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introduction
In the high-voltage inverter, in order to solve the strong and weak electrical isolation between the main control system and the power unit in the multi-level high-voltage inverter connected in series, and the electromagnetic interference between the power unit and the power unit, a fiber optic connection is proposed. The method realizes the long-distance transmission of the power-driven PWM signal.

background
Pump loads widely used in industries such as metallurgy, chemical industry, electric power, municipal water supply, and mining account for about 40% of the energy consumption of the entire electrical equipment, and electricity bills even account for 50% of the cost of water production in waterworks. This is because: on the one hand, there is usually a certain margin in the design of the equipment; on the other hand, due to the change of working conditions, the pump needs to output different flow rates.

With the development of the market economy and the improvement of automation and intelligence, the use of high-voltage frequency converters to control the speed of pump loads is not only good for improving the process and product quality, but also meets the requirements for energy saving and economical operation of equipment. The inevitable trend of sustainable development. The benefits of variable speed control of pumping loads are numerous. From the application examples, most of them have achieved good results (some energy saving is as high as 30%-40%), which greatly reduces the water production cost of waterworks, improves the degree of automation, and is conducive to the integration of pumps and pipe networks. Reduced pressure operation reduces leakage and pipe burst and prolongs the service life of the equipment.

The unit series multi-level PWM voltage source inverter adopts several low-voltage PWM frequency conversion power units in series to realize direct high-voltage output. The inverter has little harmonic pollution to the power grid, low harmonic input current, and high input power factor. It is not necessary to use input harmonic filter and power factor compensation device. For example, take the output voltage level of 6kV as an example (as shown in Figure 2). The grid voltage is stepped down by multiple isolation transformers on the secondary side and then supplies power to the power unit. The power unit is an AC-DC-AC PWM with three-phase input and single-phase output. Power type inverter structure. The output ends of adjacent power units are connected in series to form a Y-connection structure, which realizes the direct output of high voltage with variable voltage and frequency, and supplies the high voltage motor.

In the high-voltage frequency conversion system, we realize that the signal in the control unit cabinet is mainly weak current, but the working environment of the whole system is relatively harsh, the transmission distance is uncertain (long), and there are various high-voltage electrical signals. If a pure copper wire is used for transmission, it needs to face various problems such as interference, boosting, stepping down, large loss, and easy corrosion, resulting in low reliability, low practicability, and waste of resources. The use of industrial optical fiber is a good solution to the above problems, so optical fiber communication will inevitably become the main way of communication in the future.

Industrial optical fiber solves the problems of voltage transformation, loss and interference in copper wire transmission by converting electrical signals into optical signals. It is mainly used in the communication of control cabinets and frequency conversion power unit cabinets.