1. Introduction The No. 2 Steelmaking Plant of Kunming Iron and Steel Company was built and put into operation in the early 1970s. Originally designed, it consisted of three converters with a nominal capacity of 15 tons each. In 2000, the converters were expanded and the oxygen lances were upgraded. The plant's main process equipment now includes: three 20-ton converters, one 600-ton blast furnace, one 300-ton blast furnace, and three argon (nitrogen) blowing stations. The continuous casting workshop currently has four arc-shaped billet continuous casting machines. Two of these machines have an outer arc radius of 5.25 meters and a casting cross-section of 120×120 mm, with an annual production capacity of 400,000 tons. The other two machines have an outer arc radius of 7 meters and a casting cross-section of 150×150 mm, with an annual production capacity of 550,000 to 600,000 tons. In 2001, the No. 2 Steelmaking Plant of Kunming Iron and Steel Company produced a total of 906,000 tons of steel and 887,800 tons of continuously cast billets. The average tapping rate of the converter is 22 tons per heat, and the charging rate is 24 tons per heat. In 2002, the No. 2 steelmaking plant produced a total of 1.045 million tons of steel. Kunming Iron & Steel Group (Kunming Steel) further expanded the converter capacity in 2003, increasing the charging rate to 30 tons per heat and achieving an annual steel production of 1.5 million tons. To achieve a comprehensive production capacity of 1.5 million tons, in addition to expanding the converter capacity, it was necessary to improve the converter operating rate, shorten the smelting cycle, increase oxygen supply intensity, and increase the maximum decarburization rate. The converter flue gas purification and gas recovery system was implemented simultaneously with the converter expansion and renovation. The maximum decarburization rate of the converter is 0.57%c/min, and the maximum furnace gas flow rate during the gas recovery period is 20,000 m³/h (standard conditions). Based on a combustion coefficient of 10%, the flue gas flow rate of the flue gas purification system is 23,176 m³/h (standard conditions). Each of the three converters is equipped with a new, independent flue gas purification (device) system (also known as a converter primary dust removal system), sharing one set of gas recovery (device) system. An OG wet system is used to cool and purify the flue gas and recover the gas. Each system uses one D1100 gas blower (66000 m³/h, 24658 Pa, 2900 r/min, efficiency 95.5%), equipped with a YB630S1-1 motor with an installed power of 800 kW (6000 V, 90.6 A, 2950 r/min, power factor 0.89). 2. Determination of the blower speed regulation scheme During converter smelting, the gas blower operates at high speed, 2900 r/min; during the hot metal addition and tapping processes, to save energy, the gas blower operates at low speed, 800–1000 r/min. Over the past 20 years, various methods have been developed for wind turbine speed control, such as hydraulic couplings and variable frequency drives (VFDs). Due to both technical and economic reasons, hydraulic couplings have been the most widely used for many years. Several years ago, domestically produced VFDs were not very mature and could not provide high-voltage, high-power devices. Importing them would have resulted in excessively high initial investments. The biggest advantage of hydraulic couplings is their low investment cost. High-voltage AC VFD technology is a new type of electric drive speed control technology that developed in the 1990s, especially with the rapid development of domestic VFDs in the late 1990s. Currently, the core power electronic devices and control technologies required for high-voltage VFD technology include: for AC-AC high-voltage VFDs, ordinary thyristors (TH), light-controlled thyristors (LATT), and gate turn-off thyristors (GTO); while for AC-DC-AC high-voltage VFDs, IGBTs, MCTs, GTOs, integrated emitter gate thyristors (IEGT), and integrated gate strong drive thyristors (IGCT, SGCT) are used. The control method employs PWM control, and the control system uses a digital signal processor and FPGA control. Relay protection uses a PLC system, and high/low potential isolation mostly uses fiber optic transmission, with some using electromagnetic isolation. The human-machine interface uses a PMU (Power Management Unit) and other LCD touchscreen operation panels. Domestic frequency converter technology is already quite advanced, with no significant gap in technical level compared to imported products. Currently, if imported high-voltage frequency converters are used, the investment is approximately 2000-2500 RMB/kW, 10-12 times more expensive than traditional hydraulic couplers; while in recent years, domestic high-voltage high-power frequency converter technology has matured, with a price of approximately 800-1000 RMB/kW, 4-5 times more expensive than traditional hydraulic couplers. High-voltage variable frequency speed control technology is a highly complex technology integrating power electronics, microelectronics, electric drive, high-voltage technology, high/low voltage isolation, and signal transmission. Its high-performance technical indicators, such as high voltage and large capacity, practicality, low maintenance, high efficiency, and significant energy-saving effects, have led to its increasingly widespread application in various industries, especially the power industry. To save on initial