Harmonics - Rectifiers With Capacitive Filter

2.4 CAUSES OF HARMONICS - Rectifiers with Capacitive Filter

As already seen, most electronic equipment has an input stage consisting of a single-phase rectifier with capacitive filter. This type of circuit produces currents in the network impulsively, centered approximately at the peak of the sine wave. The circuit is shown in figure 1.

Figure 1 - Single-phase rectifier with capacitive filter.

Figure 2 shows the voltage and current waveforms obtained by simulation as well as the current spectrum. Note the great amplitude of the harmonics , producing, of course, a high THD.

(a) (b)

Figure 2 - (a) Input current and rectifier supply voltage supplying capacitive filter. (b) Current spectrum.

Similar situation occurs with a three-phase input, when two current pulses are observed in each half-cycle, as shown in figure 3. Note once again the significant distortion that can occur in the shape of the voltage due to the voltage drop occurs on the line reactance.

Figure 3 - Input voltage (upper) and line current (lower) in three-phase rectifier with capacitive filter.