1. Introduction Generally, we refer to medium- and large-capacity frequency converters used to drive AC motors above 1kV as high-voltage frequency converters. According to international convention and Chinese national standards, a supply voltage greater than or equal to 10kV is called high voltage, and less than 10kV is called medium voltage. Therefore, frequency converters with rated voltages of 1-10kV should be called medium-voltage and high-voltage frequency converters, respectively. However, considering that frequency converters within this voltage range share common characteristics, and that we conventionally refer to motors with rated voltages of 3kV or 6kV as "high-voltage motors," this article will also refer to them as "high-voltage frequency converters" for simplicity. By the end of 2006, China's total installed power generation capacity had exceeded 500 million kW, reaching 508 million kW. Thermal power accounted for approximately 80%, or about 400 million kW. The national annual power generation exceeded 2 trillion kWh. However, China's energy utilization rate is on average about 20% lower than that of developed countries! The total installed capacity of electric motors nationwide has reached over 400 million kW, with an annual electricity consumption of 1.2 trillion kWh, accounting for 60% of the country's total electricity consumption and 80% of industrial electricity consumption. Among these, the total installed capacity of fans, pumps, and compressors exceeds 200 million kW, with an annual electricity consumption of 800 billion kWh, accounting for approximately 40% of the country's total electricity consumption. More than 70% of fans, pumps, and compressors should operate with speed regulation, but currently only about 5% are operating at this speed. If 50% of the total installed capacity of fans, pumps, and compressors were retrofitted for speed regulation and energy saving, the retrofitted capacity would reach 100 million kW, of which 40% would be medium- and high-voltage motors, accounting for 60% of the total capacity. If we calculate based on an average motor output of 60%, 4000 hours of operation per year, and an average energy saving rate of 20-30% (average 25%), the annual energy saving potential would be 60 billion kWh! The energy saving potential of the entire motor system is approximately 100 billion kWh, and the retrofitting and upgrading are expected to require an investment of 200-300 billion RMB. According to the national energy conservation plan, my country should save or reduce energy consumption by 70 million tons of standard coal equivalent annually. Through basic construction projects and technological transformation measures, an energy-saving capacity of approximately 30 million tons of standard coal equivalent can be achieved annually. However, each ton of energy-saving capacity requires an investment of 2,000 yuan (about one-third the cost of developing the same amount of energy), resulting in an annual energy conservation investment of 60 billion yuan. During the 10th Five-Year Plan period, a total of 300 billion yuan was needed, and even more was required during the 11th Five-Year Plan period. Due to my country's rapid economic development, power generation capacity continues to grow rapidly. However, some key indicators of power operation and equipment still lag significantly behind those of developed countries: the average coal consumption of my country's thermal power units is 400 g/kWh, about 70-100 g/kWh higher than in developed countries; the plant power consumption rate of power plants in developed countries is 3.7%-6%, while in my country it is 4.7%-10.5%. Including line losses, the electricity delivered to users in my country consumes 9.5% more electricity than in developed countries, equivalent to 22,000 MW of installed capacity, or the annual power generation of 22 million MW power plants. Therefore, China's energy conservation situation is extremely serious! 2. Development History and Current Status of Variable Frequency Speed ​​Control Technology Variable frequency speed control technology involves multiple disciplines such as power, electronics, electrical engineering, information and control. With the development of power electronics technology, computer technology, and automatic control technology, modern AC speed control technology, represented by variable frequency speed control, has developed rapidly. AC variable frequency speed control drives overcome the shortcomings of DC motors, give full play to the inherent advantages of AC motors (simple structure, robust and durable, economical and reliable, good dynamic response, etc.), and effectively solve the problem of the inherent shortcomings of AC motor speed control performance. AC variable frequency speed control technology, with its excellent speed control performance, significant energy-saving effect, and wide applicability in various sectors of the national economy, is recognized as one of the most promising AC speed control methods, representing the mainstream direction of electrical drive development. Variable frequency speed control