In recent years, with the rapid development of power electronics technology, switching power supply is more and more widely used. On the one hand, switching power supply is developing towards high reliability and miniaturization; on the other hand, the power capacity and power density of power electronic devices are also increasing, such as intelligent welding machine, photovoltaic power generation, frequency  converter, servo control system and so on. But with the high frequency turn on/off operation, the big power switch tube in the switch power supply will inevitably produce the high order harmonic and the electromagnetic interference, which is injected into the power network, and endangers the stability of power network and the system. There are two main harmonic suppression methods at home and abroad: passive filter and active filter. There are many reports on harmonic analysis and suppression at home and abroad, especially the study of active power filter. Although the effect of active filter is good in theory, but because of its technical complexity and high cost, the practical application is relatively small at present. In most cases, passive filtering is still used to suppress harmonics. Passive filter can suppress harmonics on one hand, also has the function of reactive power compensation on the other.

At present, harmonics have become one of the main obstacles to the normal and reliable operation of switching power supply and electronic system in the field of application. European and American countries have formulated relevant standards in 1996 and enforced strictly. Electrical products that do not meet their standards are not allowed to enter the market. China also formulated corresponding standards in 2003, and in a few cities, the system of punishing the harmonic content exceeding the standard is introduced, so now it is very important to study the harmonic of switching power supply.

1. The harm of harmonics to power system is as follows:

(1) additional power loss and heating to the rotating motor, which may cause vibration.

(2) the resonant or harmonic current of reactive compensation capacitor banks is amplified and the capacitor is damaged by overload or overvoltage, as well as overload or overvoltage breakdown of power cable.

(3) Increase the loss of power grid. The loss can reach a considerable degree when resonance or amplification occurs.

(4) Malfunction and interference caused to relay protection, automatic control device and information machine.

(5) Harmonics increase the error of watt-hour meter itself, the user absorbs both fundamental reactive power and harmonic power.

2. The method of restraining harmonics in power supply

Harmonic suppression of switching power supply can be divided into preventive and remedial. Preventive is to use effective process and method to control switching tube according to design requirements when design switch power supply, but no matter what advanced control strategy is used, the harmonic component is unavoidable. Remedial solution is to overcome the harmonics already existed in the system. At present, the technology of harmonic suppressing is using filters (passive filters, active filters). Active filters have been studied from an ideal point of view in many literatures at home and abroad, but it is very difficult in practical applications, such as the detection of system harmonic content, the amount of harmonic content offset, real-time, and active filters contain switching devices which produce harmonics themselves, and so on. So passive filters are very suitable from the point of view of cost and control method. Considering certain margin in design, it can be designed as a more general filter device.