The most interesting reduction technique of current harmonic is a PWM (active) rectifier (Fig. 1.3). Two types of PWM converters, with a voltage source output (Fig. 1.4) and a current source output (Fig. 1.5) can be used.

An alternative to the passive filter is use of the active PWM filter (AF), which displays better dynamics and controls the harmonic and fundamental currents. Active filters are mainly divided into two different types..

The traditional method of current harmonic reduction involves passive filters LC, parallel-connected to the grid. Filters are usually constructed as series-connected legs of capacitors and chokes [the number of legs depends on number of filtered harmonics (5th, 7th, 11th, 13th) ].

Many harmonic reduction methods exist. These techniques based on passive components, mixing single and three-phase diode rectifiers, and power electronics techniques as: multi-pulse rectifiers, active filters and PWM rectifiers (Fig. 1.1). They can be generally divided as: