What is an active power filter

Active Power Filter (APF) is a new type of power electronic device used to dynamically suppress harmonics and compensate reactive power, which can compensate for harmonics and reactive power changes in magnitude and frequency.

The reason why the active filter is called active, as the name suggests, the device needs to provide a power supply (to compensate for the harmonics of the main circuit), and its application can overcome the shortcomings of traditional harmonic

suppression and reactive power compensation methods such as LC filters (the traditional can only be fixed), realize dynamic tracking compensation, and can compensate both harmonics and reactive power.

The theory of instantaneous reactive power of three-phase circuit is the main basic theory of the development of APF; APF has two types: parallel type and series type, the former is used more; parallel active filter is mainly to control current

harmonics, and series active filter is mainly to control the problems caused by voltage harmonics. Compared with passive filters, active filters have good treatment effects, and can mainly filter out multiple and higher harmonics at the same time, which will not cause resonance, but the price is relatively high.

Fundamentals of active power filters

Active power filter is a new type of special equipment for power harmonic control made of modern power electronics technology and digital signal processing technology based on high-speed DSP devices. It consists of two main parts: the
instruction current operation circuit and the compensation current generation circuit. The command current operation circuit monitors the current in the line in real time, converts the analog current signal signal into a digital signal, sends it to the
high-speed digital signal processor (DSP) to process the signal, separates the harmonic from the fundamental, and sends a driving pulse to the compensation current generation circuit in the form of a pulse-width modulation (PWM) signal, drives the IGBT or IPM power module, generates a compensation current equal to the amplitude of the harmonic current of the power grid and opposite in polarity, and injects it into the power grid, compensates or cancels the harmonic current, and

actively eliminates the power harmonics.

How active power filters work

The active filter is to use the current transformer to collect the current on the DC line, after sampling, the obtained current signal is processed by the harmonic separation algorithm to obtain the harmonic reference signal, as the modulation signal, compared with the triangle wave, so as to obtain the switching signal, use the switch signal to control the single-phase bridge, according to the principle of technology, the switching signal of the upper and lower bridge arms is reversed, and the harmonic current that is equal to the size of the harmonic signal on the line and the opposite direction can be obtained, and the harmonic current on the line is canceled. This is the feedforward control part. The harmonic component of the line current behind the access point of the active filter is then fed back as the input of the regulator to adjust the error of the feedforward control：

The main hazards of harmonics:
1. Increase the load of power facilities, reduce the power factor of the system, reduce the effective capacity and efficiency of power generation, transmission and electrical equipment, and cause equipment waste, line waste and power loss;
2. Cause resonance and harmonic current amplification of reactive power compensation capacitors, resulting in damage or failure of the capacitor bank due to overcurrent or overvoltage;
3. The motor vibrates due to the pulsating torque, which affects the product quality and the life of the motor;
4. Due to eddy current and skin effect, motors, transformers, transmission lines, etc. will produce additional power loss and overheat, waste electric energy and accelerate insulation aging;
5. The harmonic voltage enhances the electric field strength of the insulating medium in the form of being proportional to its peak voltage, and reduces the service life of the equipment;
6. The zero sequence (multiple times of 3) harmonic current will cause the center line of the three-phase four-wire system to be overloaded, and produce a circulation current in the transformer winding of the triangle connection, so that the winding
current exceeds the rated value, and even cause an accident in serious cases. 7. Harmonics will change the action characteristics of the protection relay, cause the malfunction of the relay protection facilities, and cause the work of automatic devices such as relay protection to be disordered;
8. The harmonic change changes the rate of change and the peak value of voltage or current, delays the extinguishing of the arc, and affects the breaking capacity of the circuit breaker;
9. Make the metering instrument, especially the induction electric energy meter, produce measurement error;
10. Interfere with nearby power electronic equipment, industrial control equipment and communication equipment, and affect the normal operation of the equipment.