1. Introduction In recent years, energy consumption and prices have been rising, significantly increasing cost pressures on enterprises. As an important means of energy conservation and consumption reduction, variable frequency speed control (VFD) technology is increasingly welcomed by manufacturers. AC motor VFD technology is an energy-saving technology that smoothly changes the motor speed by controlling the stator supply voltage and frequency of the AC motor, thereby achieving stepless speed regulation. It has been widely used in motor speed control and energy-saving systems across various industries. 6kV/10kV high-voltage asynchronous motors are widely used in industries such as power, chemical, water supply, and metallurgy, with capacities typically ranging from several hundred to several thousand kVA, consuming approximately 30% of China's power generation. Using high-voltage VFDs to control the speed of high-voltage motors can effectively save energy (around 40% for fan and pump loads), improve motor lifespan, and enhance product quality. Field test reports show that the energy-saving effect of VFD technology on high-voltage motors is very significant, with fan speeds ranging from 28% to 69% and pump speeds from 18% to 41%. There are two main reasons: First, the capacity margin of each auxiliary machine in thermal power plants is too large, equivalent to 1.68 to 2.54 times the actual required power; second, the units are operating at low load rates most of the time, and the phenomenon of " overpowered motors running on small loads" is serious. 2. Energy-saving technology of large-capacity asynchronous motors 2.1 Working principle of variable frequency speed regulation (1) Speed ​​regulation methods of AC asynchronous motors There are many speed regulation methods for AC asynchronous motors, such as voltage regulation, pole changing speed regulation, cascade speed regulation, slip speed regulation, variable frequency speed regulation, etc. Among them, variable frequency speed regulation has the characteristics of significant energy saving and stable performance, and is the most widely used speed regulation method in the world today. (2) Working principle of variable frequency speed regulation The synchronous speed of AC asynchronous motor, that is, the speed of rotating magnetic field, is: no=60f1/p Where, no represents synchronous speed (r/min); f1 represents stator frequency; p represents number of pole pairs; And the shaft speed of asynchronous motor is: n=n0(1-s)=60f1(1-s)/p Where, s——slip of asynchronous motor; s=(n0-n)/n0 Changing the power supply frequency of asynchronous motor can change its synchronous speed and realize speed regulation operation. 2.2 Circuit topology classification of high voltage frequency converter (1) Intermediate DC link ● No intermediate DC link Conventional "AC-AC" frequency converter directly converts power frequency AC into AC with controllable frequency and voltage. Its disadvantage is that the output frequency is low, generally below 30hz. Due to the limitation of the highest frequency, it can only be used in some low-speed, high-capacity special occasions. ● The AC-DC-AC type frequency converter with intermediate DC link converts the industrial frequency AC into smooth DC through a rectifier, and then uses a three-phase inverter composed of semiconductor devices (GTO, GTR or IGBT) to convert the DC into AC with variable voltage and variable frequency. Its characteristics are that it directly outputs high voltage, does not require an output transformer, has high efficiency, and a wide output frequency range. It is the most widely used in industrial electric systems. (2) Voltage output mode ● "High-low-high" indirect frequency conversion mode The "high-low-high" indirect frequency converter is a variable frequency speed control system that adds a step-down transformer on the input side of a low-voltage general frequency converter and a step-up frequency converter on the output side of the frequency converter to supply power to the high-voltage motor. Its structure is shown in Figure 1: Figure 1 High-low-high scheme This method has disadvantages such as large current, low efficiency and large size in the intermediate low-voltage link. It is more suitable for speed control of small-capacity high-voltage motors of about 200kw to 500kw. Therefore, its application in industry is not very widespread. ● The "high-high" direct frequency conversion method, with the development of high-voltage and high-capacity power semiconductor devices, makes it possible to directly increase the voltage of frequency converters with large and medium power capacities. This method can eliminate the output transformer, reduce losses, and improve equipment efficiency, and is the development direction of high-voltage frequency converters. There are two ways to realize it. One is to use power devices directly connected in series to form an AC-DC-AC speed control system, such as a GTO series AC-DC-AC current-type frequency converter, or an IGBT direct series high-voltage frequency converter. The other is to use a multi-level inverter topology. Compared with traditional two-level voltage-type inverters, multi-level inverters have many significant advantages [1-4]: more output levels and lower dv/dt; no need for device series connection, and higher voltage output can be achieved using low-voltage power devices; by combining multiple levels to approximate the reference waveform, the output voltage has better harmonic performance. Therefore, multi-level inverters are considered by the industry to have the most promising application prospects in the field of high voltage and high capacity. Using a multi-level structure has become an effective way to achieve high voltage and high capacity. Since the early 1980s, when Nabae et al. proposed the three-level neutral point clamped (NPC) circuit[5], multilevel inverter technology has been greatly developed. From the perspective of current industrial applications, multilevel inverters mainly have three types of topologies: