I. Passive harmonic filter　　

The passive filter is mainly composed of the inductor and the capacitor and is widely used due to its low cost, economic and simple properties. Passive filters can be classified into parallel filter and series filter. 　　

1. Passive parallel filter 　　

Most of the existing harmonic filters use passive parallel filters. A set of filters shall be used for each frequency of harmonic and sets of filters shall be used usually to filter different frequencies of harmonics. With complex construction and high costs, sets of filters cannot fully filter all harmonics due to harmonic components of infinite frequencies contained in the general system. Moreover, the parallel filter has very low impedance to the harmonic, so the harmonic source will produce greater harmonic current generally and the resonances will mutually interfere in different frequencies of filters. For example, a septic filter may amplify fifth harmonic. Comparison between the harmonics before and after installation of the parallel filter shows that the harmonic current affecting the system is decreased after installation of the filter, but the sum of the harmonic current in each filter and entering the system is far above that before installation of the filter and the harmonic current generated by the harmonic source also exceeds that before installation of the filter. 　　

Broadly speaking, the components with the frequency unequal to the power frequency are harmonics. Therefore, power frequency is a single frequency, but harmonics have infinite frequencies, indicating that the harmonics are of infinite complexity and the use of parallel filters is obviously unable to cope with the harmonics of infinite frequencies. 　　

2. Passive series filter 　　

LC series filter composed of the inductor and capacitor has a series resonance point with very low impedance. A series filter with a series resonance point as the power frequency may be created and connected in the circuit in series to filter all harmonics. It is the series filter introduced here. The series filter is composed of the inductor and the capacitor in series and is connected between the power supply and the load in series, so "series" of the series filter has double meaning, i.e. series of inductor and capacitor and series connection in the circuit. 　　

Series filters are connected in the three-phase circuit. Series connection of band-pass filter has a small impedance to the fundamental current and large impedance to the harmonic current, so only a set of filters can be used to filter the harmonics of all frequencies. 　　

The series filter has an extremely low resistance to the current at the resonance point frequency. For the current deviating from the resonance point frequency, the impedance is increased and is more with more deviation. For the current components higher than the resonance point frequency, the inductor impedance is based; for the current components lower than the resonance point frequency, the capacitor impedance is based. The harmonic components are usually higher than the fundamental frequency, so the inductor mainly filters the harmonics, while the capacitor mainly offsets the impedance of the inductor to the power frequency fundamental harmonics. 　　

The harmonics are mainly filtered by the inductor, so the harmonic filter effect is better when the inductance value is greater. But in this case, the price and the loss are higher. So the inductance value is the smaller the better in terms of cost and loss. When the fundamental harmonic inductive reactance of the inductor is less than 50% of load equivalent fundamental harmonic inductive reactance, good filter effect cannot be achieved (load equivalent fundamental harmonic inductive reactance is the ratio of the effective value of the load phase voltage to the effective value of the phase current). So the fundamental harmonic inductive reactance of the inductor must be greater than 50% of load equivalent fundamental harmonic inductive reactance. 　　

The selection methods of the capacitor and inductor are different. The number of turns of the inductor can be designed arbitrarily, while the withstand voltage of the capacitor is fixed in several grades. For example, only the power capacitors with the withstand voltage of 230V and 400V are optional in the low-voltage distribution system. The capacitor is connected in the circuit in series, so the current in the capacitor is the load current. When the actual working voltage of the capacitor is equal to its rated voltage, the current through the capacitor is equal to the rated current of the capacitor and the capacitor is fully used, so the capacitor has the lowest cost when the actual working voltage of the capacitor is equal to its rated voltage. 　

The actual series filter cost is mainly composed of the cost of the inductor and the capacitor. The inductor and the capacitor with series resonance have the same impedance to the fundamental harmonic and the same current, so the working voltage of the fundamental harmonic of the inductor and the capacitor is the same. As mentioned above, when the actual working voltage of the capacitor is equal to its rated voltage, the capacitor has the lowest cost, so the actual working voltage of the inductor shall be equal to the rated voltage of the capacitor. The rated voltage grade of the capacitor mostly matches the grid voltage.

If the actual working voltage of the inductor is equal to the rated voltage of the capacitor, i.e. the inductor impedance equal to the load impedance, the best cost performance can be achieved. If the inductor inductive reactance is increased on this basis, the filter effect can be increased slightly, the inductor cost is increased and the capacitors shall be connected in series, resulting in rapid increase in the cost and decrease in cost performance, so it is of no practical significance when the fundamental harmonic inductive reactance of the inductor is greater than 200% of the load equivalent fundamental harmonic inductive reactance.

If the inductor inductive reactance is decreased, the inductor cost is reduced and the capacitor capacity is increased, resulting in increase in the cost and decrease in the cost performance. For sufficient reliability, the actual working voltage of the inductor and the capacitor shall be slightly lower than the rated voltage of the capacitor. 　　

When the harmonic current inflows from the external network, affecting the normal operation of the load equipment in the internal network, a series filter can be connected between the power supply and the load equipment to resist the harmonics and ensure normal operation of the load equipment. 　　

When the harmonic is generated from the internal network equipment, affecting the system, the equipment generating the harmonic is the harmonic source and a series filter can be connected between the harmonic source and the power supply to significantly reduce the harmonic current generated by the harmonic source. It shall be noted here that the series filter, reducing the harmonic current generated by the harmonic source, equivalent to reducing the pollution generated by the pollution source, is a means to take radical measures. The series filter provides a low-impedance channel for the harmonic current rather than reducing the harmonic generated by the harmonic source to prevent harmonic current from polluting the system and is a means to take temporary solutions. Moreover, the parallel filter has low impedance to the harmonic and will generally make the harmonic source generate greater harmonic current. 　　

When the series filter is connected between the power supply and the harmonic source, the input voltage waveform of the harmonic source will be seriously distorted, so that the current of the harmonic source is close to the sine wave. Such distortion may affect the normal operation of the harmonic source control circuit. In this case, the power supply of the control circuit shall be connected to the front end of the series filter. 

II. Active harmonic filter 　　

Active harmonic filter is developed on the basis of the passive harmonic filter. 　　

1. Advantages of active harmonic filter 　　

The active harmonic filter can achieve timely compensation and good filter effect without increasing the capacitive components in the grid. Within the rated reactive power range, the filter effect is 100%. 　　

2. Disadvantages of active harmonic filter 　　

The active harmonic filter is restricted by the withstand voltage and rated current of the power electronic elements and is more complex than the passive harmonic filter, so its cost is very high. 　　

3. Principle of active harmonic filter 　　

In the active harmonic filter, a circuit is constituted by power electronic components to generate a harmonic current of the same frequency and amplitude but reverse phase with the system harmonic to offset the harmonic current in the system. 　　

4. Applications of active harmonic filter 　　

The active harmonic filter is mainly applied in the power supply system of the computer control system, especially the power supply system of the office building and the factory computer control supply system. 　　

5. Current status of active harmonic filter 　　

The development of the power electronic technology and the application of variable frequency rectifier and nonlinear loads affect the power quality more and more obviously and the costs are also decreased, so the active harmonic filter is chosen by more users.