Page 81 - Power Quality in Electrical Systems

64 Chapter Five

Harmonic
AC source
source
R s jX s V PCC
LOAD
I h
(a)
R s jX s

v s V PCC I h

(b)
Figure 5.1 Illustration of a harmonic current source affect-
ing power-quality. (a) Asingle-line diagram of an AC source
with source impedance R s jX s and a load that draws har-
monic current I h . (b) A single-phase equivalent circuit.

Single-Phase Rectifiers

Rectifiers are used in all sorts of power system and power electronic sub-
systems to convert AC power to DC power. In low-power applications
using single-phase power, rectifiers are used as the front-end of switch-
ing power supplies and small motor drives.
A single-phase, full-wave rectifier with current source load is shown
in Figure 5.2a. This circuit is an idealized model of systems where the
1
load draws approximately constant current. Figure 5.2b shows the line
current. In this simplified model, the line current is a square wave. In
Figure 5.2c, we see the spectrum of the line current, where the first har-
monic has amplitude 1.0, the third harmonic has amplitude 1/3, the fifth
harmonic has amplitude 1/5, and so on. Note that the line current drawn
by this rectifier circuit is very harmonic-rich, with a THD of 48.3 percent.
Another rectifier is the full-wave rectifier with capacitive filter
(Figure 5.3). In this type of circuit, the diodes are only on for a frac-
tion of a 60 Hz cycle, and the capacitor charges near the peaks of the
input sine wave voltage. Therefore, the line current contains significant
harmonic distortion, as we’ll see in the next example.

Example 5.1: Line-current harmonics. In this example, we’ll examine the
line-current harmonics drawn by the single-phase full-wave rectifier with

1
For instance, this current source models a series LR load where L/R 1/60.