diode clamping structure[5-7], H-bridge cascade structure[8-10], and floating capacitor structure[11-13]. Figure 2 shows the single-phase circuit diagrams of these three types of multilevel topologies. Figure 2 Three topologies of multilevel inverters The three-level NPC inverter is the most thoroughly studied among diode clamping structures and is also a practical topology. However, when the number of levels exceeds 3, the DC capacitor voltage of the diode clamping inverter will not be fully controlled. The ACS1000 of ABB in Switzerland and the Simovert series of medium and high voltage variable frequency speed control devices of Siemens in Germany are based on the three-level NPC inverter[14-15]. Speed ​​control devices based on three-level NPPC inverters can output 4.16kV line voltage when only one device is used in each switching position. However, to output higher voltage, devices need to be connected in series [16]. H-bridge cascaded multilevel inverters are currently the most mature topology in industrial applications. Currently, many domestic and foreign companies have frequency converters based on H-bridge cascaded multilevel inverters, such as the Harmony series frequency converters from Robicon [17]. The grid voltage is reduced to the allowable voltage by a transformer. In each phase of the inverter, a single-phase frequency converter is connected in series. After frequency conversion by the low-voltage single-phase frequency converter, high-voltage output is achieved and directly supplied to the high-voltage motor. This method does not require an output transformer, and the current waveform is close to sine. The range of its output voltage is determined by the number of single-phase frequency converters connected in series. Currently, this method has been widely adopted. Foreign companies have formal products applied in production and are continuously promoting it. Several domestic units are also conducting research, development and production in this area. It should be said that this scheme is currently the most feasible scheme for domestic 6kV and 10kV medium-voltage high-power frequency converters. Because of the direct high voltage output, the internal step-up transformer is eliminated, so it has the advantages of small size, high efficiency and wide output frequency range, and is widely used. In the engineering application of floating capacitor multilevel inverters, only Aalstom has actual products at present [18]. 2.3 Control strategy of high voltage frequency converter Since the introduction of PWM control technology into the field of power electronics in the 1960s, it has been a research hotspot. At present, the PWM methods widely used in multilevel inverters are mainly the optimization PWM method [19-20], carrier modulation PWM method [21-22] and space vector modulation (SVM) method [23-24]. (1) Optimization PWM method It is a method to solve the PWM pulse waveform based on the Fourier series expression of the output voltage waveform, with the objective function of eliminating low-order harmonics, minimizing total harmonic distortion and minimizing torque ripple. Among them, the selected harmonic eliminated modulation (SHEM) method is the most commonly used optimization PWM method. However, since optimizing the PWM method requires the use of numerical methods to calculate a large number of switching angles, real-time online calculation is difficult. In addition, since the switching mode has been preset, this method has poor control flexibility. (2) Carrier modulation PWM method Both the carrier modulation PWM method and the SVM method are two types of algorithms based on the volt-second average equivalence principle. The carrier modulation method is simple in principle and easy to understand, and the algorithm is relatively mature. At present, multi-level carrier modulation PWM mainly includes the SH-PWM (subharmonic PWM) algorithm and the carrier phase-shifting PWM algorithm. The former is suitable for multi-level topologies with a single phase voltage switch combination, such as diode clamped multi-level inverters; the latter is suitable for multi-level inverters with a large number of phase voltage redundant switch combinations, such as floating capacitor multi-level inverters and H-bridge cascaded multi-level inverters. (3) Space vector modulation (SVM) method Compared with carrier modulation, the SVM method has many advantages such as high DC voltage utilization, good harmonic performance, and easy digital implementation. Therefore, it has been widely studied and applied in two-level inverters and three-level NPPC inverters, with its application in two-level inverters being relatively mature. However, with the increase in the number of levels, the number of inverter space voltage vectors increases dramatically, increasing the difficulty of selecting space voltage vectors in SVM methods, making most multi-level SVM methods very complex and requiring a lot of computation time to implement. Currently, in high-voltage, high-capacity frequency converters, the speed control strategy for motors mostly adopts a constant V/F control method. For loads such as fans and pumps, this method is sufficient to meet the needs of speed control. Vector control and direct torque control, which have better speed control performance, are also gradually being applied to high-voltage frequency converters. 3. Evaluation of Different Technical Solutions for High-Voltage Variable Frequency Speed ​​Regulation Currently, many companies at home and abroad have developed high-voltage variable frequency speed regulation products, which are widely used in industrial fields. Among them, foreign manufacturers mainly include Siemens, ABB, Rockwell, and Robicon; domestic companies or research institutes such as Leadway, Chengdu Jialing, and the Metallurgical Automation Research Institute are mainly producing or developing high-voltage variable frequency products. The technical solutions adopted can be mainly divided into two categories: current-source inverters and multi-level inverters. Rockwell's Powerflex 7000 inverter based on SGCT uses a current-source inverter. 