1 Introduction

The ideal voltage and current waveforms in power systems are all sine waves with a frequency of 50Hz, but they are widely used in non-linear power equipment (such as high-power thyristors, frequency converters, UPS, switching power supplies, mid-frequency furnaces, and energy-saving lamps), in which a large number of distorted harmonic currents are generated and harmonic currents are coupled to produce harmonic voltages on the line. The Fourier series decomposition is performed on the non-sinusoidal distortion current, where the component of the same frequency with the power frequency is the fundamental wave, and the component of the integral multiple of the fundamental wave frequency is the

harmonic wave. This kind of harmonic "pollution" will cause great harm to the power grid and the user, so the harmonic suppression is called a "green project"!

Due to the development of power electronics technology, active power filters (APFs) have begun to be applied on a large scale. Compared to traditional harmonic suppression and reactive power compensation devices, APFs respond quickly and trace each harmonic in real time to compensate for harmonics, which have the functions of harmonic compensation, reactive power compensation and balance of three-phase current. At the same time, APF is free form system impedance, without hidden resonance problems, of high compensation efficiency, which has been widely used in

all walks of life power supply system. The product meets the requirements of the enterprise standard Q31/0114000129C033-2017 ANAPF Active Power Filter.

1.2 Basic Principles of ANAPF

ANAPF series active power filters are connected in parallel in a low-voltage power distribution system with harmonic loads, enabling fast and real-time tracking and compensation of dynamically changing harmonic currents. The principle is: ANAPF series active power filter collects harmonic current through CT, and quickly calculates and extracts the content of each harmonic current through the controller, generates harmonic current command, and then generates the compensation currents, which are of the same amplitude and the opposite direction with the harmonics current through power execution device. At last, the compensation currents are injected into the power system to cancel out the harmonic currents

generated by the non-linear loads.

The composition of the ANAPF system

Figure 1-2 shows the system schematic of the ANAPF. In the figure, the current source is the AC voltage of the power system, and the non-linear load is a harmonic source, where the harmonic and reactive currents are generated during operation. ANAPF is mainly composed of load current detection, command current calculation, bridge arm current output control, drive circuit and main circuit. By detecting the harmonic current component in the load current, the required command current for actual compensation is obtained. The IGBT drive circuit and the main circuit are the compensation current generation circuit, whose main function is to generate the actual compensation current based on the harmonic current compensation signal obtained by the instruction operation circuit. The main circuit is mainly composed of a voltage-type PWM converter, and an inductor and a DC-link capacitor (DC-Link) connected thereto.

ANAPF not only filters out harmonic currents, but also compensates reactive power appropriately and balances the current in three-phase systems.