investment, this project decided to first use a high-voltage frequency converter (with trial use) in the dust removal system of converter No. 1. For converters No. 2 and No. 3, a hydraulic coupling speed control scheme will be used. After one year of system operation, the system will be replaced with frequency converters. The design must fully consider the feasibility of future replacement with high-voltage frequency converters and minimize the replacement time. Therefore, the equipment foundation design includes provisions for secondary grouting holes for the motor foundations and installation positions for the frequency converters. The technical parameters of the hydraulic coupling used in the flue gas purification and gas recovery systems of converters No. 2 and No. 3 are as follows: Model: YotGC450 explosion-proof speed-regulating type; Input speed: 2900 r/min; Power transmission range: 430～900 kW; Rated slip rate: 1.5～3%; Hydraulic coupling with electric actuator: DKJ (DKZ) type, input signal: 0～10mA, 220V. 3. Technical Performance of High-Voltage Frequency Converters The gas blower is the power hub of the converter flue gas purification and gas recovery system. If the gas blower fails to operate normally, it will affect production and cause huge economic losses. 3.1 Requirements for Gas Blower Variable Frequency Speed ​​Control Devices (1) The frequency converter should have high reliability and should basically operate without failure for a long time. (2) The frequency converter should have a complete bypass function. Once a fault occurs, it can first switch to unit bypass operation, and at the same time, it can switch the motor to power frequency operation. (3) The speed regulation range should be large and the efficiency should be high. (4) It should have logic control capability and can automatically increase or decrease speed according to the smelting cycle under interlocking state. (5) It should have resonance point jump function, which can make the motor avoid resonance point operation and prevent the blower from surging. The high-voltage frequency converter used in the No.1 converter flue gas purification and gas recovery system of this project can basically meet the above requirements. 3.2 Technical performance parameters of high voltage frequency converter Model: dfcvert-mv-1000/6b (1) Input parameters Rated voltage: three-phase AC 6.3kV±10%; Frequency: 50Hz; Input current distortion rate: <4% (above 30% load); Input power factor: >0.96 (above 20% load); Output current distortion rate: <3%; Efficiency: 96%. (2) Output parameters: Capacity: 1000kVA (compatible with motor power 800～850kw); Rated output voltage: 6kV; Rated output frequency: 50hz; Output frequency range: 0.1～50hz; Frequency resolution: 0.01hz; Acceleration/deceleration time: 1～3000s adjustable; Current waveform: completely sinusoidal; (3) Other parameters: Protection level: IP31; Ambient temperature: 0～40℃; Ambient humidity: 90%, non-condensing; Altitude: 1860m; High and low speed logic control function (acceleration and deceleration time can be set according to process requirements); Standard PID control function; Fault query function, and can print faults after connecting with the host computer; Supports DCS and PROFIBUS network operation; Supports remote operation display; Input and output protection: Input phase loss, undervoltage, overvoltage, overcurrent; Output overcurrent, phase loss, imbalance, etc. Internal protection: overload, overheating, communication failure, automatic bypass unit failure, etc. 3.3 High-voltage frequency converter power supply principle The high-voltage frequency converter power supply principle is shown in Figure 1. Figure 1 High-voltage frequency converter power supply principle diagram In the diagram, k1, k2, and k3 are the bypass cabinets of the frequency converter. k1, k2, and k3 are interlocked. The frequency conversion modification of the motor requires minimal changes to the original power supply system, and the modification work can be completed in a short time. The function of k3 is to allow the frequency converter to bypass the power frequency in case of a fault. 3.4 Comparison of operation between hydraulic coupling and high-voltage frequency converter The high-voltage frequency converter of the No. 1 converter flue gas purification and gas recovery system was installed in August 2003 and put into operation on September 20th, simultaneously with the modified No. 1 converter and flue gas purification and gas recovery system. The No. 2 and No. 3 converters, which use hydraulic couplings, were put into operation in October and November, respectively. The simultaneous operation of two different speed control devices in the same fan room created conditions for comparing the operating effects of the two types of speed control devices. The hydraulic couplings of converters No. 2 and No. 3 failed one after another a few months after being put into operation, and were replaced with high-voltage frequency converters in March and April 2004, respectively. The operation process of the hydraulic coupling and the frequency converter was investigated and data analysis was conducted. The main differences are as follows: (1) When using the hydraulic coupling speed control device, the delay is obvious during the process of moving from low speed to high speed. It cannot respond quickly and generally takes 120 to 180 seconds to reach the process of moving from low speed to high speed. At the same