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.