1. Introduction With the increasing market competition, energy conservation and consumption reduction have become one of the effective means for enterprises to improve their market share and competitiveness. The growing maturity of high-voltage, high-power variable frequency speed control technology has led to its widespread application in various industries such as metallurgy, petroleum, chemical, and power. The control system for the 10 dust collector fans in the Third Steelmaking Plant of Jinan Iron and Steel Group currently uses a direct start and hydraulic coupling speed control system. The speed control and starting methods are still outdated, wasting a large amount of energy and reducing the lifespan of the machinery. Under these circumstances, the Third Steelmaking Plant of Jinan Iron and Steel Group decided to upgrade its dust collection system using high-voltage variable frequency technology. After extensive research and investigation, the high-voltage variable frequency speed control system produced by Guangzhou Zhiguang Company was selected first to upgrade the speed control and energy-saving performance of the No. 1 refining dust collector fan. 2 Overview of the No. 1 Refining Dust Removal Fan The No. 1 refining dust removal fan motor of Jinan Iron and Steel Group No. 3 Steelmaking Plant is a 1250kW high-voltage asynchronous motor produced by Xiangtan Electric Machinery Plant. The main parameters of the unit are as follows: (1) Refining dust removal motor parameters Model: YKK630-6; Power: 1250kW; Voltage: 10kV; Current: 87.1A; Speed: 995r/min; Frequency: 50Hz; Connection: Y; Protection level: IP54; Cooling method: IC611; Insulation level: F; Manufacturer: Xiangtan Electric Machinery Plant. (2) Process parameters of refining dust removal fan Model: Y5-2 48-14NO2LF; Type: Double inlet centrifugal type; Arrangement: Inlet 135°/Outlet 0°; Support method: Double support; Processing capacity: 3.7×105m3/h; Total pressure: 6100Pa (70℃); Speed: 980r/min; Shaft power: 934.7kW (20℃); Manufacturer: Nantong Jinfeng Fan Co., Ltd. 3. Energy Saving Benefit Analysis and Product Application of High Voltage Frequency Converter 3.1 Energy Saving Benefit Analysis of High Voltage Frequency Converter Based on the square torque load relationship formula for fans and pumps: P/P0=（n/n0）3, (where P0 is the power at rated speed n0; P is the power at speed n), the No. 1 refining dust removal fan has a power of 1250kW. The operating condition is calculated as 24 hours of continuous operation, of which 18 hours are operated at 90% load (frequency calculated at 46Hz) and 6 hours are operated at 50% load (frequency calculated at 25Hz). The annual operating time is calculated as 300 days. Then the annual power saving during high voltage frequency conversion speed regulation is Wb: Power saving during high speed operation: Wb1 =1250×18×[1-（46/50）3]×300 =1493375kW·h Power saving during low speed operation: Wb2 =1250×6×[1-（25/50）3] ×300 =1968750 Therefore, the total energy saving of high-voltage frequency converter speed regulation is: Wb = Wb1 + Wb2 = 1493375 + 1968750 = 3462125 kW·h. If calculated at 0.5 yuan/kW·h, the annual electricity cost saving of No. 1 refining dust removal fan after adopting high-voltage frequency converter speed regulation is: 3462125 × 0.95 × 0.5 = 1.64 million yuan. 3.2 Current Application Analysis of High-Voltage Frequency Converter Products In recent years, various high-voltage frequency converters have emerged continuously. To date, high-voltage frequency converters do not have a nearly unified topology like low-voltage frequency converters. Based on the high-voltage composition method, they can be divided into direct high-voltage type and high-low-high type. Based on the presence or absence of an intermediate DC link, they can be divided into AC-AC frequency converters and AC-DC-AC frequency converters. Among AC-DC-AC frequency converters, based on the different intermediate DC filtering links, they can be divided into voltage source type and current source type. The following will analyze several widely used high-voltage frequency converters and point out their respective advantages and disadvantages. Current-source inverters are technologically mature and can operate in four quadrants, but their application is limited due to issues such as voltage equalization problems in series connection of components at high voltages, the impact of input harmonics on the power grid, and the impact of output harmonics on the motor. Furthermore, the inverter's performance is dependent on motor parameters, resulting in poor versatility, high harmonic content in the current, significant pollution and losses, and high common-mode voltage, which affects motor insulation. AB's Power Flex 7000 series uses SGCT power transistors with a withstand voltage of 6.5kV, and the highest voltage achieved is only 6.6kV. Voltage-source inverters, due to significant dv/dt on the output side, require output filters. Due to limitations in component withstand voltage levels, the highest voltage they can achieve is only 4160V. Unit-series multi-level PWM voltage-source inverters offer advantages such as low harmonic pollution to the power grid, high input power factor, and no need for input harmonic filters and power factor compensation devices. They produce a good output waveform and do not suffer from problems caused by harmonics, such as additional motor heating and torque pulsation, noise, output dv/dt, and common-mode voltage, allowing the use of ordinary asynchronous motors. The output voltage of a unit-series multilevel frequency converter can reach 10kV or even