1. Introduction China currently has over 40 million fans and pumps, consuming 30% to 55% of the total electricity generated. Converting these fans and pumps to speed-regulating operation could result in annual energy savings of up to 20 billion kWh, based on a 20% energy saving rate. The energy-saving effect is significant after fans and pumps adopt AC frequency converters for speed regulation. Relevant statistics are shown in Table 1. [align=center] Table 1 Energy Saving of Fans and Pumps Based on Speed ​​Regulation [/align] In recent years, guided and supported by national policies, some enterprises have invested in energy-saving renovations of fans and pumps, achieving certain results. This has also made enterprises fully aware that adopting AC frequency conversion speed regulation technology for technological transformation is an effective way to reduce costs and improve efficiency, aligning with the fundamental interests of long-term enterprise development. However, currently, it is mainly applied to low-voltage, small-capacity (380V, below 315 kW) equipment, while the voltage level of large-capacity fans and pumps in China is 6 kV or 10 kV. They are numerous and widely distributed. Examples of high-voltage, high-capacity fan and pump equipment include water pumps in the water supply industry, fans and pumps in the metallurgical industry, oil pumps and water injection pumps in the petroleum industry, fans and compressors in the chemical industry, boiler fans and pumps in the power industry, and fans in mines. Upgrading these high-voltage, high-capacity fans and pumps using AC variable frequency drive (VFD) technology will yield significant energy-saving benefits. Furthermore, the investment cost for energy saving is much lower than that for power generation (see Table 2), with energy-saving investment only 1/10 to 1/100 of the investment for power generation. In today's increasingly energy-constrained environment and with the promotion of a circular economy, it is clear that energy-saving upgrades for "high-power-consuming" equipment such as fans and pumps have enormous potential, significant meaning, and are quick and effective. Some domestic companies have already experimented with the application of AC-DC-AC high-voltage, high-capacity inverters and achieved excellent results, with energy savings exceeding 50%. Therefore, developing high-voltage, high-capacity AC VFDs is both necessary and urgent. [align=center] Table 2 Comparison of Energy Saving and Power Generation Investment Costs [/align] 2. Development of High-Voltage High-Capacity AC Variable Frequency Technology Abroad Currently, the main manufacturers abroad capable of producing high-voltage high-capacity AC variable frequency drives include: Siemens, CEGELEC, Allen-Brad, Robicon, ROSS HILL, and a few other companies. The main solutions are as follows: (1) High-Low-High Mode This is currently the most widely used method. The so-called high-low-high mode involves adding a step-down transformer before the input side of a general-purpose variable frequency drive (low voltage) and a step-up transformer after the output side to form a high-voltage variable frequency drive system. For example, Siemens previously used the SCR current-type SIMOVERTA series AC variable frequency drive, and now it uses the new generation IGBT voltage-type S1MOVERT Master Drives series AC variable frequency drive, plus step-down/step-up transformers to form a high-voltage AC variable frequency system. The characteristics of this method are: 1) Easy to implement, because the main body of the frequency conversion system adopts a readily available low-voltage general-purpose frequency converter. The voltage can be selected from 380 to 1000VAC depending on the situation. Through the step-down/step-up transformer, it can be used for frequency conversion speed control drive of 3000 to 10000VAC motors. 2) The input transformer generally adopts a single-winding transformer. The two secondary windings are 30° electrical angle apart to achieve 12-pulse working mode and reduce harmonics in the main circuit. 3) Since the output of the frequency converter contains high-order harmonics and DC components, the output (step-up) transformer must be specially designed. (2) Direct high voltage method This method is to directly rectify the high voltage power supply and then invert it for output without step-down/step-up transformer. Its performance characteristics are: 1) Applicable to 400 to 1000OHP, 2300 to 7200VAC AC motors, output frequency: 2 to 75Hz, quasi-sine wave output. 2) PWM-CSI pulse width modulation current type inverter design, full digital vector control. 3) The latest GTO inverter technology. 4) No step-down or step-up transformers, resulting in a compact structure and low losses. 5) Due to harmonic issues, a harmonic filter may be required. (3) Multiplexing method This method achieves high-voltage output by superimposing multiple low-voltage power units in series. The rated output voltage of each power unit is 865 V. With 4 power units per phase, the phase voltage is 3460 V, and the corresponding line voltage is 6000 V. Each power unit is powered by the secondary coil of an integrated input isolation transformer. The secondary coils use multiple connection methods and have a phase difference to achieve input multiplexing, thereby eliminating the harmonics generated by each power unit. In addition, each power unit is a three-phase input, single-phase output inverter. By using multiplexing pulse width modulation technology, a near-perfect sine wave output voltage is synthesized. Therefore, ROBICON calls it a perfect harmonic-free high-voltage inverter. The characteristics of this method are: 1) Very little harmonic pollution to the power grid, see Table 3. [align=center] Table 3 Comparison of Harmonics between Multi-mode Inverters and Current-type Inverters [/align] 2) High efficiency: Because multi-mode inverters have no output transformer, they also do not require power factor compensation devices, harmonic filters, etc. Simultaneously, due to the small harmonic components, the losses in the power transformer and motor are small, resulting in high system efficiency (see Table 4). [align=center] Table 4 Comparison of Efficiency between Multi-mode Inverters and CSI Inverters (%) [/align] 3) Due to the adoption of multi-mode pulse width modulation technology, a sinusoidal output waveform is provided, eliminating torque pulsation caused by harmonics and problems with motor heating and noise. Moreover, regardless of the cable length from the inverter to the motor, the motor can be protected from common-mode voltage and dv/dt damage. 4) More complex technology, using more power components. 3. Domestic Production Scheme and Approach Based on the actual situation of China's industry and looking towards the future, multi-mode high-voltage inverters, with their good performance, high efficiency, and no harmonic pollution, are the ideal solution for developing high-voltage inverters. 1) The multi-functional high-voltage frequency converter system consists of a high-voltage incoming cabinet, an input isolation transformer cabinet, a power unit cabinet, and output and control cabinets. Currently, except for the control section, most of these components can be domestically produced and supplied, thus significantly reducing the production and installation costs of the cabinets. 2) The input isolation transformer uses domestically produced high-voltage transformers, which fully meet the requirements and are inexpensive. 3) Power components can be sourced domestically to reduce and control the overall cost. Domestic SCR and GTO production technologies are relatively mature, with stable processes and reasonable prices. With the rapid development of power electronic devices such as GTR, IGBT, and IPM, even better conditions will be created for the development and production of the complete system. 4) The power units are drawer-type, and each power unit uses common low-voltage components, facilitating production, testing, and maintenance. In conclusion, through technology introduction and the selection of reasonable solutions, it is entirely possible to gradually achieve domestic production, which can greatly reduce costs, save foreign exchange, and simultaneously promote the development of power electronics technology and electrical energy-saving technology in my country.