1. Introduction As is well known, AC motors are widely used in electric drives because they are simpler in structure, easier to manufacture, cheaper, and easier to maintain than DC motors. However, AC motors are more troublesome or difficult to start and regulate speed compared to DC motors. The synchronous speed of an AC motor. It can be seen that as long as the frequency f of the power supply can be continuously changed, the speed of an AC motor can be smoothly adjusted (where p is the number of pole pairs of the motor, which can only be changed in stages and to a limited extent). Due to the development of power electronics technology, static frequency converters composed of power semiconductor devices are now very mature, and variable voltage variable frequency (VVVF) speed regulation of AC motors has become increasingly popular, and its advantages are widely known. However, variable frequency speed regulation is still mainly used in medium and small capacity and low voltage motors. In industrial sectors such as mining, metallurgy, chemical, petroleum, and building materials, as well as water plants and power plants, a large number of medium and high voltage fans, pumps, compressors, and mixers are used. The power of these machines is in the hundreds of kilowatts or even thousands or tens of thousands of kilowatts, and their energy consumption is considerable. Previously, most of these machines used constant speed AC transmission, and the output was adjusted by means of baffles, valves or empty discharge return, resulting in a large loss of electrical energy. Therefore, using frequency conversion speed regulation on these machines and changing the output power by electrical means according to the output requirements is of great significance for energy saving. 2. Several problems of high voltage frequency conversion speed regulation power supply (1) Series connection In theory, when the device requires high voltage but the device has limited voltage withstand capability, the device can be connected in series to meet the requirements. However, the use of devices in series has steady-state and dynamic voltage equalization problems due to the different dynamic resistance and polar capacitance of each device. If the voltage equalization measures of parallel R and RC with the device are adopted, the circuit will become more complicated and the loss will increase. At the same time, the requirements of the drive circuit are also greatly increased by the device series connection. It is necessary to make the devices in series conduct and turn off at the same time. Otherwise, due to the different on and off times of each device, the voltage is uneven, which will lead to device damage or even the collapse of the entire device. (2) Harmonics This is a common problem of all AC-DC-AC frequency converters, but it is more prominent in high power frequency conversion speed regulation. Harmonic pollution of the power grid will affect other equipment on the same power grid and even affect the normal operation of the power system. Harmonic currents also cause motors to heat up, increase losses, and decrease the power factor, so they have to be used at reduced capacity. (3) The greater the power of the efficiency device, the more important the efficiency problem becomes. In order to improve efficiency, it is necessary to try to minimize the losses in power switching devices and frequency converters. Based on the above considerations, the high-voltage frequency converter for AC motor variable frequency speed regulation composed of fully controlled power electronic devices IGBT adopts a new structure of "power unit" series connection, that is, using multiple low-voltage pulse width modulation (PWM) inverters as power units and assembling them into a high-voltage frequency converter in a multi-level format, which can better solve these problems. 3. IGBT high-voltage frequency converter Figure 1 (a) and (b) are the main circuit topology and connection diagram of a 6000V frequency converter. Each phase consists of 5 power units with a rated voltage of 690V connected in series, so the phase voltage is 5 × 690V = 3450V, and the corresponding line voltage is 6000V (if each phase uses 4 power units with a rated voltage of 480V connected in series, the output line voltage is 3300V). Each power unit is powered by 15 secondary windings of the input isolation transformer. The 15 secondary windings are divided into 5 groups, and there is a phase difference between each group. In Figure 1(b), with the middle delta connection as a reference (0°), there are two sets of 4 windings above and below, leading (+) and lagging (-) by 12° and 24° respectively. The required phase difference angle can be achieved by different connection groups of the transformer. Each power unit in Figure 1 is a three-phase input, single-phase output low-voltage PWM voltage-type inverter composed of insulated-gate bipolar transistors (IGBTs). The main circuit is shown in Figure 2. Each power unit outputs a voltage with three voltage levels: 1, 0, and -1. With five units per phase superimposed, eleven different voltage levels can be generated: ±5, ±4, ±3, ±2, ±1, and 0. Figure 3 shows the sinusoidal output voltage waveform synthesized for one phase. High-voltage frequency converters constructed using this multiplexing method are also called unit-series multilevel PWM voltage-type frequency converters. The high-voltage frequency converter shown in Figure 1, because each phase consists of five 690V power units connected in series, does not use traditional device series connection to achieve high-voltage output, thus eliminating the device voltage equalization problem. 