technology provides a crucial means for energy conservation and consumption reduction, improving control performance, and increasing product output and quality. Variable frequency speed control theory has formed a relatively complete scientific system, becoming a relatively independent discipline. The 20th century was a golden age for the birth and development of power electronic variable frequency technology. The initial theory of AC variable frequency speed control emerged in the 1920s. However, it wasn't until the 1960s, with the development of power electronic devices, that variable frequency speed control technology began to move towards practical application. The oil crisis that swept through industrialized countries in the 1970s prompted them to invest significant human, material, and financial resources in researching high-efficiency frequency converters, leading to substantial development and widespread application of variable frequency speed control technology. By the 1980s, variable frequency speed control had been commercialized, with continuously improving performance, leveraging the advantages of AC speed control and finding widespread application in various industrial sectors, partially replacing DC speed control. In the 1990s, the development and performance improvement of new power electronic devices such as IGBTs (Insulated Gate Bipolar Transistors) and IGCTs (Integrated Gate Commutated Thyristors), as well as the development of computer technology, such as the evolution from 16-bit to 32-bit machines and the emergence and development of DSPs (Digital Signal Processors) (e.g., field-oriented vector control and direct torque control), greatly improved the technical performance of variable frequency speed control. This promoted the development of variable frequency speed control technology, enabling frequency converters to far surpass other conventional AC speed control methods in terms of speed range, drive capability, speed control accuracy, dynamic response, output performance, power factor, operating efficiency, and ease of use. Their performance indicators have also surpassed those of DC speed control systems, reaching the point of replacing DC speed control systems. At present, AC variable frequency speed regulation is widely welcomed by various industries due to its excellent performance. It can be seen in the transformation of traditional industries such as power, steel rolling, papermaking, chemical, cement, coal, textile, railway, food, shipbuilding, and machine tools, as well as in the development and application of high-tech industries such as aerospace. Variable frequency speed regulation technology has achieved significant economic benefits. The current status of variable frequency speed regulation technology has the following characteristics: (1) In terms of power devices, in recent years, the production of high-voltage, high-current devices such as SCR, GTO, IGBT, and IGCT, as well as the application of parallel and series technologies, have made the production and application of high-voltage, high-power frequency converters a reality. (2) In terms of microelectronics technology, the rapid development of 16-bit and 32-bit high-speed microprocessors, as well as DSP and ASIC (Application Specific IC) technologies, has provided hardware means to achieve high precision and multi-functionality of frequency converters. (3) In terms of control theory, new control theories such as vector control, flux control, torque control, and intelligent control have provided relevant theoretical foundations for the development of high-performance frequency converters. (4) In terms of product manufacturing, the rapid development of basic industries and various manufacturing industries has promoted the socialized and specialized production of inverter-related components. 3. Classification, comparison and application of high-voltage inverters at home and abroad Currently, high-voltage inverters in the world do not have a mature and consistent main circuit topology like low-voltage inverters. Instead, due to the contradiction between the voltage withstand capability of power devices and high-voltage operating conditions, inverter manufacturers at home and abroad use different power devices and different main circuit topologies to adapt to different voltage levels and the requirements of various driven equipment. Therefore, there are also differences in various performance indicators and applicable ranges. Generally speaking, in the case of high-voltage power supply and limited voltage withstand capability of power devices, the method of connecting power devices in series can be used to solve the problem. However, when power devices are used in series, there are static voltage equalization and dynamic voltage equalization problems because the dynamic resistance and inter-electrode capacitance of each device are different. If voltage equalization measures such as connecting R and Rc in parallel with the devices are adopted, it will complicate the circuit and increase losses. At the same time, the series connection of devices also greatly