Because of the vigorous development of frequency conversion technology, the use of frequency converters in various industries is getting wider and wider, but the harmonic problems of frequency converters have been unable to circumvent. No matter what kind of frequency converter, a large number of non-linear power electronic components such as thyristors are used. No matter which rectification method is used, the way the frequency converter draws energy from the power grid is not a continuous sine wave, but a pulsating intermittent In this way, the current is obtained from the power grid. This pulsating current and the impedance of the road along the power grid together form a pulsating voltage drop superimposed on the voltage of the power grid, and the voltage is distorted. According to the Fourier principle analysis, this asynchronous sine wave current is due to the same frequency The harmonic composition of the wave and frequency is greater than the fundamental frequency.

First, the method of suppressing the harmonics of the inverter

1. Multipleization of converter circuit

For large-capacity inverters, a dedicated power input transformer can be installed at the input end of the inverter, and the power-side converter can be divided into two. The transformer can be used to shift the phase of the input current, and the inverter can be suppressed by multiplexing. Higher harmonics to the power supply side.

2. Install AC / DC reactor

Installing a reactor actually increases the impedance of the power supply of the inverter from the outside. Installing a reactor on the AC side or DC side of the inverter or installing them at the same time can suppress harmonic currents. After using the AC / DC reactor, as shown in Figure 2, the incoming current (voltage distortion rate) is reduced by about 30% to 50%, which is about 1/2 of the harmonic current without the reactor.

3. Install active power filter

In addition to the traditional LC debugging filters that are still in use before, an important trend of current harmonic suppression is the use of active power filters, which are connected in series or in parallel to the main circuit to detect harmonic currents from the compensation object in real time A compensation current equal to the harmonic current and opposite in direction is generated by the compensation device, so that the grid current only contains the fundamental current. This kind of filter can track and compensate the harmonics whose frequency and amplitude change. Its characteristics are not affected by the system and there is no danger of harmonic amplification. Therefore, it has attracted much attention and has been widely used in Japan and other countries.

4. Increase the internal impedance of the inverter power supply

Under normal circumstances, the internal impedance of the power supply device can buffer the reactive power of the DC filter capacitor of the inverter. This internal impedance is the short-circuit impedance of the transformer. The smaller the power supply capacity relative to the inverter capacity, the larger the internal impedance value, and the smaller the harmonic content; the larger the power supply capacity relative to the inverter capacity, the smaller the internal impedance value, and the greater the harmonic content. Therefore, when choosing a transformer power supply transformer, it is best to choose a transformer with a large short-circuit impedance.

5. Install the output reactor

The method of adding an AC electric machine between the frequency converter and the motor can also be used. The main purpose is to reduce the electromagnetic radiation generated by the line during the energy transmission of the output of the frequency converter.

Second, the method of the equipment itself to reduce harmonics and interference

1. Use isolation transformer

The use of isolation transformers is mainly to deal with conducted interference from the power supply, as shown in Figure 4. Using an isolation transformer with an isolation layer can isolate most of the conducted interference before the isolation transformer. At the same time, it can also have the role of power supply voltage conversion. Isolation transformers are commonly used in the anti-conduction interference of instruments, PLCs and other low-voltage low-power electrical equipment in the control system.

2. Use filtering modules and components

At present, there are many filter modules or components in the market that are specifically used to resist conducted interference. These filters have strong anti-interference ability, and at the same time can prevent the interference of the electrical appliance itself from being transmitted to the power supply, and some also have peak voltage absorption functions. , Has many benefits for various types of electrical equipment.

3. Ground anti-interference

Grounding is an important means to suppress noise and prevent interference. A good grounding method can largely suppress the coupling of internal noise, prevent the intrusion of external interference, and improve the anti-interference ability of the system. The grounding method of the inverter has several forms such as multi-point grounding, one-point grounding and grounding via busbar. It should be adopted according to the specific situation. Be careful not to cause interference to the equipment due to poor grounding.

4. Make anti-interference of signal line

The signal line is responsible for the transmission of detection signals and control signals. There is no doubt that the quality of signal transmission directly affects the accuracy, stability and reliability of the entire control system. Therefore, it is necessary to do anti-interference of the signal line.

The harmonic interference of the inverter will cause the communication system to not work properly, affect the working accuracy of the electronic equipment, make the quality of the finishing equipment unstable, reduce the service life of the equipment, and greatly reduce the use of household appliances. Therefore, it is very important to correctly understand the harmonic interference of the inverter and the method of suppressing harmonics.