Abstract: This paper introduces the working principle, application performance, and practical application effects of high-voltage green frequency converters in large fluid machinery, based on application examples. Variable frequency speed control is the most ideal speed regulation and energy-saving technology for fluid machinery, but to date, the application of variable frequency speed control in large-scale fluid machinery such as fans, pumps, and compressors in China is still very limited. This is because most large-scale fluid machinery in China with a power output of 200-315kW and above uses or entirely uses high-voltage motors. Traditional high-voltage frequency converters suffer from significant harmonic pollution. In recent years, with the emergence of high-voltage green frequency converters, the energy-saving problem of large-scale fluid machinery is expected to be solved. This paper will provide a comprehensive introduction to this product and its applications, hoping to contribute to the advancement of energy-saving technology in fluid machinery. I. Working Principle High-voltage green frequency converters are also known as multi-level frequency converters or unit-series multi-level PWM voltage source frequency converters. Each phase consists of multiple low-voltage PWM frequency conversion power units connected in series to achieve input voltage multiplexing. Taking the HAR2MONY series frequency converter from Robicon Corporation of the United States as an example, the topology of a 6kV green frequency converter is shown in Figure 1. The three-phase system has 15 power units, each powered by one of the 15 secondary windings of the input isolation transformer. These 15 secondary windings are divided into 5 groups, each with a 12° phase difference, as shown in Figure 2. Each power unit is a PWM inverter with three-phase input and single-phase output, as shown in Figure 3. It uses a three-phase bridge rectifier circuit composed of diodes, and after rectification, it is inverted into single-phase AC output by four low-voltage IGBTs. [align=center]Figure 1 Figure 2[/align] It should be noted that this inverter is not a true direct high-voltage inverter, as it can be directly connected to high-voltage motors and high-voltage power grids; therefore, we classify it as a high-voltage inverter. II. Performance Characteristics In addition to the advantages of previous high-voltage inverters, such as significant energy saving, smooth stepless speed regulation, soft start, and stable process adjustment performance, this inverter also has the following characteristics. 1. Low harmonic pollution to the power grid and high power factor: Since the input is 30 pulses, theoretically, harmonics below the 29th order can be canceled, resulting in a total voltage distortion rate of only 1.12% and a total current distortion rate of only 0.18%, meeting the requirements of IEEE 519-1992. The input power factor can reach over 0.195, eliminating the need for power factor compensation. Table 1 lists the harmonic comparison between multi-mode inverters and current-mode inverters. [align=center] Figure 3 [/align] 2. High reliability and convenient maintenance: Due to the filter capacitor, the system can withstand a power supply voltage drop of -30% and a power loss of 5 cycles. The power unit uses conventional low-voltage IGBTs, with a simple drive circuit, mature and reliable technology, and a modular structure that allows for interchangeability. Each power unit is connected to the system via IPO terminals, making maintenance and replacement very convenient. Due to the series structure, bypass technology and redundancy design can also be adopted. For example, my country's pioneering HVF high-voltage inverter is designed with hot backup, hot-swappable, and main circuit direct-close functions. When a module fails, the faulty unit can be automatically switched to bypass mode, enabling hot backup. When the entire machine fails, it can switch from inverter mode to mains frequency mode within milliseconds, preventing motor shutdown due to inverter failure. 3. No special restrictions on output cable length for ordinary high-voltage asynchronous motors. The output voltage step of this inverter is the DC bus voltage of the unit, with very small dvPdt. Simultaneously, due to the use of phase-shifted PWM, the equivalent switching frequency of the motor voltage is greatly increased, and the number of output levels is increased, reducing output harmonics. Motor heating, noise, torque pulsation, and common-mode voltage caused by harmonics are significantly reduced. Therefore, this inverter can directly use ordinary high-voltage motors without reducing the rated capacity, and there are no special restrictions on the length of the output cable. 4. Low loss and high efficiency. This inverter does not require power factor compensation, has very small harmonic components, and greatly reduces losses caused by harmonics. The IGBT drive power is very small, with a peak power of about 5W and an average power of less than 1W. The efficiency of the inverter section is over 98%, and the efficiency of the entire inverter system is over 96%. Table 2 lists the efficiency comparison between multi-level frequency converters and CSI-PWM frequency converters. III. Research and Development Status in China In July 1997, Beijing Xianxing New Electromechanical Technology Co., Ltd. developed China's first high-voltage green high-power variable frequency speed control system with independent intellectual property rights, and simultaneously obtained a national invention patent for this technology. In August 1998, it passed the expert appraisal commissioned by the Beijing Municipal Economic Commission and hosted by the China Electrical Equipment Industry Association and the China Electrotechnical Society. This product was included in the 1998 National Torch Program of the Ministry of Science and Technology and the National Technological Innovation Project, and obtained the National Key High-tech Product Certificate in 1999. It has been practically applied in units such as the Beijing Yanshan Petrochemical Corporation Water Supply Plant, Dagang Oilfield Hydropower Plant, Wuhan Iron and Steel Group, Guohua Sanhe Power Plant, Daqing Oilfield, and Liaohe Oilfield, with significant comprehensive benefits. Beijing Leadway Technology Co., Ltd., relying on the National Key Laboratory of Tsinghua University, has developed the HARSVERT-A series of high-voltage green frequency converters ranging from 300 to 2500kW in three voltage levels: 3kV P, 6kV P, and 10kV. One 1000kV P, 6kV high-voltage frequency converter has been successfully put into operation at the Tianjin Xinkaihe Water Plant. In August 2000, it passed the testing of the National Electrical Control and Distribution Equipment Quality Supervision and Inspection Center, obtained the grid access test report from the China Electric Power Research Institute, and passed the product appraisal organized by the former State Machinery Bureau in September 2000. IV. Application Examples The No. 9 boiler at the Shanghai Minhang Power Plant is equipped with two YA-73No. 28D centrifugal induced draft fans, with a rated air volume of 380,000 m³/h and a total pressure of 3973 Pa, and is equipped with JSQ-156-6 type motors with a rated power of 700kW and a rated voltage of 6kV. After installing two 750kW high-voltage green frequency converters manufactured by Robicon Corporation of the United States, the effect was significant. Compared with the previous inlet diverter regulation, it saved 2.67 million kWh of electricity annually, saving more than 640,000 yuan in electricity costs. The Energy Conservation Information Dissemination Center of the State Economic and Trade Commission held an on-site meeting in July 2000 to promote its use. In July 1997, a 6kV, 290kVA HVF high-voltage green frequency converter speed control system was installed on the water pump motor of the third water supply workshop of the Yanshan Petrochemical Group Power Plant. The National Power Testing Center conducted a two-week comparative operation test of the system using both frequency conversion and direct current, and the results showed that the overall energy saving effect of the system averaged 41%. The HVF high-voltage green frequency converter has not only been successfully applied to asynchronous motors, but also to synchronous motors. In early 2001, the first 10kV, 400kW HVF high-voltage green frequency converter in China was successfully put into operation on the air compressor of the high-speed wire rod workshop of the large rolling mill of Wuhan Iron and Steel Group. The synchronous motor used in this air compressor is model TK350-14P1430, with a rated voltage of 10kV, a rated current of 2417A, and a rated speed of 428r/min. After installing a high-voltage frequency converter, the starting current decreased from 100A to 12A, the operating frequency is 35-40Hz, and the average power saving is 40%.