I. Introduction Currently, with increasingly fierce competition among cement enterprises, the level of production costs determines the competitive position of cement companies in the market. A large portion of the costs for cement production enterprises is wasted on energy consumption, and reducing electricity consumption in the cement production process has increasingly attracted the attention of the industry. In the cement production process, fans are widely used, but these fans consume a large amount of power and have high starting current. Furthermore, the use of electric valves and baffles to regulate airflow results in low efficiency, and the opening of valves generates whistling noise and vibration, frequently leading to accidents. To meet the maximum requirements of the production environment, the airflow and pressure in the duct system design are often too high, inevitably resulting in energy waste. Variable frequency drive (VFD) technology, as an advanced motor speed control method, is well-known for its excellent performance and considerable economic benefits. Practice has proven that integrating a VFD system into the fan system, using VFD technology to change the motor speed to regulate airflow and pressure instead of valve-controlled airflow, can achieve significant energy-saving effects. This article analyzes and summarizes the application of the SH-HVF series high-voltage VFD in Huaxin Jinmao Cement (Suzhou) Co., Ltd. [b]II. Energy-Saving Principle of Frequency Converters[/b] The synchronous speed formula for a motor is: n1 = 60f/p. However, there is a slip relationship between the asynchronous motor speed n and the synchronous speed n1: n = n1(1-s) = 60f/p(1-s). From the above formula, we can see that the speed of an asynchronous motor can be changed by changing f, p, and s. For a specific motor, p is constant, and the speed adjustment space by changing S is very small. Therefore, frequency conversion speed regulation, by changing the stator power supply frequency f, is the most reasonable speed regulation method for asynchronous motors. If the power supply frequency f is changed uniformly, the synchronous speed of the motor can be changed smoothly. Frequency conversion speed regulation of asynchronous motors has the advantages of a wide speed range, high smoothness, and relatively stiff mechanical characteristics. Currently, frequency conversion speed regulation has become the most important speed regulation method for asynchronous motors and has been widely used in many fields. According to the similarity law of fluid mechanics: Q1/Q2 = n1/n2, the output air volume Q is directly proportional to the rotational speed n; H1/H2 = (n1/n2)2, the output pressure H is directly proportional to the rotational speed n2; P1/P2 = (n1/n2)3, the output shaft power P is directly proportional to the rotational speed n3. When the fan air volume needs to be changed, such as adjusting the opening of the damper, a large amount of electrical energy will be wasted on the resistance of the valve and pipeline system. If frequency conversion speed regulation is used to regulate the air volume, the shaft power can be significantly reduced as the flow rate decreases. When the fan speed is lower than the rated speed during frequency conversion speed regulation, the theoretical energy saving is E = [1 - (n′/n)3] × P × T (kWh), where: n—rated speed, n′—actual speed, P—motor power at rated speed, T—working time. It can be seen that modifying the fan by frequency conversion not only saves energy but also greatly improves the operating performance of the equipment. The above formula provides sufficient theoretical basis for frequency conversion energy saving. [b]III. Principle and Characteristics of SH-HVF Frequency Converter[/b] The SH-HVF series high-voltage frequency converter is a new generation of high-voltage frequency converter developed by Hubei Sanhuan Development Co., Ltd. It adopts a direct high-to-high voltage conversion method, a multi-level series voltage multiplication technology, and an optimized PWM control algorithm to achieve high-quality sinusoidal voltage and current output from variable frequency variable voltage (VVVF). 1. Principle of SH-HVF High-Voltage Frequency Converter The SH-HVF series high-voltage frequency converter uses a voltage source type AC-DC-AC frequency converter. The transformer uses phase-shifting technology to make the grid current close to a sinusoidal current. The main circuit of the power unit consists of fuses, a three-phase full-bridge rectifier module, filter capacitors, and IGBT modules. The low-voltage AC entering the power unit is rectified by the rectifier module and filtered by the capacitors to become intermediate DC. Under the control of the control system, the IGBT inverter unit inverts the intermediate DC into an alternating pulse width modulation output. Each power unit outputs a voltage with three voltage levels: 1, 0, and -1. By superimposing K units per phase, 2K+1 different voltage levels can be generated. High-voltage frequency converters constructed using this multiplication technology are also called unit-series multilevel PWM voltage-type frequency converters. They achieve high-voltage output by connecting power units in series, rather than using traditional device series connection, thus eliminating the issue of voltage equalization among components. K power units are connected in series on the inverter side to form one phase. The output levels of each power unit are superimposed, and