The Working Principle of Active Power Filter

Active power filter is a new type of power electronic device used to dynamically suppress harmonics and compensate reactive power. It can quickly track and compensate harmonics of different sizes and frequencies. Source filter can only passively absorb harmonics of fixed frequency and size, the active power filter can control and actively output the magnitude, frequency and phase of the current by sampling the load current and separating the harmonics and reactive power, and respond quickly to offset the corresponding current in the load, realizing dynamic tracking compensation. And it can not only make up harmonics but also make up reactive power and unbalance.

1. Theoretical basis of active power filter

Compared with the passive filter, the active power filter design has a good governance effect. It can mainly filter out multiple and high-order harmonics at the same time without causing resonance, but the price is relatively high. The actual application safety factor is very low. The international common practice is to use transformer boosting to ensure reliability. Relevant state departments also require the combination of transformer boosting and active filters to control high-voltage harmonics.

2. The active power filter working principle

The active power filter detects the load current through the current transformer, and calculates through the internal DSP, extracts the harmonic components in the load current, and then sends it to the internal IGBT through the PWM signal to control the inverter to generate a harmonic current with the size of the load. Equal and opposite harmonic currents are injected into the power grid to achieve the purpose of filtering.

The function of the command current detection circuit is mainly to separate the harmonic current component and the fundamental wave reactive current from the load current, and then reverse the polarity to generate the command signal of the compensation current. The function of the current tracking control circuit is to calculate the trigger pulse of each switching device of the main circuit according to the compensation current generated by the main circuit, and this pulse acts on the main circuit after the driving circuit. In this way, the power supply current only contains the active component of the fundamental wave, so as to achieve the purpose of eliminating harmonics and performing reactive power compensation. According to the same principle, the active power filter can also compensate the negative sequence current component of the asymmetric three-phase circuit.

The main circuit of the active power filter is generally composed of a PWM inverter. According to the different energy storage elements on the DC side of the inverter, it can be divided into voltage-type active filters (energy-storage elements are capacitors) and current-type active filters (energy-storage elements are inductors). The voltage-type active filter needs to control the capacitor voltage on the DC side during operation, so that the voltage on the DC side remains unchanged, so the output of the AC side of the inverter is a PWM voltage wave. The current-type active filter needs to control the inductor current on the DC side during operation, so that the current on the DC side remains unchanged, so the output of the AC side of the inverter is a PWM current wave. The advantages of the voltage-type active filter are that the loss is less and the efficiency is high. The current will produce a large loss in the internal resistance of the inductor.

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