Active PWM filter

Submitted by bssuperadmin on Sun, 07/27/2014 - 21:10

1.4.4 Active PWM filter

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 active shunt filter (current filtering) (Fig.1.2) and the active series filter (voltage filtering).


Fig-1.2: Active PWM shunt filter


The three-phase two-level shunt AF consists of six active switches and its topology is identical to the PWM inverter. AF represents a controlled current source   which when added to the load current   yields sinusoidal line current .



Active Filter provide:

  • compensation of fundamental reactive components of load current,

  • load symetrization (from grid point of view),

  • harmonic compensation much better than in passive filters.


In spite of the excellent performance, AFs possess certain disadvantages as complex control, switching losses and EMC problems (switching noise is present in the line current and even in the line voltage). Therefore, for reduction of these effects, inclusion of a small low-pass passive filter between the line and the AF is necessary.



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Power electronics in broader aspect actually controls the flow of power to make it suitable to be used by diverse type of instruments and appliances. The ability to control the flow of power is derived from using solid state switches along with their controllers.

PWM Rectifier

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.

Multi-pulse rectifier

1.4.3 Multi-pulse rectifier.

Mixing single and three phase diode rectifier

1.4.2 Mixing single and three phase diode rectifier

Traditional harmonic reduction technique

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) ].