the grid will increase the current flowing through
capacitors as the capacitive reactance is inversely proportional
to the frequency, consequently capacitors could be subjected to
overload. An overload on capacitors could cause the premature
failure of the capacitors due to increased voltage and thermal
stress on the capacitor’s dielectric.
The inductive reactance (X L) of a reactor, however, is directly
proportional to frequency and the magnitude of an inductive
reactance will increase with frequency, thus blocking the high
harmonic current. Placing reactors in series with capacitors offers
a higher impedance to harmonics, thus eliminating the risk of
overloading the capacitors.
To avoid resonance between the capacitor and inductance,
detuned reactors are used. The inductance value of a detuned
reactor is selected such that the resonant frequency is less than
90% of the dominant harmonic in the circuit.
For example: If the fifth harmonic of the fundamental (5 x 50
Hz) is dominant, a series LC circuit having a resonance frequency
of 90% of 250 Hz (i.e., 225 Hz), is categorised as a detuned filter
or a detuned capacitor.
In any plant containing power PFC capacitors, harmonic
currents and voltages can be magnified considerably due to the
interaction of the capacitors with the distribution transformer,
known as harmonic resonance or parallel resonance. For a typical
plant containing power factor correction capacitors, the resonant
th
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frequency often falls in the vicinity of the 5 to the 13 harmonic.
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Since nonlinear loads typically inject currents at the 5 , 7 , 11 , and
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13 harmonics, a resonant or near-resonant condition will frequently Figure 3: Shunt harmonic filter (Eaton)
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