Abstract: This paper mainly discusses the necessity, feasibility, economy, and practical operating experience of using high-voltage variable frequency speed control for cement plant fans. Some suggestions are offered regarding the selection and use of high-voltage variable frequency speed control devices for cement plant applications. For a long time, the Chinese government has attached great importance to energy conservation. China's "15th Five-Year Plan" for energy conservation and comprehensive resource utilization identified high-voltage, high-power variable frequency speed control as one of the key energy-saving technologies, requiring vigorous promotion of high-voltage, high-power variable frequency speed control demonstration projects. Currently, competition in the cement industry is fierce, but the key is still competition in manufacturing costs. Electric motor power consumption accounts for nearly 30% of the cost, and the high-voltage motors used to drive fans constitute a large proportion of the motor cost. Therefore, improving the efficiency and reducing the energy consumption of motors is extremely important. Currently, many cement plants suffer from the serious problem of oversized fans. Utilizing variable frequency speed control technology to change the operating speed of the equipment to adjust the air volume can not only meet production requirements but also save energy, while reducing the economic losses caused by wear and tear on baffles and pipes due to baffle adjustments and frequent downtime for maintenance. Therefore, adopting variable frequency speed control technology for cement plant fans can significantly improve energy efficiency, increase production efficiency, and bring substantial economic and social benefits to the cement plant. Depending on the specific circumstances, the energy saving rate after adopting variable frequency speed control for fans ranges from 30% to 50%, and the total investment, including the cost of the frequency converter and other installation costs, can typically be recovered within 1.5 to 2 years. Problems with Traditional Baffle Adjustment The traditional method of fan adjustment is to adjust the opening of the inlet baffle to regulate the airflow. This is an outdated method with poor economic efficiency, high energy consumption, serious equipment damage, difficult maintenance, and high operating costs. The main problems are as follows: 1. When using baffle adjustment, a large amount of energy is lost during the baffle's flow interception process. For fans, the most effective energy-saving measure is to adjust the flow rate by speed regulation. Since most fans are square torque loads, the shaft power is roughly cubic in relation to the rotational speed. Therefore, when the fan/pump speed decreases, the power consumption decreases significantly. Figure 1 shows the power consumption versus airflow curves when various adjustment methods are used for the fan. Curve 1 represents the power consumed by the motor when the output damper is controlled, curve 2 represents the power consumed by the motor when the input damper is controlled, curve 3 represents the power consumed by the motor when using slip speed control (using a slip motor and hydraulic coupling), curve 4 represents the power consumed by the motor when using frequency conversion speed control, and the bottom curve represents the actual shaft power required by the fan (i.e., the motor shaft output power) when using speed control. It is evident that among the various adjustment methods, frequency conversion speed control offers the best energy-saving effect. 2. The medium exerts a significant impact on the baffle valve and pipelines, causing severe equipment damage. 3. The baffle action is slow, making manual operation difficult, and improper operation can cause fan vibration. The baffle actuator is generally a high-torque electric actuator, prone to failure, unable to adapt to long-term frequent adjustments, exhibiting poor linearity, making closed-loop automatic control difficult, and resulting in unsatisfactory dynamic performance. 4. When an asynchronous motor is directly started, the starting current generally reaches 6 to 8 times the motor's rated current, causing a significant impact on the power grid and also causing the motor to overheat. The strong impact torque has many adverse effects on the mechanical life of the motor and fan. Some wound-rotor motors use water resistance starting, which has disadvantages such as complex equipment and low reliability. In the past, cement motors also used hydraulic couplings for speed regulation. The disadvantages of hydraulic coupling devices are large size, high noise, narrow speed range, low efficiency, and complex oil system maintenance. The advantages of using variable frequency speed regulation are : 1. Variable frequency speed regulation can save a large amount of energy lost in the original baffle throttling process, greatly improving economic efficiency. Variable frequency speed