increases the requirements of the drive circuit. It is essential to ensure that the series devices are turned on and off simultaneously; otherwise, due to inconsistent switching times and uneven voltage distribution, it can lead to device damage or even the collapse of the entire device. Harmonic problems are common to all frequency converters, especially in high-voltage, high-power variable frequency speed control. Harmonics pollute the power grid, affecting other electrical equipment on the same grid and even impacting the normal operation of the power system. Harmonics also interfere with communication and control systems, potentially causing communication interruptions and system paralysis. Harmonic currents also increase motor losses, leading to increased heat generation, decreased efficiency and power factor, and ultimately necessitating derating. Efficiency is also a concern; the larger the capacity of the variable frequency speed control device, the more critical the efficiency of the speed control system becomes. Different main circuit topologies, the types and quantities of power devices used, and the use of transformers and filters all affect system efficiency. To improve system efficiency, it is essential to minimize the losses of power switching devices and the variable frequency speed control device. Reliability and Redundancy Design Issues: High-voltage, high-power drive systems generally require high system reliability, especially in important sectors of the national economy such as power, energy, metallurgy, mining, and petrochemical industries. Equipment failure can cause huge losses of life and property. Therefore, the ease of adopting redundancy design and bypass control functions in the design of high-voltage frequency converters is crucial. Based on the presence or absence of a DC link, high-voltage frequency converters can be divided into AC-AC converters and AC-DC-AC converters. Based on the nature of the DC link filter element, they can be divided into current-source converters and voltage-source converters. Current-source converters can be further divided into load-commutated thyristor converters (LCI) and current-source converters using self-turn-off devices (GTO, SGCT). Voltage-source converters can be divided into: a) power device series two-level direct high-voltage converters, b) multi-level voltage-source converters using HV-IGBT, IGCT, c) unit series multiplexed voltage-source converters using LV-IGBT, etc., as shown in Figure 1. Figure 1. Classification of High-Voltage Frequency Converters. Before the late 20th century, high-voltage, high-power frequency converters were all imported brands. For example, the Power Flex™ 7000 high-voltage frequency converter from Rockwell Automation (USA) is a current source type converter using SGCT power devices in series (AC/DC/AC). Its performance is related to the characteristics of the motor, making debugging difficult; and its large du/dt ratio significantly impacts motor insulation. It was one of the earliest products introduced into energy-saving retrofit projects in my country's thermal power plants. The unit-series multilevel frequency converter from Robicon (USA) uses low-voltage IGBT power devices and is touted as a perfect harmonic-free frequency converter. It was also one of the earliest and most widely used products in my country. Its advantages include good voltage and current waveforms, low harmonic content, and minimal impact on the motor. The ACS1000 high-voltage frequency converter from ABB (Europe) is a three-level frequency converter using IGCT devices, with a maximum voltage of 4.16kV. If used on 6kV high-voltage motors in my country, a star-delta connection conversion is required, which is not conducive to power frequency bypass switching (a delta-star conversion must be performed before switching), thus limiting its use in Chinese thermal power plants. Currently, its usage in Chinese power plants is very limited. Siemens' SIMOVERT MV series high-voltage frequency converters can achieve 2000kW at 6kV. They actually use a high-low-high configuration. Their core inverter is a three-level frequency converter using high-voltage IGBT devices, with an output voltage of 2.3kV. An integrated boost filter boosts the voltage to 6kV and also provides filtering. This was one of the earlier products introduced to my country, and currently, over 200 units are in use. Alstom's ALSPA CDM6000 series high-voltage frequency converters are four-level frequency converters using flying capacitors with IGBT devices. They can operate in four quadrants, have a better output waveform, and lower harmonic content and du/dt (du/dt < 500V/µs). High-voltage frequency converters are widely used in metallurgy and mining, but rarely in my country's power plants. Foreign products generally offer high quality and reliability, but are also expensive, poorly adapted to my country's power grid, have poor user interfaces (not localized to Chinese), and lack after-sales service. Spare parts are also scarce and expensive. These factors cause significant inconvenience to domestic users. Since the