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Power electronic control in electrical systems 353

Fig. 8.72 Typical type ofharmonic current source: (a) phase-controlled thyristor rectifier; and (b) single-phase
equivalent circuit.

current source and represented as a current source shown in Figure 8.72(b). Other
common sources of periodic, non-sinusoidal signals, and hence harmonics, are
adjustable speed drives, inverters, and compact fluorescent lamps (Peng, 1998).
At present, another common harmonic source is that of diode rectifiers with
`smoothing' DC capacitors as shown in Figure 8.74(a). Figure 8.75 shows the current
and voltage waveforms. Although the current is highly distorted, its harmonic
amplitude is greatly affected by the impedance of the AC side whereas the voltage
at the rectifier's input terminals is characteristic and less dependent upon the AC
impedance.
Therefore, a diode rectifier with a `smoothing' capacitor behaves like a voltage
source rather than a current source. Figure 8.74(b) shows the equivalent circuit of the
diode rectifier system, where the diode rectifier is represented as a harmonic voltage
source. Accordingly, the harmonic current originates from the rectifier voltage, and
its content is determined by, and dependent, upon the rectifier voltage and the AC
impedance. Conventionally, passive L±C filters have been used to eliminate line