higher. Comparing the three types of high-voltage frequency converters mentioned above, the unit-series multilevel frequency converter has seen rapid development and widespread application in my country in recent years, especially in energy-saving fields such as fans and pumps, where it has almost achieved a monopoly. In the No. 1 refining dust removal high-voltage motor of the Jinan Iron and Steel Group's Third Steelmaking Plant, which is a 10kV ordinary squirrel-cage asynchronous motor, the unit-series multilevel voltage source frequency converter is the most suitable choice. The unit-series multilevel voltage source frequency converter was invented and patented by Siemens Robicon, named the Perfect Harmonic-Free Frequency Converter. Since Robicon's patent application is only in the United States, this technology is publicly known in my country. The high-voltage frequency converters produced by Beijing Leadway also use this structure. Besides the two companies mentioned above, many other manufacturers, including Shandong Xinfengguang, Shanghai Aipa, Hekang Yisheng, Hubei Sanhuan, and Guangzhou Zhiguang, produce this type of frequency converter both domestically and internationally. After extensive investigation, comparison, and bidding, Jinan Iron and Steel Group (Jinan Steel) selected the high-voltage variable frequency speed control system manufactured by Guangzhou Zhiguang Company. 4. Features of the Guangzhou Zhiguang ZINVERT High-Voltage Variable Frequency Speed ​​Control System The Guangzhou Zhiguang ZINVERT series high-voltage variable frequency speed control system is a high-tech product integrating the most advanced high-voltage high-power power electronic control technology, microelectronics technology, optoelectronic communication technology, computer technology, automation control technology, and high-voltage technology. It is a direct high-voltage output type high-voltage frequency converter. It adopts power unit series technology, solving the problem of device withstand voltage. The inter-stage SPWM signal phase shifting and superposition improves the output voltage harmonic performance and reduces the output voltage dv/dt. Current multiplexing technology reduces input-side harmonics, minimizing harmonic pollution to the power grid. The main controller uses dual digital signal processors (DSPs), ultra-large-scale integrated circuit programmable devices (CPLD and FPGA) as its core, combined with data acquisition, unit control, fiber optic communication loops, and a built-in programmable logic controller (PLC) to form the system control part. 4.1 Composition and Principle of ZINVERT High-Voltage Variable Frequency System The ZINVERT series high-voltage variable frequency speed control system consists of a rectifier transformer, a power inverter cabinet, and a control cabinet. In actual use, a manual or automatic power frequency bypass switching cabinet can be added according to user requirements. 4.1.1 Rectifier Transformer The secondary windings of the rectifier transformer are isolated from each other and adopt a phase-shifted extended delta connection to ensure that the power factor on the grid side remains above 0.96 when the system operates at more than 20% load. The input transformer adopts a multi-stage design to reduce harmonic current. With a high input power factor, there is no need for input harmonic filters and power factor compensation devices. Taking a 6kV frequency converter as an example, the 15 secondary windings of the transformer are connected in an extended delta configuration, divided into 5 different phase groups. They are 12° apart, forming a 30-pulse diode rectifier circuit structure. Therefore, theoretically, harmonics below the 29th order can be eliminated, and the input current waveform is close to a sine wave. The total harmonic current distortion can be less than 1%. 4.1.2 Power Unit Electrical Principle The power unit mainly consists of a three-phase full-bridge rectifier, a filter capacitor bank, and an IGBT inverter bridge (H-bridge). It also includes a control circuit comprising power device drive, protection, signal acquisition, and fiber optic communication functions. By controlling the operating state of the IGBTs, a PWM voltage waveform is output. Its electrical principle is shown in Figure 1. 4.1.3 Series Connection of Power Units The ZINVERT series high-voltage variable frequency speed control system is composed of multiple power units connected in series through phase shifting. The voltage superposition principle is similar to the "battery pack superposition" technology. Taking the 6kV six-unit-per-phase series connection shown in Figure 2 as an example, the effective value of the AC voltage output by each unit is 577V, the phase voltage reaches 3450V, and the line voltage is 6kV. The harmonic content of the output phase voltage is very small, close to a standard sine wave. 4.2 ZINVERT High-Voltage Variable Frequency Control System The ZINVERT series high-voltage variable frequency speed control system adopts open-loop, voltage-frequency ratio control, featuring a simple structure and reliable operation. The main control section uses dual digital signal processors (DSPs) as the control core, supplemented by ultra-large-scale integrated circuit programmable logic devices (CPLD and FPGA), analog input and output (AI/O) units, and digital output interfaces (DI/O). The human-machine interface consists of a large-screen Chinese-language LCD, digital tube display (LED), and touch buttons. The unit control section uses a programmable device (CPLD) as its core, equipped with dedicated IGBT drive and protection modules, and input voltage, output current, DC voltage, and device temperature detection circuits. Control signals from the main control section and unit control sections are transmitted via optical fiber, effectively avoiding electromagnetic interference and ensuring the reliability of system control signal transmission. 