regulation of asynchronous motors achieves speed regulation by changing the stator power supply frequency f to change the synchronous speed. During speed regulation, a small slip rate can be maintained from high speed to low speed, thus consuming little slip power and achieving high efficiency, making it the most reasonable speed regulation method for asynchronous motors. From the formula N=60f/p(1-s), it can be seen that if the power supply frequency f is changed uniformly, the synchronous speed of the motor can be smoothly changed. Variable frequency speed regulation of asynchronous motors has the advantages of a wide speed range, high smoothness, and relatively stiff mechanical characteristics. Currently, variable frequency speed regulation has become the most important speed regulation method for asynchronous motors and has been widely used in many fields. For centrifugal fans, fluid mechanics holds the following principles: output airflow Q is directly proportional to rotational speed; output pressure H is directly proportional to rotational speed n²; output shaft power P is directly proportional to rotational speed n. This direct proportionality means that when the fan airflow needs to be changed, such as by adjusting the damper opening, a large amount of electrical energy is wasted on the damper and pipeline system resistance. However, if variable frequency speed control is used to adjust the airflow, the shaft power can decrease significantly with decreasing flow rate. During variable frequency speed control, when the fan speed is below the rated speed, the theoretical energy saving is substantial. The above formulas provide a sufficient theoretical basis for variable frequency energy saving. 2. After adopting variable frequency speed control, soft starting can be achieved, eliminating the impact on the power grid and mechanical load, while extending the lifespan of the motor and fan. Simultaneously, after adopting variable frequency speed control, the reactive power of the motor is instantaneously compensated through the filter capacitor in the DC link of the frequency converter, and the input power factor of the frequency converter can be greater than 0.95. Compared to direct power frequency operation of the motor, the power factor is greatly improved, especially for low-speed motors. After implementing variable frequency speed control, the fan often operates below its rated speed, significantly reducing wear on the fan and baffles, bearings, and seals, thus reducing maintenance workload. Motor vibration and noise are also significantly reduced. 3. With variable frequency speed control, closed-loop control can be easily established for automatic adjustment. The 4-20mA signal output from the regulator is sent to the frequency converter (or controlled via a communication interface). Adjusting the motor speed through the frequency converter allows for smooth airflow regulation with good linearity and fast dynamic response, enabling safe and stable operation of the equipment in a more economical manner. Aipa High Voltage Frequency Converter Principle and Features The Innov-ert series high voltage frequency converter (unit series multi-level PWM voltage source frequency converter) from Shanghai Aipa Power Electronics Co., Ltd. is a direct high voltage output voltage source frequency converter. It uses several low-voltage PWM frequency conversion power units connected in series to achieve direct high voltage output. This frequency converter has low harmonic pollution to the power grid, high input power factor, and eliminates the need for input harmonic filters and power factor compensation devices. The output waveform quality is excellent, eliminating problems such as additional motor heating, torque pulsation, noise, output du/df, and common-mode voltage caused by harmonics. No output filter is needed, allowing the use of ordinary asynchronous motors. The Innovert series employs sensorless vector control technology (a domestic first), featuring fully digital control, high starting torque, high speed accuracy, and strong resistance to grid fluctuations and load disturbances. Its principle is shown in Figure 2 (taking a 3kV high-voltage frequency converter as an example). The grid voltage (e.g., 6kV) is stepped down by a secondary-side multiplexed isolation transformer to power the power units. The power units are three-phase input, single-phase output AC-DC-AC PWM voltage source inverters. The output terminals of adjacent power units are connected in series, with their center points connected to form a Y-connection. The other three terminals achieve high-voltage output through voltage and frequency conversion, supplying the motor. Each phase of the 3kV output voltage level frequency converter consists of three power units with a rated voltage of 690V connected in series. By changing the number of power units connected in series in each phase, different high-voltage output levels can be achieved. Each phase of the 6kV frequency converter