Abstract: The first high-voltage, high-power variable frequency speed control device produced by Shenzhen Winner S&T Co., Ltd. is a product developed and designed based on the absorption of previous technical experience and advantages, using a unit series multilevel method. It is a relatively mature technology solution in the market. Keywords: High-voltage frequency converter [b]1. Introduction[/b] Shenzhen Winner S&T Co., Ltd. is a professional manufacturer with independent intellectual property rights, producing 20,000 low, medium, and high-voltage frequency converters annually. With over 20 years of experience in frequency converter design and production, it has manufactured the first 6kW and 400kW high-voltage variable frequency speed control devices. Winner Technology enriches your future. In recent years, domestic manufacturers of high-voltage frequency converters have been rapidly emerging. Currently, the use of frequency converters for variable speed drive of AC motors has been widely applied in industrial control. Especially for square torque loads such as fans and water pumps, since power is proportional to the cube of the speed, using speed control instead of valves or baffles to regulate flow is the best energy-saving control solution. High-voltage AC variable frequency speed control technology is a new type of electric drive speed control technology that has developed rapidly in the 1990s. Its technology and performance surpass any previous speed control method (such as: voltage reduction speed control, pole changing speed control, slip speed control, internal feedback series pole speed control and hydraulic coupling speed control, etc.). Variable frequency speed control has been recognized by users and the market due to its significant energy-saving benefits, high speed control accuracy, wide speed range, perfect power electronic protection functions, and easy-to-implement automatic communication functions. In terms of safe and reliable operation, installation and use, maintenance and repair, it has brought great convenience and fast service to users, making it the first choice for enterprises to adopt motor energy-saving methods. 2. Several high-voltage variable frequency speed control methods High-voltage high-power frequency converter speed control is the most advanced high-voltage motor speed control method in modern times, but so far, there is no unified main circuit topology structure like low-voltage frequency converters. According to the different composition methods of high voltage, high-voltage frequency converters can be roughly divided into the following three types: (1) High-low-high type high voltage frequency converter uses a transformer to step down the input voltage and step up the output voltage at both ends of the motor. In essence, it is still a low voltage frequency converter. It is a workaround that is specially adopted due to the limitations of the voltage rating of power devices. The main advantage of this method is low equipment cost, but it has many disadvantages, such as large current in the intermediate link, low system efficiency, poor output waveform, poor reliability, large footprint, and increased operating cost. At the same time, the power factor is low and the harmonic pollution of the power grid is large. With the development of power electronics technology, it has been basically eliminated. (2) High-high three-level type high voltage frequency converter uses diode clamped three-level structure in both input rectifier circuit and inverter circuit. The midpoint of the two converters is connected and vector control is used to achieve four-quadrant operation. The system has good dynamic performance, but this three-level voltage source frequency converter has problems such as output harmonics and dv/dt. Generally, an output filter must be set up. Otherwise, a special motor must be used. (3) The unit-series multilevel PWM voltage source inverter uses several low-voltage PWM inverters connected in series to achieve direct high-voltage output. This inverter features low input harmonics, high input power factor, high efficiency, and good output waveform. The inverter's input side uses a multi-winding phase-shifting transformer, making the input current closer to a sine wave, combining multilevel and multiplexing characteristics. (See Figure 1). [align=left] Figure 1: Unit-series multilevel inverter topology. Key evaluation indicators for high-voltage inverters include cost, reliability, harmonic pollution to the power grid, input power factor, output harmonics, dv/dt, common-mode voltage, system power, and whether it can operate in four quadrants. For high-power motors such as fans and pumps that generally do not require four-quadrant operation, the unit-series multilevel PWM voltage source inverter has significant advantages in terms of input and output harmonic power and input power factor. Currently, domestic manufacturers producing this type of high-voltage inverter mainly include Beijing Leadway, Hekang Yisheng, Xindian Chuangtuo, and Chengdu Dongfang Kaiqi. 3. Microenergy High-Voltage Frequency Converter Microenergy Company, absorbing the technical experience and advantages of predecessors and overcoming the shortcomings encountered in actual operation, is developing, designing, and producing a direct high-voltage frequency converter using a unit-series multi-level PWM voltage type AC-DC-AC converter. The system principle is shown in Figure 1. The frequency converter adopts sinusoidal PWM technology with space voltage vector control and employs a multi-stage design, where several low-voltage PWM power units in each phase are connected in series. The number of power units connected in series varies depending on the output voltage. Each power unit is powered by a multi-winding isolation transformer, and control is achieved using a high-speed microprocessor and optical fiber isolation drive. This multi-stage design fundamentally solves the harmonic problem generated by general 6-pulse and 12-pulse frequency converters, enabling perfect harmonic-free frequency conversion speed regulation. Figure 2 shows the main power supply topology of a 6kW frequency converter. Each phase is powered by 5 voltage-rated 690V units. Each power unit is powered by 15 secondary windings of the input isolation transformer. The 15 secondary windings are divided into 5 groups, with a 12° phase difference between each group. Each power unit in Figure 2 is a three-phase input, single-phase output low-voltage PWM voltage inverter composed of 1GBTs, as shown in Figure 3. Figure 3 shows the structure of each power unit. Each power unit outputs voltage at three levels: 1, 0, and -1. With 5 units superimposed in each group, 11 different voltage levels can be generated: ±5, ±4, ±3, ±2, ±1, and 0. High-voltage output is achieved by connecting power units in series instead of using traditional device series connection, thus eliminating the problem of voltage equalization. Each power unit bears the entire output current but only 1/5 of the output phase voltage and 1/15 of the output power. The inverter uses multiple PWM technology, modulating the fundamental voltage with 5 pairs of triangular carrier waves with a 12° phase shift. The 5 signals obtained from the A-phase fundamental modulation control A1 to A5, the 5 power units, and after superposition, an 11-level stepped phase voltage waveform can be obtained. Theoretically, harmonics below the 19th order can be controlled. To compensate, the total voltage and distortion rate are below 1.2% and 0.8% respectively, making it a near-perfect harmonic-free (HAR) inverter. Its power factor can reach over 0.95, eliminating the need for input filters and power factor compensation devices. If the IGBT switching frequency of each power unit is 600Hz, then when five power units are connected, the equivalent output phase voltage switching frequency is 6kHz. Besides reducing harmonics, the improved waveform also reduces noise, dv/dt value, and motor torque ripple. Although this main circuit topology increases the number of components, the low IGBT drive power and the elimination of voltage equalization circuits, absorption circuits, and output filters allow the inverter efficiency to reach over 96%. The input and output waveforms are shown in Figure 4 (with a 6kV, 315kW motor). Figure 4 Input and Output Harmonics 4. Conclusion This high-voltage frequency converter has the following characteristics: The product has low input harmonic current, fully meeting the harmonic requirements of IEEE519-1992 and GB/T14549-93; No power factor compensation device is required, with a power factor greater than 0.97; No input/output filters are required, and there are no special requirements for the motor; System power efficiency reaches up to 96%; User-friendly interface; Full Chinese WINDOWS NT operating platform, large-screen true-color LCD display, quick and convenient function parameter settings, and the ability to save up to 16 sets of parameter settings; Automatic recording of each operation and fault, viewing and printing output records and reports, and real-time viewing of the frequency converter's output and input waveforms; Flexible control methods; Local control can be achieved through the touch screen, remote control can be achieved through the built-in PLC I/O, and upper-level control can be achieved by connecting to DCS and other systems via the RS485 interface protocol.