4.2.1 Control Strategy The ZINVERT series high-voltage variable frequency speed control system adopts an output voltage-frequency ratio control method. Based on the steady-state mathematical model of the asynchronous motor, as long as the ratio of the motor's back electromotive force (EMF) to the stator frequency remains constant, the motor can operate at its rated magnetic flux, achieving optimal efficiency. Simultaneously, neglecting stator resistance and leakage inductance, the motor's back EMF equals the stator terminal voltage. Since the back EMF is difficult to control directly, a strategy of maintaining a constant ratio between the motor terminal voltage and the stator frequency is adopted under approximate conditions. Based on constant voltage-frequency ratio control, the ZINVERT series high-voltage variable frequency speed control system provides two types of V/F curves, as shown in Figure 3. Users can set functional parameters according to site conditions, select a suitable V/F curve to compensate for the stator voltage, and choose the more efficient curve based on the load type. The system's specially designed unit DC bus detection circuit can provide real-time DC bus voltage values ​​for each unit, promptly correcting the modulation coefficient and further improving the system's control performance. For fan-type loads, this control method provides sufficiently high precision. If PID control frequency is used, it fully meets the flow and pressure requirements of the process site. 4.2.2 Power-off Restart: Users' high-voltage power supplies may experience momentary voltage loss due to bus switching, grid faults, etc. The ZINVERT series high-voltage variable frequency speed control system is specifically designed with a power-off restart function, with a maximum waiting time of 30 seconds. After the voltage returns to normal, the system can automatically restart the high-speed rotating motor within 0.1 to 1 second, with a smooth and shock-free starting current, ensuring the continuous and stable operation of the user's load. For different applications, users can reasonably set the maximum possible power-off recovery time according to their own power grid. 4.2.3 Power Frequency Bypass Function: When the variable frequency speed control system fails and shuts down or is under maintenance, the power frequency bypass operation mode can improve the equipment utilization rate of the user's load and ensure the continuous operation requirements of the user's production. Depending on different user needs, both manual and automatic power frequency bypass modes are available. 4.2.4 Unit Bypass Function Each power unit box of the ZINVERT series high-voltage variable frequency speed control system is equipped with a bypass function. When a power unit fails, the output terminal can be short-circuited by controlling the bypass mechanism to bypass the unit, thereby taking it out of operation. The frequency converter can operate at a reduced rating, thus avoiding losses caused by downtime in many situations. The power unit bypass technology greatly improves the reliability of the unit series multilevel frequency converter, largely compensating for the problem of reduced reliability caused by a large number of components. 5 1# Refining Dust Removal Fan High-Voltage Frequency Converter Control System 5.1 Selection of High-Voltage Frequency Converter The high-voltage motor used in the 1# refining process is a 10kV ordinary squirrel-cage asynchronous motor. Therefore, the selected high-voltage frequency converter is the ZINVERT series 10kV high-voltage frequency converter, model ZINVERT-A9H 1500/10YA10, whose technical parameters are shown in the attached table. 5.2 Control System Scheme To ensure continuous equipment operation during system shutdowns or maintenance of the variable frequency drive (VFD) system, an automatic power frequency bypass mode is selected. The control principle is shown in Figure 4. The symbols in the figure have the following meanings: KM: High-voltage vacuum circuit breaker for powering the VFD; KJ1, KJ2, KJ3: Vacuum contactors; BPQ: ZINVERT-A9H 1500/10YA10 VFD; DJ: 1250kW/10kV asynchronous motor. During system shutdowns or maintenance of the VFD system, KJ1 and KJ2 can be automatically disconnected, and KJ3 can be closed. This allows the system to automatically switch to the power frequency grid without stopping the load, meeting the requirement of no shutdown on site. 6 Conclusion Since its upgrade in 2006, the high-voltage VFD system of the No. 1 refining dust removal fan at Jinan Iron and Steel Group's No. 3 steelmaking plant has demonstrated stable performance and reliable operation. High-voltage frequency converter systems can meet the requirements of motor speed control in production processes while significantly saving energy and reducing production costs. The successful application of high-voltage frequency converters in the No. 1 refining dust removal system of Jinan Iron and Steel Group's No. 3 steelmaking plant serves as a valuable reference not only for the next stage of high-voltage frequency converter upgrades in the dust removal system of Jinan Iron and Steel Group's No. 3 steelmaking plant, but also for the upgrades of high-voltage dust removal systems in other steelmaking plants.