consists of 5 power units connected in series, while each phase of the 10kV frequency converter consists of 8 power units connected in series. Each power unit is powered by a set of secondary windings of the input transformer, and the power units and the secondary windings of the transformer are mutually insulated. The secondary windings adopt an extended delta connection to achieve multiplexing and reduce input harmonic current. For the 6kV voltage level frequency converter, the 15 secondary windings supplying the 15 power units are divided into 5 different phase groups of 3, with a phase difference of 12 electrical degrees, forming a 30-pulse rectifier circuit structure. The input current waveform is close to a sine wave, and the total harmonic current distortion can reach about 1%. Due to the very low input current harmonic distortion and the use of diode rectification, the overall power factor of the frequency converter input can reach above 0.95. Figure 3 shows the input voltage and current waveforms of this frequency converter. The inverter output employs multi-level phase-shifted PWM technology. Power units in the same phase output fundamental voltages with the same amplitude and phase, but the carrier waves of the series-connected units are staggered by a certain electrical angle, achieving multi-level PWM. The output voltage is very close to a sine wave. Each voltage level step is only the size of the unit's DC bus voltage, resulting in a very small du/dt ratio. The power units use relatively low switching frequencies to reduce switching losses and improve efficiency. The inverter's rated efficiency can reach up to 98.5%, and the overall efficiency, considering the input transformer, is still above 97%. 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 increases. Taking a 6kHz output inverter as an example, the output phase voltage is 11 levels, the line voltage is 21 levels, and the equivalent switching frequency is 6kHz. The increase in the number of levels and the equivalent switching frequency helps improve the output waveform and reduce output harmonics. Motor heating, noise, and torque ripple caused by harmonics are greatly reduced. Therefore, this type of inverter has no special requirements for the motor and can be directly used for ordinary asynchronous motors. Figure 4 shows the output voltage and current waveforms of this type of inverter. Compared to conventional current-source inverters and three-level voltage-source inverters that use high-voltage devices directly connected in series, this inverter uses series-connected power units. The highest voltage the devices withstand is the voltage of the DC bus within the unit, eliminating the need for series-connected devices and thus avoiding voltage equalization issues. The power units utilize conventional IGBT power modules, resulting in simple, mature, and reliable drive circuits. The modular structure of the power units allows for interchangeability of all power units within the same inverter, facilitating maintenance. Due to the series-connected power unit structure, a power unit bypass option can be implemented. In the event of a power unit failure, the control system can automatically bypass the faulty unit, allowing the inverter to continue operating at a reduced rating, significantly improving system reliability. Figure 5 shows the external appearance of the 6kV/1000kVA high-voltage inverter and a photograph of the internal power units. [align=center] [/align] Application Example In May 2006, a cement plant in southern China adopted the Innovert series high-voltage variable frequency drive (VFD) manufactured by Shanghai Aipa Power Electronics Co., Ltd. for its kiln tail exhaust gas treatment fan. To date, it has been operating well and has achieved significant energy savings. The exhaust gas treatment fan for the kiln tail is a 10kV high-voltage motor with the following parameters: wound-rotor asynchronous motor, model YRKK-630-10, rated power 800kW, frequency 50Hz, rated voltage 10000V, Y/Y wiring, rated current 62.21A, rotor open-circuit voltage 1591V, rated rotor current 307A, speed 594rpm, power factor 0.781, insulation class F, protection class IP54, manufactured by Lanzhou Electric Machinery Factory in 2004. The frequency converter is model Innovert 10/10-70, rated voltage 10kV, rated current 70A, capacity 1050kVA. In the original production process, the exhaust fan's airflow was adjusted by changing the damper opening based on the amount of material added to the kiln and the kiln's rotational speed. Due to the large design margin, during normal production, the damper opening was small, resulting in a large pressure difference on both sides of the damper and causing significant throttling losses. The current operating mode involves fully opening the damper and adjusting the motor speed via frequency converter to regulate airflow. The frequency converter is connected to the existing DCS system, which handles normal operation. The frequency converter operates at