1. Introduction The fuming furnace at Shaoguan Smelter is a furnace used to process slag from a closed blast furnace. Its main equipment, the main blower (model D210-4), provides high-pressure air for the fuming furnace blowing process. It is driven by a 6kV, 500kW three-phase AC asynchronous motor (model JK2500-2). Because the fuming furnace blowing operation is intermittent, the typical operating cycle is 120–180 minutes, including 15–20 minutes for slag discharge and 15–40 minutes for feeding. The required air volume varies greatly during each stage of the normal daily operating cycle. In the past, there was a lack of safe and effective means to regulate the air volume. In most cases, the only way to regulate the air volume was by opening and closing the vent valve, resulting in a significant waste of electrical energy. Therefore, in 1998, Shaoguan Smelter Plant cooperated with Changsha Mining Research Institute to jointly develop a high-low-high type high-voltage variable frequency speed control system using voltage-type frequency converters. This system was successfully used for speed regulation and energy-saving operation of the fuming furnace blower at Shaoguan Smelter Plant. 2. System Scheme Selection Based on the 6kV voltage level and 500kW capacity of the three-phase AC asynchronous motor in this system, the following three frequency conversion methods can be proposed: (1) Replace the high-voltage motor with a low-voltage motor and use a low-voltage frequency converter to form a "high-low" type variable frequency speed control system; (2) Use a direct high-voltage frequency converter; (3) Use a "step-down transformer + low-voltage frequency converter + step-up transformer" to form a "high-low-high" type variable frequency speed control system. Option 1 requires replacing the existing motor and adding auxiliary equipment such as a transformer. Replacing the original motor would result in too much system change and is therefore unsuitable. Option 2 is too expensive and not cost-effective for small-capacity motors. Therefore, this system ultimately adopts the third option, a "high-low-high" variable frequency speed control scheme. 3. System Design The schematic diagram of this high-voltage variable frequency speed control system is shown in Figure 1. Figure 1: High-voltage variable frequency speed control system diagram for the fuming furnace fan. The high voltage (6kV) is sent to the step-down transformer via a high-voltage circuit breaker. Based on the grid's harmonic requirements, a 12-pulse rectification is used. Therefore, the step-down transformer is a three-winding transformer, which reduces the 6kV voltage to 400V and sends it to its respective 6-pulse rectifier bridge. After pulse rectification, the voltage reaches the DC bus, passes through the pre-charge circuit and main contactor, and then is sent to the inverter for frequency conversion and voltage transformation via the DC link filter. To obtain an output voltage close to a sine wave, a sine wave filter is added to this system. The voltage output from the filter, close to a sine wave, is then stepped up by a two-winding step-up transformer before being supplied to the high-voltage asynchronous motor. High-voltage switchgear control cabinets are installed at both ends of this variable frequency speed control system to realize high-voltage power supply and switching. 3.1 Motor Data The motor used is an existing old motor. The nameplate data of the motor is: Shenyang Electric Machinery Factory: 72.12. Since this is an old motor produced in 1972, it has been used for many years and has Class B insulation, which is a low insulation level. For such a motor, it is difficult to withstand the high-low-high voltage variable frequency drive (VFD) constructed using a current-type VFD. It has strict requirements on the output voltage waveform of the VFD speed control system; otherwise, the motor will be damaged. 3.2 The use of a three-winding transformer and 12-pulse rectification, employing a three-winding transformer and 12-pulse rectification, double-connects the two bridge rectifier circuits to reduce harmonic pollution to the power grid. The step-down transformer serves as the input transformer, using a three-winding configuration to meet the 12-pulse rectification requirement. A split structure is adopted, with one high-voltage winding and two low-voltage windings. The two low-voltage windings are phase-shifted by 30° electrical angles to supply power to two sets of diode rectifier bridges. Each diode rectifier bridge is a three-phase full-wave rectifier. The generated harmonics can be calculated using the following formula: h = k * p ± 1 Where: h—harmonic order; k—natural number (1, 2, 3…); p—number of rectifier bridge pulses, in this system p = 12. Therefore, this system only contains harmonics of the 11th, 13th, 23rd, 25th, … orders, effectively eliminating the 5th and 7th harmonics, which are the most harmful and polluting to the power grid. This 12-pulse rectification can make the harmonic content basically comply with the international GB/T14549-93 "Power Quality - Harmonics in Public Power Grids" and also comply with the international IEEE-519 standard for harmonics. At the same time, the diode rectifier bridge of this voltage-type frequency converter ensures a power factor of not less than 0.93 throughout the entire speed range. 