beginning of the new century, domestic high-voltage frequency converter manufacturers have risen rapidly and captured the market at an astonishing pace. Beijing Leadway's high-voltage frequency converter sales exceeded 600 units by June 2006. Other companies such as Chengdu Dongfang Hitachi (formerly Dongfang Kaiqi), Beijing Hekang Yisheng, Shandong Xinfengguang, Chengdu Jialing, Zhongshan Mingyang, Harbin Jiuzhou, Guangzhou Zhiguang, Shanghai Keda, Shenzhen Weineng Technology, and Kangwo have also entered this field, and more companies are bound to join. This will play a positive role in helping Chinese high-voltage frequency converter brands capture the domestic market and provide a strong impetus for my country's efforts to build a resource-saving society. Domestic brands are catching up in terms of reliability and manufacturing processes. Their biggest advantage lies in their suitability for Chinese conditions and user needs, allowing for customized designs, user-friendly interfaces, ease of operation, and lower prices. Most importantly, their excellent pre-sales and after-sales service, spare parts provision, and operator training are unmatched by foreign brands. At the end of the last century, users exclusively chose imported brands, completely disregarding domestic ones. Now, the situation is reversed, with many users actively requesting domestic brands over imports. Foreign high-voltage frequency converter manufacturers, seeking to capture the Chinese market, have established assembly plants in China, with domestic suppliers providing components such as incoming line transformers. Product designs are increasingly tailored to Chinese user requirements, and prices have decreased. Most domestic products use unit-series multilevel circuits, with a few employing three-level circuits and power devices directly connected in series in two-level circuits. For Chinese users currently prioritizing energy conservation, unit-series multilevel circuits still offer a performance advantage. However, with the expansion of application areas, there is still a need to develop products with even better performance. Chengdu Jialing Electric Co., Ltd., after years of research and development, has solved the technical problem of direct series connection of power devices IGBTs, making a truly transformerless direct high-voltage frequency converter a reality. This not only greatly improves the efficiency of the frequency converter but also significantly reduces its size and weight. By employing anti-common-mode voltage technology to eliminate the input transformer, and using output filters and optimized PWM waveforms, harmonic content is greatly reduced, lowering the total harmonic distortion (THD) to below 2%. The use of a two-level inverter simplifies the circuit structure and control, reducing size and cost. Guangdong Mingyang Longyuan Power Electronics Co., Ltd.'s three-level voltage source high-voltage frequency converter is mainly used in power plants, water plants, steel plants, mines, metallurgy, chemical, and petroleum industries for energy-saving drive of high-voltage motors, controlling the energy-saving operation of equipment such as fans, pumps, and rolling mills, with significant application results. The company's ML-VERT-S series high-voltage frequency converters use a diode-neutral-clamped three-level voltage source inverter as the main circuit. The switching devices are the first in China to use ABB's advanced high-power integrated gate commutated thyristor (IGCT) series technology, which meets the requirements of direct "high-high" drive of 6kV motors in China. Currently, there are nearly 3,000 high-voltage frequency converters in operation in China, namely: (1) Robicon 450 sets (2) Siemens 300 sets (3) Rockwell (AB) 200 sets (4) ABB 160 sets (5) Leadway 650 sets (6) Hitachi 480 sets (7) Zhongshan Mingyang Longyuan 180 sets (8) Harbin Jiuzhou 120 sets (9) Chengdu Jialing 80 sets (10) Shandong Xinfengguang 60 sets (11) Shanghai Keda 50 sets (12) Guangzhou Zhiguang 40 sets (13) Hubei Sanhuan 16 sets (14) Other companies and brands about 200 sets 4 Key control technologies and their development in high-voltage frequency conversion speed regulation (1) Vector control technology In 1971, Siemens proposed vector transformation control, which is a new control idea and control theory. Its basic idea is to simulate the AC motor as a DC motor for control. It is based on rotor magnetic field orientation and uses vector transformation to achieve complete decoupling of AC motor speed and flux linkage control. To date, vector control technology has made great strides. (2) Sensorless vector control technology In recent years, high-performance asynchronous motor speed control systems have been widely used, but problems such as the installation, maintenance, and low-speed performance of speed sensors have affected the simplicity, low cost, and reliability of asynchronous motor speed control systems. Sensorless asynchronous motor control has received increasing attention and importance. Sensorless vector control frequency converters have both the advantages of high