3.1 High-voltage variable frequency speed control based on current-source inverters: This type of inverter has advantages such as fewer power devices, easy current control, and four-quadrant operation. However, its disadvantages are also obvious, such as serious grid pollution, low power factor, sensitivity to grid voltage and motor parameters, and inability to achieve true universality. Technically and economically, it is at a disadvantage compared to voltage-source inverters. 3.2 Midpoint clamped three-level voltage-source inverters using IGCT/IGBT: ABB's ACS1000 and Siemens' Simovert MV series inverters both use this circuit structure, as shown in Figure 3. Its advantages include: simple circuit structure, fewer power devices, high efficiency, and high overall reliability. However, the disadvantages are also obvious. Under the current level of power electronic device technology, achieving a voltage output of 6kV and above requires the series operation of power switching devices, and the dv/dt is still relatively large, requiring an output filter. Figure 3 shows a three-level NPC inverter. 3.3 Unit-cascaded multilevel voltage type inverter using IGBT. Robicon's Perfect Harmony series inverters use this circuit structure, as shown in Figure 4. Its advantages are: no output filter required, modular structure, redundant operation is possible, and low cost. Moreover, it has a high input power factor and low input harmonic content, and is known as a perfect harmonic-free high-voltage inverter. The disadvantages are: complex input transformer structure, complex circuit structure, a large number of power devices used, complex control circuit, and large IGBT conduction loss. Figure 4 shows a unit-cascaded multilevel inverter. 3.4 Floating capacitor clamped multilevel voltage type inverter using IGCT. Aalstom inverters from France use this circuit structure, as shown in Figure 5. Its advantages are: multilevel output, simple circuit structure, and avoidance of series operation of power switches when outputting voltages of 6kV and above. The disadvantages are the need for more capacitors, complex control technology, and a capacitor pre-charging circuit. Figure 5 Floating Capacitor Multilevel Inverter 4. Development Trends of High-Voltage Variable Frequency Speed ​​Control Technology With the advancement of power electronics technology, various new technologies will gradually be applied to high-voltage variable frequency speed control. The author believes that future high-voltage variable frequency speed control technology should include research and application of new main circuit topologies, high-power switching devices, and control strategies. 4.1 Research on New Main Circuit Topologies Multilevel inverters provide an effective way to increase the high voltage and capacity of variable frequency speed control. However, with the deepening of application and research, the above-mentioned circuit topologies all have their own shortcomings in practical applications. Therefore, in recent years, experts and scholars at home and abroad have proposed a variety of new multilevel topologies, such as general clamped multilevel, stacked multilevel, and hybrid multilevel. New multilevel topologies and their control strategies are becoming a hot research topic in the field of multilevel inverters. 4.2 Development of High-Power Switching Devices Since the invention of the first ordinary thyristor in 1957, power electronic devices have undergone several development stages, from thyristors without self-turn-off capability, to self-turn-off devices, to high-performance composite devices and power integrated circuits, and finally to various new high-power switching devices. They are now widely used. The emergence of high-power switching devices has provided conditions for the development of high-performance, high-capacity devices. Currently, the main high-power devices in practical applications are gate turn-off thyristors (GTO) and insulated-gate bipolar transistors (IGBT), while new high-power switching devices—integrated gate commutated thyristors (IGCT) and injection-enhanced gate transistors (IEGT)—have also entered the industrial application stage. Future high-power switching devices will develop towards higher blocking voltages and faster switching speeds, providing better options for the development of high-capacity power electronic devices. 4.3 Control Strategy Finding a simple, reliable, fast, and effective control strategy is one of the key technologies for realizing high-voltage variable frequency speed regulation. Current electronic technology development has laid a good foundation for the fully digital control of variable frequency speed regulation. Digital signal processing, fieldbus, configuration software, remote control, intelligent control, neural networks, adaptive technologies, parameter self-identification, and self-adjustment will be increasingly widely applied in variable frequency speed control. 5. Conclusion Using frequency converters to control fans, water pumps, and oil pumps for power control upgrades or additions in the generator industry has significant energy-saving potential. Generally, the installed capacity of medium-voltage power equipment in various power plants ranges from 360kW to 2400kW. A large number of old, high-energy-consuming power equipment still plays a crucial role, resulting in significant electricity consumption. In particular, energy storage power generation, utilizing water pumps for water storage and gas turbines for power generation, and starting power generation using electric motors for peak shaving, all involve high-power motor loads. The installed capacity is 65MW to 200MW. Therefore, energy-saving upgrades to the motor power systems in this type of power generation industry are an important means to achieve economical operation, strengthen enterprise management, build an "energy-saving and harmonious society," and promote technological progress.