The frequency converter is a kind of high-tech product, mainly composed of power electronic devices and microelectronic devices. The three-phase ac power grid is connected to the input end of the converter. The input terminal of the inverter is the rectifier circuit and the output terminal is the frequency conversion circuit. Therefore, the voltage and current on the input side and the output side contain rich harmonics, which will lead to the waveform distortion of the grid; At the same time, whether the grid voltage is symmetrical, balanced, the size of the transformer capacity and whether the nonlinear equipment is connected to the distribution bus will also affect the normal operation of the converter.

The present frequency converter has a DSP chip or a computer chip, which realizes the control, and drives the main circuit control of the frequency converter. Therefore, in order to realize power supply that can drive safe and reliable operation, measures must be taken to suppress the mutual interference between the two.

The rectifier circuit and inverter circuit of frequency converter are composed of nonlinear devices. Circuit structure can lead to distortion of network voltage and current waveform. The three-phase ac voltages ur, us and ut are converted from ac to dc through the three-phase bridge rectifier circuit. After being filtered by the electrolytic capacitor, the dc voltage is basically unchanged. The diode used in the rectifier circuit is a nonlinear device. The output voltage after rectification is charged for the filter capacitor. The charging current waveform depends on the difference between the rectifier voltage and the capacitor voltage.

When the frequency converter is in different frequency and different current working state, the input current waveform is also different. Figure 10-6 shows the input current waveform when the 55kW inverter drives the cage asynchronous motor with loads of 10Hz, 20Hz and 50Hz. It can be seen that as the frequency and current of the motor increase, the input current changes from intermittent to continuous, and the waveform distortion of the current becomes less and less. Many harmonic current components will be obtained by mathematical Fourier series decomposition of these waveforms.

The experiment also proves that the commutation of rectifier diode will cause some gap source voltage waveform and protrusion when the inverter is running.

To sum up, the voltage and current waveform of the power grid will be distorted when the converter is running. In order to comprehensively judge the effect of such distortion on the system, the comprehensive voltage distortion rate d can be calculated by the following formula

What is the influence of power grid quality on inverter life

In the workshop, the driver will be able to be used in different loads above, and the effect of different loads in the converter itself is not the same.

Under the impact load of welding machine, electric arc furnace, steel rolling machine, voltage flashover often occurs. At this point, the harmonics generated by the frequency converter have a very serious pollution to the quality of the power grid, but also have a considerable harm to the equipment itself. The light can not work normally continuously, the heavy can damage the input circuit of the equipment. This will seriously affect the service life of the product.

1. In the case of impact load, such as welding machine, arc furnace, steel mill, etc. It is suggested that users add reactive static compensation device to improve power factor and quality of power grid.

2. The concentration rectifier recommended by the inverter concentration workshop and the common bus of dc power supply. The 12-pulse rectifier mode is recommended. Harmonic small, energy efficiency, especially suitable for frequent starting, braking, motor are the advantages of motor running and power generation running.

3. Passive LC filter is installed on the input side of the converter to reduce input harmonics, improve power factor, high reliability and good effect.

4. The input side of the converter is installed on the source PFC device, which has the best effect but high cost.