performance of vector control and the advantages of general frequency converters without speed sensors. However, how to obtain speed signals during vector control is the key technology of sensorless vector control. The methods for obtaining speed signals in sensorless control systems include direct calculation, parameter identification, state estimation, and indirect measurement. Based on the stator voltage and current, which are relatively easy to measure, the quantities related to speed are calculated to obtain the rotor speed, which is then used in the speed feedback system. Commonly used methods include: using the basic equations of the motor (static and dynamic) to derive the speed equation for calculation. According to the theory of model reference adaptive control, a suitable reference model and an adjustable model are selected, and the speed is identified by the adaptive algorithm. The speed is calculated by the tooth harmonic potential of the motor. Since the proposal of the sensorless vector control strategy in 1983, it has been highly valued by the academic and industrial communities. Hitachi, Yaskawa Electric and other companies published their research results in 1987 and launched their products one after another. At present, the speed regulation range of sensorless vector control frequency converters is about 1:50, and some manufacturers have products with a speed regulation range of 1:75 or even higher. (3) Direct Torque Control Technology Direct Torque Control Technology (DTC) is a new type of high-performance AC variable frequency speed regulation technology that has been developed in the past 10 years after vector control technology. In fact, because the rotor flux is difficult to observe accurately, the system characteristics are greatly affected by the motor parameters, and the vector transformation is relatively complex, there are some situations where the theory does not match the practice. In 1985, M. Depenblock of Germany first proposed the theory of DTC. It directly analyzes the mathematical model of the AC motor in the stator coordinate system, adopts stator magnetic field orientation without decoupling current, directly controls the flux linkage and torque of the motor, focuses on the speed response of torque, and obtains efficient control performance. Compared with vector control technology, this control technology is not sensitive to motor parameters, is not affected by rotor parameters, is simple and easy to implement, and largely overcomes the shortcomings of vector control technology, and has broad development and application prospects. (4) PWM control technology With the increasing application of voltage-type inverters in high-performance power electronic devices, such as AC drives, uninterruptible power supplies and active filters, PWM control technology, as a common and core technology of these systems, has attracted great attention and has been studied in depth. The so-called PWM technology is to use the turn-on and turn-off of semiconductor devices to convert DC voltage into a voltage pulse sequence of a certain shape. It is a technology to realize frequency and voltage control and eliminate harmonics. The development of self-turn-off devices has paved the way for PWM technology, and almost all variable frequency speed control devices currently use this technology. PWM technology, used in inverter control, can significantly improve the inverter's output waveform, reduce motor harmonic losses, and decrease torque ripple. It also simplifies the inverter's structure, accelerates regulation speed, and improves the system's dynamic response performance. Besides inverter control, PWM technology is also used in rectifier control. PWM rectifiers have been successfully developed, enabling the achievement of sinusoidal input current and a high grid power factor. PWM rectifiers are considered "green" converters that do not pollute the power grid. Currently, dozens of PWM control schemes have been proposed and applied. Especially with the application of microprocessors to the digitization of PWM technology, the variety has continuously increased. From initially pursuing a sinusoidal voltage waveform, to a sinusoidal current waveform, and then to a sinusoidal magnetic flux; from optimal efficiency and minimal torque ripple to noise elimination, the development of PWM control technology has undergone a process of continuous innovation and improvement. New schemes are still being proposed, indicating that research in this technology is still in its early stages. Many methods have matured, and many have been applied in practice. PWM control technology can generally be divided into three categories: sinusoidal PWM, optimized PWM and random PWM. In terms of implementation methods, there are roughly two types: analog and digital. The digital type includes several implementation methods such as hardware, software or lookup table. In terms of control characteristics, it can be mainly divided into two types: open-loop (voltage or magnetic flux control type) and closed-loop (current or magnetic flux control type). With the continuous