with dynamic distribution technology and appropriate control algorithms, a set of stepped waves approximating a sine wave is obtained on the output side. Compared to the simple PWM method used in low-voltage frequency converters, the output dv/dt is very low, and the waveform essentially fits the sine wave very well. Combined with optimized PWM control, output harmonics are greatly reduced. Because this waveform has good sinusoidal properties and a small du/dt, it can reduce insulation damage to cables and motors, eliminate the need for output filters, and the length of output cables is almost unlimited. Motors do not need to be derated and can be directly used for retrofitting old equipment. Simultaneously, the harmonic losses of the motor are greatly reduced, eliminating the resulting mechanical vibrations and reducing mechanical stress on bearings and blades. [b]2. Control Unit Principle and Characteristics[/b] The control section of the entire system consists of a PLC, a main controller, a bypass controller, an intelligent operation panel (touchscreen), and some switches, power supplies, relays, etc. The PLC manages and processes the entire variable frequency speed control system, including start/stop logic and alarm/fault logic. The touchscreen is a Chinese-language LCD display that displays inverter parameters, operating parameters, and alarm/fault information. The main controller generates and phase-shifts the PWM signal, converting it into an optical signal that is transmitted to the power unit via optical fiber. The low-voltage and high-voltage sections are completely and reliably isolated, providing extremely high system safety and excellent electromagnetic interference resistance. The bypass controller provides high fault tolerance for the entire system. When a power unit fails during operation, the bypass control can automatically remove it from operation and put the backup power unit into operation. The design principle of the entire control system is reliability, practicality, and simplicity. [b]IV. Frequency Conversion Retrofit of Raw Material Mill Circulating Fan[/b] [b]1. Introduction to Raw Material Mill System:[/b] Huaxin Jinmao Cement (Suzhou) Co., Ltd. is a 3000T/D cement production line. Its raw material mill circulating fan motor is 6KV/1600KW. The air volume is adjusted by baffles (valves), resulting in high power consumption and significant throttling losses. Adjusting the damper to control the air volume is prone to wear due to the high pressure, which can lead to inaccurate air volume adjustment in the pipeline and have a significant impact on the raw material mill system process. The raw material mill circulating fan plays a significant role in the raw material mill system. By adjusting the circulating fan, the following parameters of the mill system can be controlled: 1> Internal ventilation volume: Vertical mills rely on air to sweep the mill, so the ventilation volume must be appropriate. Insufficient airflow prevents qualified raw materials from being carried out in time, leading to a thicker material layer, increased slag discharge, high equipment load, and reduced output. Excessive airflow results in a thinner material layer, affecting the stable operation of the mill. 2. Material Layer Thickness: Another important factor for the stable operation of a vertical mill is a stable material bed. A stable material bed ensures stable airflow, air pressure, and feed rate; otherwise, the material layer thickness must be maintained by adjusting the airflow and feed rate. Failure to adjust in time can cause increased vibration, increased motor load, or system shutdown. 3. Mill Vibration: Vibration is a common phenomenon in roller mill operation. Reasonable vibration is permissible, but excessive vibration can cause mechanical damage to the grinding disc, grinding rollers, and liners. 4. Mill Pressure Difference: Pressure difference refers to the pressure loss at the air ring. During mill operation, changes in the mill load are reflected not only by parameters such as mill current, material layer thickness, and vibration amplitude, but also by the pressure difference, which better reflects the internal condition of the mill. 5. Mill Outlet Temperature: Effective control of the outlet temperature can maintain good drying and grinding conditions. 6. Product Fineness: The amount of airflow inside the mill also has a certain impact on product fineness. Other factors affecting the stable operation of the mill include feed rate, water spray rate, grinding pressure, circulating air volume, and classifier speed. 2. Main Circuit Design To fully ensure system reliability, based on the relevant parameters of Huaxin Jinmao Cement's load, a SH-HVF-Y6K/2000 series high-voltage frequency converter is selected. The frequency converter configuration adopts a one-to-one method: 6KV high-voltage power supply goes through the user's switch cabinet high-voltage switch QF to the knife switch cabinet, then through the input knife switch QS1 to the high-voltage frequency converter. The frequency converter output is sent to the motor through the outgoing knife switch QS2; the high-voltage power supply can also directly start the motor through the bypass knife switch QS3. The function of the incoming/outgoing knife switch QS2 and the bypass knife switch QS3 is: in case of a frequency converter failure, the incoming/outgoing knife switch QS2 can be immediately disconnected to isolate the frequency converter, and the bypass knife switch QS3 can be manually closed to start the motor under the power frequency supply. QF retains the original circuit breaker used by the user. QS1, QS2, and QS3 are installed in a switch cabinet and supplied in conjunction with the frequency converter. QS2 and QS3 are mechanically interlocked to prevent misoperation. [b]3. Control Methods:[/b] This equipment has three control methods: 1> Closed-loop control based on pressure and flow: Automatic control is achieved based on the input 4-20mA analog value. 