3.3 Using a voltage source frequency converter The voltage source frequency converter is used to reduce the price of the frequency converter and obtain a sinusoidal output current. This system uses the Italian Ansaldo Industrial Systems SVTL860FN cabinet-type frequency converter. The frequency converter consists of three cabinets, namely the rectifier cabinet, the inverter cabinet and the auxiliary power supply cabinet. Its circuit principle is shown in Figure 2. Figure 2 Schematic diagram of the frequency converter rectifier-inverter circuit (1) The rectifier circuit mainly consists of two sets of three-phase knife switches, two sets of diode rectifiers, main contactors, pre-charge circuit, voltage and current transformers, etc. When the system is initially powered on, the pre-charging circuit is closed, and the DC filter capacitor is pre-charged through the current-limiting resistor r to avoid overvoltage on the capacitor when the switch is closed, which would break down the capacitor. When the voltage at the capacitor terminal is detected to exceed 80% of the rated voltage, the pre-charging circuit is opened, the system is ready, and it is prepared to receive control commands. When faults such as overcurrent, undervoltage, overvoltage, and overheating are detected, the main contactor trips, the system enters the protection state, and the system is safe. (2) The inverter circuit is mainly composed of an IGBT inverter bridge, a trigger unit, a control unit, an operation panel, a DC filter circuit, and other auxiliary units. The DC power output from the rectifier is filtered by the DC filter circuit and then sent to the inverter bridge. The inverter output is controlled by the control unit, the trigger unit, etc., according to the input control parameters. (3) The main technical parameters of the inverter are: power supply voltage: 380V～415V AC ±10% input frequency: 50～60Hz ±3Hz output frequency: 0.1～480Hz overload: 110%～150% 1min control mode: vector control/scalar control switching frequency: 2～16kHz grid side power factor: >0.95 efficiency at rated power >0.98 protection level: IP31 (standard) standard functions: speed/torque vector control (this system adopts speed control) acceleration/deceleration time 0.1～3000s maximum and minimum frequency limits slip compensation (V/Hz control) three jump frequencies jump frequency width range 0～30Hz protection functions mainly include: CPU fault overcurrent overvoltage/undervoltage overheating, motor overload: motor stall protection speed/torque 4～20mA signal loss DC overvoltage output circuit short circuit and grounding protection, etc. (4) A sinusoidal filter is used to ensure the output voltage approximates a sine wave. To reduce the harmonic content of the inverter's output voltage, a PWM (Pulse Width Modulation) control method is employed. The inverter output is a set of high-frequency square waves, which approximate a sine wave. The output voltage is controlled by adjusting the pulse width; the higher the modulation frequency, the lower the harmonic content, while du/dt increases with increasing modulation frequency. If the inverter's output voltage (see Figure 3) is directly sent to the output transformer for step-up, the waveform of the output transformer will be significantly distorted, generating large voltage spikes. This will seriously threaten the motor's insulation, even to an unacceptable degree. This is why voltage-type inverters were previously considered unsuitable for high-low-high variable frequency speed control systems. Figure 3 shows the current and voltage output waveforms without a sinusoidal output filter. To solve this problem, a sinusoidal filter is installed on the inverter's output side to filter out high-frequency harmonic components. With appropriate parameter selection, the output current waveform can approximate a sine wave, as shown in Figure 4. Figure 4 shows the current and voltage output waveforms with a sinusoidal output filter. By comparing the two waveforms, it can be seen that without the output filter, the voltage waveform contains discontinuous and very high du/dt values, and the voltage waveform