advancement of computer technology, digital PWM has gradually replaced analog PWM and become the core technology used in power electronic devices. The continuous improvement of AC motor speed regulation performance is largely due to the continuous advancement of PWM technology. At present, the quasi-optimized PWM method developed on the basis of regular sampling PWM is widely used, namely the third harmonic superposition method and the voltage space vector PWM method. These two methods have the characteristics of simple calculation and easy real-time control. (5) Digital control technology Digital control technology is the core technology of static frequency converter and is the future development trend. At present, almost all frequency converters on the market have fully realized digital control. Due to the high performance and miniaturization of components, the frequency converter has achieved high precision control. The use of DSP and ASIC has realized fast calculation and high precision control, which can obtain good current waveforms and greatly reduce the noise of the frequency converter. Due to the application of microelectronics and ASIC technologies, the number of components in the device has been greatly reduced, thereby significantly improving the reliability of the frequency converter. In the early days, due to the limitations of CPU processing speed and the influence of discretization delay time, the current control response was several milliseconds and the speed control response was about ten milliseconds. In recent years, the improvement of CPU processing speed and the application of DSP and ASIC control have greatly shortened the scanning time. Currently, the current response is 0.1 to 0.7 ms and the speed response is 2 to 4 ms, which is sufficient to meet the control requirements of the transmission field. (6) Self-tuning technology is increasingly widely used in frequency conversion speed control systems. It can automatically adjust the parameters of the speed control system according to the changes in speed and load, so that the system has a fast dynamic response. Self-tuning technology is divided into offline and online types. The research results of offline self-tuning have been applied in many products. It is to run a self-tuning program before running the system program, identify relevant data, and modify the relevant parameters of the system program in order to obtain good system control performance. The disadvantage of offline self-tuning is that the system parameters cannot be modified in real time after the system is running, so the system cannot obtain the best control performance. Online self-tuning can modify the controller parameters in real time, thus achieving optimal control performance. Research topics for self-tuning technology include: expanding the application scope, improving accuracy, and online self-tuning. At the same time, improving control technology and enhancing system robustness are also closely related to self-tuning technology. (7) Intelligent control of AC drive systems Modern control theory and intelligent control theory are very active in the field of AC drive. There are already quite mature research results in the detection and estimation of control quantities (such as flux linkage, speed, torque, and magnetic pole position), and some of these results have been applied in products. In particular, the application of observer theory to construct system state observers to estimate physical quantities in the system that are difficult to detect with sensors has improved the system control performance and achieved good results. Intelligent control theory combining fuzzy logic, neural networks, and variable structure control has good application prospects in AC drive systems. Intelligent control originated and developed from the practice of solving engineering and technical problems. With the increasing automation and widespread adoption of automation, controlled objects have become increasingly complex. For many processes where mathematical models are difficult to obtain or the models are complex, applying classical and modern control theories often fails to achieve satisfactory control results, or is even completely ineffective. However, skilled operators can handle manual control with ease. Therefore, it is natural to consider incorporating the experience of skilled personnel into automatic control technology. The development of computer control technology has made this possible. Computers, with their functions in logical reasoning, judgment, recognition, decision-making, and learning, can perform control tasks based on the experience and methods of skilled operators and experts. On the other hand, many academic fields exploring how to realize the functions of human brain thinking, such as artificial intelligence, expert systems, neural networks, and fuzzy logic, have also made encouraging progress. These research results have proposed various methods for control that mimic human knowledge and thinking from different perspectives, collectively known as intelligent control. Its development has also brought new ideas and methods to the control strategies of AC speed control systems, making intelligent control of AC drive systems a current research hotspot. 