Page 80 - Power Quality in Electrical Systems

Chapter
5

Harmonic Current Sources

In this chapter, we shall discuss the circuits and magnetics
that create harmonics (multiples of the line frequency) and/or
interharmonics (other high-frequency components).
Harmonics are generated by rectifiers, line-frequency
converters, and nonlinear magnetics. Interharmonics are
created by high-frequency switching power supplies. For most
of what we shall do in this chapter, the fundamental
frequency used will be 60 Hz.

Background
A typical setup that shows how harmonic currents can affect power
quality is shown in Figure 5.1a. An AC voltage source is displayed, with
, and resistance, R . This AC source can
its associated line reactance, X s s
be single-phase or three-phase. The line inductance depends on the
length of the line and the geometry of the conductors. The line resist-
ance, on the other hand, depends on the length of the wire and the wire
gauge used. The AC source voltage then supplies a nonlinear load that
draws harmonic current. Typically, this harmonic source is a rectifier
or other converter.
In Figure 5.1b, we see the single-phase equivalent circuit. Note that
the voltage labeled V pcc (for voltage at the point of common coupling or
PCC) has harmonic components due to the harmonic current I drawn
h
by the load running through the line impedance. If this voltage at the
PCC feeds additional equipment other than the harmonic generating cir-
cuit, the resulting voltage distortion can disrupt operation of the equip-
ment if the harmonic distortion is too high. Harmonic limits are
discussed in great detail in IEEE Std. 519.
In the following, we’ll discuss numerous pieces of equipment that
generate harmonic currents.

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