distortion rate is very high. When the output transformer is stepped up, peak voltage will be superimposed on the output voltage waveform, damaging the motor insulation and causing premature motor failure. However, after adding the output filter (sinusoidal filter), the output voltage waveform is close to a sine wave, the voltage waveform distortion rate is less than 3%, and the current waveform distortion rate is very low. This makes the motor torque pulsation very small throughout the entire operating speed range. At the same time, the additional heating of the motor caused by high-order harmonics and the extra temperature rise of the motor are also very small. Therefore, we can use the original motor without having to select a motor specifically for the frequency converter. (5) Special design of the step-up transformer to ensure that the output current waveform is not distorted. The output transformer is a two-winding transformer with a Δ/y connection method. Since the output voltage waveform of the frequency converter is a high-frequency modulated pulse waveform, although it is close to a sine wave after passing through the sinusoidal filter, there are still some peaks and spikes, and the output frequency is 5-50 Hz. Therefore, this transformer requires special design to ensure that the output current waveform is not distorted, does not generate high-frequency oscillations, reduces noise, and ensures that the transformer heats up within the specified range. This transformer meets these requirements. 4. Application Effect of the Flue Gas Furnace Blower Variable Frequency Speed ​​Control System 4.1 Experimental Data of the Flue Gas Furnace System for a Certain Air Supply Cycle After Using This System The experimental data of the flue gas furnace system for a certain air supply cycle after using this system is shown in Table 1. Table 1 Experimental Data 4.2 Energy Saving Calculation of This System The comparative analysis of power consumption before and after using this system is shown in Table 2. Table 2 Comparison of System Power Consumption After Using This System From the statistics in Table 2, the annual power consumption of the flue gas furnace blower (calculated based on 230 days of operation per year) and energy saving data can be calculated before and after using the high-voltage variable frequency speed control system. The original system's annual power consumption was 9944.917 × 230 = 2287330.91 (kWh). The new system's annual power consumption was 211.315 × 24 × 230 = 1166458.8 (kWh). The annual power saving was 2287330.91 - 1166458.8 = 1120872.11 (kWh). Based on the current electricity price of 0.61 yuan/kWh at Shaoguan Iron and Steel Group, the annual power saving was 0.61 × 1120972.11 = 683,731 yuan. The system has been in operation since August 1999 and has been performing well. It has the following advantages: (1) It meets the process requirements of the fuming furnace. The control system has high reliability and meets the strict requirements for air volume and pressure in the smelting process of the fuming furnace, thus ensuring the production of the fuming furnace; (2) The use of the frequency converter has a significant energy-saving effect, saving more than 680,000 yuan in electricity costs every year; (3) It has good speed regulation performance. The frequency conversion speed control system makes the dynamic response of the fan good, with high sensitivity and a wide speed range; (4) Since the frequency converter adopts a soft start method, the mechanical damage to the motor and fan is minimal, which protects the equipment well, greatly reduces the equipment failure rate, and reduces maintenance costs; (5) The use of buttons and speed control potentiometers for operation improves the degree of automation and facilitates the operation of workers. 5. Conclusion The design of the high-voltage variable frequency speed control system for the blower of the Shaoxing metallurgical furnace was successful, and it passed the trial run and acceptance on the first attempt. It resulted in annual energy savings of 1.12 million kWh, a power saving rate of 49%, and a payback period of less than 18 months, demonstrating significant economic benefits. Through more than five years of production practice, the high-voltage variable frequency speed control system has proven to be stable and reliable in operation. This proves the success of the system design. The use of voltage-type frequency converters to construct a high-low-high variable frequency speed control system is an innovation in China, setting a precedent for using high-performance, relatively inexpensive, and highly reliable atmospheric voltage-type frequency converters in high-voltage variable frequency speed control systems.