5 Development trend of high voltage variable frequency speed regulation technology At the end of the 20th century, AC motor variable frequency speed regulation technology, with power electronic power conversion technology and microelectronic control technology as the core, has achieved amazing development. Looking ahead to the 21st century, variable frequency speed regulation technology will have even greater development. ● High frequency and low loss, self-turn-off, modularization, high withstand voltage and large capacity of power converters; ● Promotion of the emergence of matrix frequency converters ● Application of frequency converters in synchronous motors ● Digitalization, vector control and direct torque control of control technology ● Sensorless vector control ● Networking of operating systems ● Hardware generalization and software-based debugging and maintenance ● Harmonic-free frequency converters, adopting multi-level, multi-level and local compensation ● Modeling of working load parameters ● The emergence of new theories, new mechanisms and new materials will lead to new concept power conversion devices and new concept frequency converters. The following are introductions: (1) In terms of switching devices: IGBT frequency converters have become the mainstream of variable frequency speed regulation technology in the 1990s and will remain the dominant frequency converter in the field of electrical drives for a considerable period of time in the early 21st century. In the 21st century, IPM and intelligent frequency converters will see great development. The integration of power conversion, driving, detection, control and protection functions has facilitated the intelligentization of power devices and frequency converters, achieving high efficiency, energy saving, multi-functionality, high performance and high added value. At the same time, new power electronic devices such as IGCT, IEGT (Integrated Emit Gate Thyristor), GaAs (Gallium Arsenide), SiC (Silicon Carbide Composite Device), light-controlled IGBT and superconducting power devices will be researched and developed. (2) In terms of frequency converter circuit topology: Green frequency converter circuit based on dual PWM energy feedback is the development trend of frequency conversion speed regulation technology. That is, the rectifier section also adopts power electronic self-turn-off devices and performs PWM control on it. On the one hand, the AC input current waveform is sinusoidal and the power factor is 1; on the other hand, energy is fed back to the grid to ensure the four-quadrant operation of the frequency converter. In addition, the PWM rectifier circuit also helps to reduce the capacity of the DC link filter capacitor. With the continuous improvement of the performance of power semiconductor devices and the continuous decline in price, this structure will be widely promoted and applied. (3) In terms of frequency converter control circuits: Frequency converters have almost achieved digital control, but the microelectronic digitalization of control technology is still the future development trend. The digital technology of frequency converters has gradually developed from the mid-1980s to 16-bit and 32-bit microprocessors, and DSP is now widely used. (4) Vector control technology and direct torque control technology: Vector control is still the mainstream control strategy for high-performance AC motor speed control systems. The key technologies it includes are: control theory and methods, such as PWM technology, magnetic flux observation, speed identification, sensorless control; motor iron loss compensation, parameter identification, parameter change compensation; the main circuit uses new power semiconductor devices to increase the switching frequency and improve the voltage or current waveform, and at the same time uses DSP, CPU, ASIC and other technologies provided by microelectronics. Direct torque control technology still has many problems in the low-speed range, especially the identification of stator resistance, which has become an obstacle to its further development and has troubled scholars in various countries. There are corresponding solutions for vector control in the low-speed range, which have significant and practical guiding significance for the low-speed performance of direct torque control systems. Practice has proven that it is no longer possible to improve direct torque control technology from the motor itself, and we must find another way. The development of modern control theory has provided a solid theoretical foundation for the control of AC speed-regulating electric drive systems, and scholars from various countries are increasingly applying modern control technology to the speed control of AC motors. As a new and more advanced technology, direct torque control requires various advanced auxiliary technologies as support. The promotion and application of various new technologies have injected