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 frequency converter uses multiplexing PWM technology. The frequency converter in Figure 1 modulates the fundamental voltage using five pairs (each pair containing positive and negative phase signals) of triangular carrier waves with a 12° phase shift. The five signals obtained from the fundamental modulation of phase A control power units A1 to A5 respectively, and their superposition yields the phase voltage waveform with 11 steps shown in Figure 3. It is equivalent to a 30-pulse frequency converter, theoretically canceling harmonics below the 29th order. The total voltage and current distortion can be as low as 1.2% and 0.8% respectively, making it a truly "harmonic-free" frequency converter. Its input power factor can reach over 0.95, eliminating the need for input filters and power factor compensation devices. In this series of frequency converters, the power units in the same phase output the same fundamental voltage. The carrier waves between the series-connected units are staggered by a certain phase. If the IGBT switching frequency of each power unit is 600Hz, then when there are 5 power units in series per phase, the equivalent output phase voltage switching frequency is 6kHz. Using a low switching frequency for the power units reduces switching losses, while a high equivalent output switching frequency and multi-level operation significantly improve the output waveform. Besides reducing output harmonics, waveform improvement also reduces noise, du/dt value, and motor torque ripple. Therefore, this type of frequency converter has no special requirements for the motor used in speed control power supplies; it can be used with ordinary high-voltage motors without derating and has no special limitations on the length of the output cable. Voltage-type power units, with sufficient filter capacitors, allow the inverter to withstand a -30% power supply voltage drop and 5 cycles of power loss. While this main circuit topology increases the number of components, the low IGBT drive power (around 5W peak, less than 1W average) and the elimination of voltage equalization circuits, voltage absorption circuits, and output filters result in an inverter efficiency exceeding 96%. Another approach to constructing a high-voltage inverter using power units is to employ high-voltage IGBT devices to reduce the number of series-connected power units. For example, a PWM voltage source inverter using two power units connected in series with 3.3kΩ IGBT devices can output 4160V medium voltage; for a 6000V high-voltage output, only three units need to be connected in series. This reduction in the number of power units and components also reduces losses and failure rates. Figure 4 shows the electrical connection diagram of an inverter with two power units connected in series. Due to the reduced number of output voltage levels, an output filter is required for optimal performance in this case. Each power unit of the aforementioned frequency converter receives commands from a central controller composed of a microprocessor. Control and communication signals are transmitted via optical fiber, maintaining 5kV insulation and ensuring good anti-interference and reliability. Furthermore, due to its modular structure, all power units are identical and interchangeable. Each power unit connects to the device via only three AC inputs, two AC outputs, and three communication plugs, making unit maintenance and replacement very convenient. If power unit bypass technology is employed, the frequency converter can continue to operate at a derated rate even if a power unit fails. 4. Conclusion In industry, medium and high voltage motors are widely used. To achieve variable frequency speed control, the so-called "high-low-high" scheme was previously commonly used. This involved using a low-voltage frequency converter in the middle, while step-down and step-up transformers were used at its input and output to adapt to the voltage levels. This allowed the use of relatively inexpensive and widely used low-voltage frequency converters. However, this method resulted in high current in the intermediate stages, and coupled with the losses of the step-up and step-down transformers, led to low system efficiency, large size, and reduced reliability. Currently, with the aforementioned two types of high-voltage frequency converter products available, direct frequency conversion speed regulation is undoubtedly a more reasonable choice, especially for high-power motors above 1000kW. Furthermore, it can be predicted that with technological advancements, the 21st century will be an era in which high-voltage frequency converters will truly shine in the field of electric drives.