Page 49 - Energize September 2022

TECHNICAL

PF and harmonics: Compensation

and correction

Efficient operation requires that the maximum amount of power is delivered to the load for a given
circuit capacity. The PF of a load can affect this capability and result in less power delivery than the
network is capable of. PF problems can be corrected in various ways.

By Mike Rycroft, Energize

ower Factor (PF) is the ratio of true power to apparent
power in a circuit or distribution system. Any AC circuit
Pconsists of real, reactive, harmonic and apparent (total)
power. True power is the power, in W or kW, used by motors,
lights and other devices to produce useful work. Reactive power
is the power, in VAR or kVAR, stored and released by inductors
and capacitors.
Reactive power shows up as a phase displacement between the
current and voltage waveforms. Harmonic power is power, in VA or Figure 1: Effect of reactive components on current (Electrical 4U)
kVA, lost to harmonic distortion. Apparent power is the power, in
VA or kVA, which is the vector sum of true power, reactive power
and harmonic power. Apparent power is not a simple summation the voltage or lag the voltage as shown in Figure 1. Most PF
but a vector summation. problems encountered in networks involve a lagging PF due to
The PF problem is that less power is delivered to the load, and inductive loads.
a higher current is required to deliver the same power as the ideal The PF of these leading or lagging loads is an expression of the
case (where PF = 1). Higher current means higher I R losses, larger amount that the current was out of phase with the voltage, hence
2
transformers, cables and switch gear for the same power delivered. the term displacement PF. By applying the same magnitude of
Generators are also affected by low PF circuits. corrective PF (in the case of lagging PF, a leading PF device needs to
be connected and vice versa) the original poor PF is cancelled out
PF and linear electric loads and the overall PF improves. On most sites a practical target PF is
Linear electrical loads fall into one of three categories from a PF between 0,95 and 0,99 at 50 Hz.
perspective:
• Resistive – These are loads which display resistive characteristics The real power delivered to the load is the product of voltage and
only. There is no inductance or capacitance to impact on PF. With current, and because the current is out of phase with the voltage
resistive loads the voltage and the current are in phase, meaning this is given by:
that no PF correction is required. Examples of these loads are
incandescent light bulbs and heating elements. P r = V*I*Cos θ (1)
• Inductive – These are loads which display both resistive and
inductive characteristics. The inductive component of the load where θ is the phase angle between voltage (V) and current (I).
causes the current to lag behind the voltage, hence the term
lagging PF. These inductive loads are very common in industry The apparent power is the product of the values of voltage and
and examples are AC motors, lighting ballasts and induction current, ignoring the phase angle:
furnaces.
• Capacitive – These are loads which display both resistive and P a = V*I (2)
capacitive characteristics. The capacitive component of the load
causes the current to lead the voltage, hence the term leading PF is defined as the ratio of real power to apparent power and is
PF. These capacitive loads are uncommon. An example is an over- equal to the cosine of the phase angle.
excited synchronous motor or capacitor.
PF disp = Cos θ (3)
Reactive components cause the current flowing in the circuit
to be out of phase with the voltage i.e., the current may lead This is referred to as displacement PF.

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