time, the current is large at this time. If the setting is not good, it will cause high voltage tripping and affect the stability of the system. The high-voltage frequency converter has a fast response speed and generally reaches high speed from low speed in 1 to 3 seconds. During the process of moving from high speed to low speed, the hydraulic coupling generally takes 120 to 180 seconds; the high-voltage frequency converter generally takes about 90 seconds. Otherwise, the motor feeds power to the grid, the high-voltage system trips quickly, and the CRT shows a high-voltage fault. (2) The hydraulic coupling has poor control accuracy. After setting the speed, the speed keeps jumping and cannot be fixed at the set value. This is mainly due to the slip of the hydraulic coupling and the up-and-down movement of the scoop tube caused by equipment vibration. The high-voltage frequency converter can keep the speed constant at the set speed and is not affected by equipment vibration. (3) The speed regulation range of the hydraulic coupling is narrow, usually between 30% and 97%; the high-voltage frequency converter can be between 5% and 100%. It is well known that the hydraulic coupling cannot achieve synchronous speed between the motor and the fan, while the frequency converter can achieve synchronous speed. (4) When the motor starts, the inrush current is large when using the hydraulic coupling, which affects the stability of the power grid. The frequency converter does not have this problem and can achieve a smooth transition from zero current to rated current. (5) When the fan is running at high speed, the hydraulic coupling has a slip phenomenon (this has also occurred in other projects), which affects the dust collection effect. The frequency converter runs more stably. (6) Hydraulic couplings generate mechanical losses and slip losses during speed regulation, resulting in low efficiency and wasted electrical energy. In contrast, frequency converters achieve direct connection between the fan and motor, eliminating this problem. (7) When a hydraulic coupling is in operation, the amount of oil in the working chamber is adjusted by a scoop tube, thereby changing the transmitted torque and output speed to meet the operating requirements. Therefore, the working chamber and oil supply system require frequent maintenance and repair. In contrast, the maintenance cost of a hydraulic coupling is relatively high after a period of use. Frequency converters have relatively low maintenance costs. (8) When a hydraulic coupling fails, the fan it drives cannot be operated by other means and must be shut down for repair. When a frequency converter fails, it can bypass the circuit without affecting the operation of the fan. (9) When a hydraulic coupling is used, the operating noise of the fan and motor is high, reaching about 90 dB(A), which affects the health of the operators. 3.5 Economic Analysis of Hydraulic Coupler and High-Voltage Frequency Converter Operation After the converter modification, the smelting cycle is 23 minutes, of which oxygen blowing time is 10-12 minutes and other times are 13-11 minutes. The frequency converter operates at 43 Hz (2500 r/min) at high speed and 18 Hz (1000 r/min) at low speed. When the gas blower operates at its rated speed, the blower vibration is significant, shortening the impeller maintenance cycle and affecting steel production. Therefore, the manufacturer decided to set the blower's high speed to 2500 r/min. The average time required for the gas blower to operate at high speed in each smelting cycle is 10 minutes, and the average time required at low speed is 13 minutes. Based on an annual operating time of 8000 hours, the annual time spent in high-speed mode is approximately 3480 hours, and in low-speed mode approximately 4520 hours. The measured values ​​of the motor input voltage and current when the gas blower operates at high speed and low speed using the frequency converter and hydraulic coupler are shown in the attached table. According to formula (1), the actual power consumption of the motor when the fan is running at high and low speeds using a high-voltage frequency converter and a hydraulic coupling can be calculated as shown in the attached table. Attached table: Comparison of parameters when the fan is running at high and low speeds. In the formula: p — actual power consumption of the motor, kW; i — current of the input motor, a; v — voltage of the input motor, kV; cosφ — power factor. The energy saving rate of using frequency converter speed regulation, hydraulic coupling speed regulation, and frequency converter speed regulation is calculated according to formula (2). In the formula: η — energy saving rate of high-voltage frequency converter, %; pe — actual power consumption of the motor when using a high-voltage frequency converter, kW; po — actual power consumption of the motor when using a hydraulic coupling, kW. As can be seen from the attached table, using frequency converter speed regulation has a good energy saving effect, saving 593,840 kWh of electricity per year. Calculated at 0.54 yuan/kWh, this translates to an annual electricity saving of 320,000 yuan. The investment in equipment exceeding the cost can be recovered in about 3 years. 