new vitality into direct torque control technology and promoted its continuous improvement and development. Recently, artificial neural networks have begun to be applied to direct torque control technology. This is a beneficial attempt and a good start. Applying modern control theory to the research of direct torque control technology is undoubtedly the development trend of this new technology and a topic worthy of in-depth research. The commercialization process of direct torque control frequency converters will make great progress. (5) PWM and multi-level technology: The best way to eliminate mechanical and electromagnetic noise is not to blindly increase the operating frequency. Random PWM technology can provide a new approach. Since the switching losses of PWM inverters increase rapidly with the increase of power and frequency, there is still a lot of work to be done in terms of high frequency and high power. At present, one way to increase the switching frequency is to use harmonic technology and soft switching technology developed on this basis. In terms of high-power devices, in addition to adopting optimized PWM mode as much as possible, multi-level inverters are also receiving increasing attention. At this time, the switching loss problem is transformed into the voltage equalization problem of multiple tubes in series. (6) The network configuration of the system-based frequency converter is mainly based on three levels: equipment layer, control layer and information layer. Among them, the frequency converter, as an actuator, can be connected to the most basic RS232/RS485 serial communication protocol, Profibus and other fieldbus protocols, as well as Internet local area network protocols. Different network protocols are configured and selected for different control systems and different user requirements. The frequency converter with network configuration has the following significant characteristics: ● High-precision frequency setting; ● Basic elements of remote control and factory informatization; ● Remote diagnostic system. Setting frequency via a network is a high-precision frequency setting method, offering advantages such as high response speed, stability, reliability, and simple wiring. In analog control, the output goes through a digital-to-analog converter (DAC), then through wires to the input (inverter), and finally through another DAC before participating in control. Differences in the bit depth of the two converters and wire losses can cause errors. In contrast, communication transmits digital signals directly without conversion, eliminating errors and losses during transmission, and offering a much faster response time. Inverters are frequently used in complex systems, harsh environments, high-load conditions, and long-term operation, such as unattended pump stations and oilfield pumping units. The failure rate of inverters is naturally higher in such environments, typically requiring reactive maintenance. However, with the development of electronic technology, traditional maintenance methods are shifting towards fault prediction and online maintenance. It is necessary to monitor their online operating status and conduct comprehensive analysis of common fault mechanisms to enable proactive fault diagnosis. Significant progress has been made in fault diagnosis, remote monitoring systems, and intelligent control for high-power inverters, and these technologies are already in practical operation. In today's increasingly networked world, compared with ordinary point-to-point hard-wired connections, frequency converter systems connected by high-speed communication can minimize system maintenance time, improve production efficiency, and reduce operating costs. Currently installed fieldbus modules include Profibus DP, Interbus, DeviceNet, CAN Open, and Modbus Plus. Users have greater freedom to choose PLC models and brands according to the production process and integrate them into the existing network very easily. Moreover, through fieldbus modules, the model of the frequency converter can be disregarded, and different power ranges and models of frequency converters can be configured using the same language, such as power, speed, torque, current, and setpoints. Due to the use of communication, configuration and system maintenance can be easily performed through a PC, including uploading, downloading, copying, monitoring, and parameter reading and writing. (7) Application with synchronous motors AC synchronous motors have become a rising star in AC adjustable speed drives, especially permanent magnet synchronous motors. The motor has a brushless structure, high power factor, and high efficiency, and the rotor speed is strictly synchronized with the power supply frequency.同步电机变频调速系统有他控变频和自控变频两大类，自控变频同步电机在原理上和直流电机极为相似，用电力电子变流器取代了直流电机的机械换向器，如采用交-直-交变压变频器时叫做“直流无换向器电机”或称“无刷直流电动机”。传统的自控变频同步机调速系统有转子位置传感器，现正开发无转子位置传感器的系统，且已经取得重大进步和在市场的成功应用。同步电机的他控变频方式也可采用矢量控制，其按转子磁场定向的矢量控制比异步电机更为简单。 参考文献： [1] 徐甫荣. 高压变频调速技术应用实践[M]. 中国电力出版社.2007（2）. [2] 张承慧等. 变频调速及其控制技术的现状与发展趋势[J]. 变频器世界. 2001（12）. [3] 李方圆. 浅谈变频器应用和发展的几个趋势[A]. 第三届变频器行业企业家论坛论文集. 2004（8）.