Method for suppressing harmonics of frequency converter

No matter which type of inverter, non-linear power electronic components such as thyristors are used in large quantities. Regardless of which rectification method is adopted, the way in which the inverter draws energy from the grid is not a continuous sine wave, but a pulsed discontinuity. The method obtains current from the grid, and the pulsating current and the impedance along the path of the grid form a pulsating voltage drop superimposed on the voltage of the grid to distort the voltage. According to the Fourier principle analysis, the asynchronous sinusoidal current is due to the same frequency. The harmonics of the wave and frequency are greater than the fundamental frequency.

1. Multiplexing of the 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 phase of the input current is staggered by the transformer, and the inverter is suppressed by multiplexing. Higher harmonics to the power supply side.

2. Install AC/DC reactor

The installation of the reactor is actually to increase the impedance of the inverter power supply from the outside. Install a reactor on the AC side or DC side of the inverter or install it at the same time to suppress the harmonic current. After using the AC/DC reactor, the incoming current (voltage distortion rate) is reduced by about 30% to 50%, which is about 1/2 of the harmonic current of the reactor.

3. Install active power filter

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

4, increase the internal impedance of the inverter power supply

Normally, the internal impedance of the power supply device can act to 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 is compared with the inverter capacity, the larger the internal impedance value is, and the smaller the harmonic content is. The larger the power supply capacity is compared with the inverter capacity, the smaller the internal impedance value is and the larger the harmonic content is. Therefore, when selecting the inverter power supply transformer, it is best to choose a transformer with a short circuit impedance.

5, install the output reactor

It is also possible to use a method of adding an AC motor between the frequency converter and the motor. The main purpose is to reduce the electromagnetic radiation generated by the line during the energy transmission of the output of the frequency converter.

6, using an isolation transformer

The use of an isolation transformer is mainly to deal with conducted interference from the power supply, as shown in Figure 4. Using an isolation transformer with an isolation layer, the conducted interference of the dead part can be isolated before the isolation transformer. At the same time, it can also have the function of power supply voltage conversion. Isolation transformers are commonly used to control the conducted interference of meters, plc, and other low-voltage, low-power consumers in control systems.

7, using filter modules and components

At present, there are many filter modules or components dedicated to anti-conduction interference. These filters have strong anti-interference ability, and can also prevent the conduction of the electric device itself from being transmitted to the power supply. Some of them also have the function of peak voltage absorption. There are many benefits to various types of electrical equipment.

8, grounding anti-jamming

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 mode of the inverter has several forms such as grounding, one-point grounding and busbar grounding. It should be used according to the specific conditions. Be careful not to interfere with the equipment due to poor grounding.

9, do the signal line anti-interference

The signal line bears the transmission task of the detection signal and the control signal. It is undoubted that the quality of the signal transmission directly affects the accuracy, stability and reliability of the entire control system. Therefore, it is necessary to do the anti-interference of the signal line.