3.6 Reflections on Speed ​​Regulation with Hydraulic Couplers An AC speed regulation system that does not change the synchronous speed no of the asynchronous motor is called a slip speed regulation system. The hydraulic coupler speed regulation system is a typical slip speed regulation system. This type of speed regulation system has slip loss δps. The rated power saving value g(s) after speed regulation is: Where: n — motor operating speed, r/min; no — motor synchronous speed, r/min. The relationship between δps, g(s) and speed under load torque m (e.g., fan) is shown in Figure 2. Figure 2: Relationship curves of δps, g(s) and n/n0 under different loads. As can be seen from Figure 2, the slip loss is greatest at 0.667 of the rated speed, at which point the hydraulic coupler efficiency is lowest. Therefore, this speed regulation state should be avoided in engineering design and system operation management. Generally, the low speed should be 0.3 of the rated speed, and the high speed should be the rated speed. When using a high-voltage frequency converter, the low speed can be set to 0.1 of the rated speed. 3.7 Advantages of high voltage frequency converters (1) High voltage frequency converters operate stably and are safe and reliable. When using hydraulic couplings, the bearings must be replaced every 40 days or so, and the furnace must be shut down for about half a day each time, resulting in a great economic loss. Frequency converters are maintenance-free, and the ventilation filter on the cabinet door can be replaced periodically without shutting down the machine, ensuring the continuity of production. (2) The energy-saving effect is more significant, greatly reducing the energy consumption per ton of steel. (3) The motor achieves true soft start and soft stop. The frequency converter provides the motor with a sinusoidal current without harmonic interference, reducing the number of motor failures. At the same time, the frequency converter sets the resonance point jump frequency, avoiding the possibility of the fan running at the resonance point, making the fan work smoothly, reducing the wear of the fan bearings, extending the service life and maintenance cycle of the motor and fan, and improving the service life of the equipment. (4) The frequency converter has complete self-protection functions. Compared with the original relay protection, it has more protection functions and is more sensitive, greatly strengthening the protection of the motor. (5) The frequency converter adopts a reliable connection method with the field signal, which is convenient to control, reliable in performance, and meets the needs of steelmaking production. The frequency converter has a built-in PLC, and the field signal access is flexible. In terms of control logic, the field (converter) provides a pair of high-speed and low-speed nodes for the frequency converter, and the frequency converter automatically runs back and forth at high speed and low speed according to the status of the nodes; the speed is measured by the frequency output of the frequency converter itself, which can eliminate the original speed measuring device connected to the motor, and the frequency converter directly provides the motor speed indication to the field. (6) The equipment has a strong ability to adapt to grid voltage fluctuations. Sometimes the grid voltage is as high as 6.9kV, or the voltage is as low as 5.5kV, the frequency converter can still operate normally. (7) Compared with the hydraulic coupling, the noise is greatly reduced during acceleration, which weakens the noise pollution. Since there is no need to replace the bearings regularly or maintain the hydraulic coupling, the pollution of the environment by the oil is avoided, which greatly improves the field environment of the fan room. (8) Since the motor runs at reduced speed and operates in the high-efficiency range, the temperature rise of the motor and bearings is significantly lower than that of the system using the hydraulic coupling, which can extend the service life of the fan system. 4. Conclusion High-voltage frequency converters offer significant energy savings, boasting high efficiency, high precision, a wide speed range, and comprehensive protection and automatic control functions. They have been widely adopted in China in recent years. Converter steelmaking plants often require speed-controlled dust removal systems, such as converter flue gas purification and gas recovery systems (primary dust removal systems), converter secondary dust removal systems, and dust removal systems for blast furnaces and ladle stations. Currently, the use of high-voltage frequency converters for speed control in these dust removal systems is relatively limited in China, due to both awareness and technical/economic issues. Using high-voltage frequency converters for speed control allows for precise adjustment of fan operating conditions to meet process requirements, saving electricity, cooling water, and reducing production costs, resulting in substantial economic benefits for enterprises. Furthermore, in converter gas recovery, it can increase gas recovery volume, reduce energy consumption per ton of steel, and improve energy utilization efficiency. The switching time from high speed to low speed of high-voltage frequency converters needs further improvement. The key issue is to solve the problem of power supply to the grid. At the same time, the dust protection requirements should be reduced to improve the environmental adaptability of the frequency converters. The heat dissipation problem of the frequency converters should be solved, and the ventilation and heat dissipation methods should be further improved.