approximately 40Hz. We have compiled relevant operating data before and after this frequency converter upgrade, and some data analysis is as follows: Before the upgrade, the average power consumption of the motor was 680kW; after the upgrade, the average power consumption was 410kW, a reduction of 270kW. Based on 330 operating days per year, the annual energy saving is: 270 × 24 × 330 = 2,138,400 kWh. At 0.5 yuan per kWh, the annual energy saving benefit is 2,138,400 × 0.5 = 1,069,200 yuan. The energy saving rate is: 270 / 680 = 39.7%. A comparison of the fan performance before and after the upgrade is shown in Table 1. This inverter is also designed with a power frequency bypass circuit (Figure 6). In case of inverter failure, the motor can be automatically switched to the original power frequency power supply circuit and started using the original water resistor to ensure normal motor operation without affecting production. During power frequency operation, the fan speed will increase and the air pressure will change significantly. Therefore, the fan damper should be adjusted in time on the DCS to reduce the fan output air volume to the required value. [align=center] [/align] Issues to be noted when using variable frequency speed control 1. Reliability considerations. The continuous production nature of the cement industry determines that the high-voltage inverter used in cement plants needs to have high reliability to ensure safe production. The Innovert series adopts the mainstream power unit series technology scheme, instead of direct series connection of power devices, avoiding the voltage equalization problem of direct series connection of devices, which essentially ensures the reliability of the system. At the same time, the product's unique sensorless vector control technology improves the starting torque and speed accuracy, while improving the ability to resist power grid fluctuations and load disturbances, greatly improving reliability. 2. The impact of inverter input harmonics on the power grid. If the input current harmonics of a frequency converter are large (such as traditional current source frequency converters), the following hazards may occur: Malfunction of the relay protection devices in the power supply system may lead to large-scale power outages. Increased errors in measuring instruments may affect metering accuracy and control performance. It may affect the normal operation of other power electronic devices, computer systems, and communication equipment. Harmonics increase the losses of electrical equipment such as motors, transformers, and capacitors, and in severe cases, may cause overheating or burnout. Innovert series high-voltage frequency converters have extremely low input current harmonic distortion, producing virtually no harmonic pollution to the power grid, and meet IEEE 519-1992 and 6B/T14549-93 standards. Large and medium-sized cement plants have high levels of automation, and most use automated instruments and computer control systems, which have very high requirements for harmonic control in the power system. Innovert frequency converters have a significant advantage in this regard. 3. The impact of the frequency converter output waveform on the motor. Since a large part of the application of frequency conversion speed regulation in cement plants is the retrofitting of existing equipment, the original ordinary motors were designed to run directly on the power grid, and the power grid voltage waveform is basically a sine wave. Poor output waveform quality from the frequency converter can negatively impact the motor. Output harmonics can cause additional heating and torque pulsation in the motor, increase noise, and affect motor insulation due to output dv/dt and common-mode voltage. Innovert series high-voltage frequency converters, with their superior output waveform quality, eliminate the need for output filters, allowing the use of existing ordinary asynchronous motors. Conclusion: Currently, many cement plants suffer from a severe problem of oversized fans. Fan airflow is primarily adjusted via baffles, resulting in high energy consumption, poor economic efficiency, and significant equipment damage. Advanced high-voltage variable frequency speed control is urgently needed to reduce electricity consumption and improve economic efficiency. The application of high-voltage variable frequency speed control technology for cement industry fans, as demonstrated by the practical application of high-voltage frequency converters from Shanghai Aipa Power Electronics Co., Ltd., is necessary, feasible, and yields significant economic benefits. The Innovert series high-voltage frequency converter boasts high reliability and superior input/output waveform quality, making it suitable for variable frequency speed control of cement plant fans. It improves equipment reliability, saves significant energy, and brings substantial economic and social benefits to cement plants, making it highly valuable for widespread adoption.