2> Open-loop control based on speed: In this method, the user can set the speed according to the working conditions through remote operation (DCS or remote operation box). The frequency converter uses this speed as the control value. In this method, the frequency change is based on the analog value input by the user; 4mA corresponds to 0 speed, and 20mA corresponds to the rated speed. 3> Open-loop control based on frequency: In this method, the output frequency is set directly from the touch screen through local operation (operation on the equipment itself). The frequency converter uses this frequency as the control target value. Users can set the above three control methods through the human-machine interface (touch screen) to meet different working conditions. 4. Operational Data of Huaxin Jinmao Cement Raw Material Mill System after Variable Frequency Drive Retrofit: (On-site Measurement) The above measurement data shows that, based on meeting the process requirements of the vertical raw material mill, such as material layer thickness, raw material fineness, mill internal pressure difference, and vibration, adjusting the speed of the variable frequency drive and the classifier allows the entire raw material mill system to achieve a dynamic balance. This achieves good energy saving while ensuring the raw material process requirements are met. V. Benefit Analysis after Variable Frequency Drive Retrofit: 1. Direct Benefits: Before Retrofit: During operation, the valve opening is 56%-58%. When the valve opening is 58%, the current is 147A; when the valve opening is 5%, the current is 101A; when the valve opening is 20%, the current is 116.5A; when the valve opening is 55%, the current is 142-145A; when the valve opening is 60%, the current is 150-152A. After frequency conversion modification: Valve opening is 100%, frequency converter speed is 1300-1360 r/min, and frequency converter current is 102-110A. Average power before modification: 1310KW Average power after modification: 1008KW According to on-site measurements, the energy saving rate of the equipment after frequency conversion is: ΔP＝（P1－P2）/P1=（1310－1008）/1310＝23% The energy saving power after frequency conversion modification is: PB=P1-P2=1310-1008=302kW The above data shows that the application of SH-HVF-Y6K/2000 high-voltage frequency converter on Jinmao cement circulating fan has significant energy-saving effect. 2. Indirect benefits: 1> After frequency conversion modification, the motor achieves soft start, the starting current is less than the rated current value, and the start is smoother. 2> The speed of motor and load decreases, the system efficiency is improved, and energy saving effect is achieved. 3. Significantly reduces equipment maintenance, saving human and material resources. 4. The change in motor and load speed regulation alters operating characteristics, improving equipment conditions and extending service life. 5. The power factor improves from around 0.8 to over 0.95, eliminating the need for power factor compensation devices and reducing line losses. 6. Factory noise pollution is reduced. 7. The automation and process levels of the entire system are improved. 8. Energy saving and emission reduction reduce greenhouse gas emissions, protecting the environment. 9. After load frequency change, the inverter uses unit series phase-shifting technology, theoretically eliminating harmonics below the 35th order. Due to actual manufacturing limitations, the total harmonic content of the grid-side voltage can be controlled within 2%, and the total harmonic content of the current is less than 2%, extending motor lifespan. 10. The inverter output uses PWM technology control, resulting in an output voltage waveform close to a sine wave with a total harmonic content of less than 1%. All these indicators meet the IEEE-519 international power quality harmonic standard requirements, extending motor lifespan. 10. Using frequency converter regulation enables real-time constant parameter operation, improving the safety and stability of system operation. 11. Due to the adoption of automatic control, the automation level of equipment operation control and system operation management is further improved, thus truly achieving automatic adjustment and greatly enhancing operational safety and reliability. [b]VI. Conclusion[/b] The SH-HVF-Y6K/2000 high-voltage frequency converter is used in the circulating fan motor of the raw material mill system at Huaxin Jinmao Cement (Suzhou) Co., Ltd. It is not only convenient and easy to operate with minimal maintenance, but also has a significant energy-saving effect. The application of the SH-HVF high-voltage frequency converter at Huaxin Jinmao Cement has enhanced operational safety and reliability, increased the output of the raw material mill, and achieved good energy-saving results. Therefore, the adoption of high-voltage frequency converter technology in the circulating fan of the raw material mill system in cement plants should be advocated and promoted.