1. Overview Currently, competition in the cement industry is fierce, but the key factor is still manufacturing cost. Electric motors account for nearly 30% of the cost, and the high-voltage motors driving the fans constitute a large proportion of the total motor cost. Therefore, improving the efficiency and reducing the energy consumption of electric motors is extremely important. Many cement plants currently suffer from the problem of oversized fans. Utilizing variable frequency drive (VFD) technology to change the operating speed of the equipment and adjust the airflow can meet production requirements while saving energy. It also reduces wear and tear on baffles and pipes caused by adjusting baffles, as well as economic losses due to frequent downtime for maintenance. Therefore, adopting VFD technology for cement plant fans can save significant energy, improve production efficiency, and bring substantial benefits to the cement plant. Depending on the specific circumstances, the energy saving rate after adopting VFD technology for fans ranges from 30% to 50%, and the total investment, including the VFD equipment and other installation costs, can typically be recovered within 1.5 to 2 years. 2. Problems with Traditional Baffle Adjustment The traditional method of adjusting a blower 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 This is an outdated method with poor economic efficiency, high energy consumption, serious equipment damage, difficult maintenance, and high operating costs.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 throttling process. For blowers, the most effective energy-saving measure is to adjust the flow rate by speed regulation. Since most blowers are square torque loads, the shaft power is roughly cubic in relation to the rotational speed. Therefore, when the blower/pump speed decreases, the power consumption decreases significantly. Figure 1 shows the power consumption versus airflow curves when various adjustment methods are used for blowers. 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 blower (i.e., the motor shaft output power) when using speed control. It is evident that among the various adjustment methods, variable frequency speed control offers the best energy-saving effect. 2) The medium exerts a significant impact on the baffle valve and pipeline, causing severe equipment damage. 3) The baffle action is slow, making manual operation difficult and prone to causing fan vibration if not handled properly. The baffle actuator is typically a high-torque electric actuator, prone to failure, unable to adapt to frequent and prolonged 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 leading to motor overheating. The powerful impact torque has many adverse effects on the mechanical lifespan 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, some cement plants also used hydraulic couplings for speed regulation. The disadvantages of hydraulic coupling devices are their large size, high noise, narrow speed range, low efficiency, and complex oil system maintenance. [ALIGN=CENTER] Figure I Comparison of power consumption of fans using various adjustment methods [/ALIGN] 3. Advantages of using variable frequency speed regulation 1) Variable frequency speed regulation can save a lot of energy that was originally lost in the baffle interception process, greatly improving economic efficiency. The variable frequency speed regulation of asynchronous motors is achieved by changing the synchronous speed by changing the stator power supply frequency f. In the speed regulation, a small slip rate can be maintained from high speed to low speed, so the slip power consumption is small and the efficiency is high. It is the most reasonable adjustment method for asynchronous motors. As can be seen from the formula, if the power supply frequency f is changed uniformly, the synchronous speed of the motor can be changed smoothly. The variable frequency speed regulation of asynchronous motors has the advantages of wide speed regulation range, high smoothness and hard mechanical characteristics. At present, 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, the following principles of fluid mechanics apply: the output orifice volume Q is proportional to the speed n; the output pressure H is proportional to the speed n2[/SUP]. The output shaft power P is directly proportional to the rotational speed n[SUP]3[/SUP]. That is, when the fan airflow needs to be changed, such as by adjusting the opening of the damper, a large amount of electrical energy will be wasted on the damper and pipeline system resistance. If variable frequency speed regulation is used to adjust the airflow, the shaft power can be significantly reduced as the flow rate decreases. When the fan speed is lower than the rated speed during variable frequency speed regulation, the theoretical power saving is given by the formula: n is the rated speed; n' is the actual speed; P is the motor power at the rated speed; T is the working time. The above formula provides sufficient theoretical basis for variable frequency energy saving. 2) After adopting variable frequency speed regulation, soft start can be achieved, which greatly reduces the impact on the power grid and mechanical load, and extends the life of the motor and fan. At the same time, after adopting variable frequency speed regulation, the reactive power of the motor is instantaneously compensated by the filter capacitor of the DC link of the frequency converter, and the input power factor of the frequency converter can reach more than 0.95. Compared to direct power frequency operation of electric motors, the power factor is significantly improved, especially for low-speed motors. After implementing variable frequency speed control, the fan often operates below its rated speed, greatly 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 formed for automatic adjustment. The 4-20mA signal output from the regulator is sent to the frequency converter, which adjusts the motor speed, allowing for smooth airflow regulation with good linearity and fast dynamic response, enabling the equipment to operate safely and stably in a more economical manner. 4. Principle and Characteristics of AIPA High-Voltage Frequency Converters The Lnnovert 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. It produces a good output waveform, avoiding problems such as additional motor heating, torque pulsation, and noise caused by harmonics, eliminating the need for output filters and allowing the use of ordinary asynchronous motors. The Innovert series employs sensorless vector control technology and fully digital control, featuring high starting torque, high speed accuracy, and strong resistance to power grid fluctuations and load disturbances. Its principle is shown in Figure 2 (taking a 3 kV high-voltage frequency converter as an example). [ALIGN=CENTER] [B]Figure 2 Principle of Unit Series Multilevel Frequency Converter a) Main Power Unit Topology b) Power Unit Structure[/B][/ALIGN] The grid voltage (e.g., 6 kV) is stepped down by a secondary-side multiplexed isolation transformer to supply power to the power unit. The power unit is a three-phase input, single-phase output AC-DC-AC PWM voltage source inverter structure. The output terminals of adjacent power units are connected in series, with their center points connected to form a Y-connection structure. The other three terminals achieve high-voltage output for variable voltage and frequency conversion, supplying the motor. A 3 kV output voltage level frequency converter consists of three power units connected in series per phase, each with a rated voltage of 690V. By changing the number of power units connected in series per phase, different voltage levels of high-voltage transmission can be achieved. A 6 kV frequency converter consists of five power units connected in series per phase, and a 10 kV frequency converter consists of eight power units connected in series per phase. Each power unit is powered by the secondary winding of the input transformer, and the power units and the transformer secondary windings are mutually insulated. The secondary windings adopt an extended delta connection to achieve multiplexing and reduce input harmonic current. For a 6 kV voltage-level frequency converter, the 15 secondary windings supplying power to 15 power units are grouped into 5 different phase groups of 3, with a 12° electrical angle difference between each other, forming a 30-pulse rectifier circuit structure. The input current waveform is close to a sine wave, and the total harmonic current distortion is 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 over 0.95. Figure 3 shows the input voltage current waveform of this frequency converter. The inverter output adopts multi-level phase-shifted PWM technology. The power units of the same phase output the fundamental voltage with the same amplitude and phase, but the carrier waves of the series-connected units are staggered by a certain electrical angle to achieve multi-level PWM, and the output voltage is very close to a sine wave. Each level step of the output voltage is only the size of the unit's DC bus voltage, so du/dt is very small. The power units use a relatively low switching frequency to reduce switching losses and improve efficiency. The rated efficiency of the frequency converter can reach 98.5%, and the overall efficiency after considering the input transformer is still over 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 6 kV output inverter as an example, the output phase voltage is 11 levels, the line voltage is 21 levels, and the output equivalent switching frequency is 6 kHz. The increase in the number of levels and the equivalent switching frequency helps to improve the output waveform and reduce output harmonics. The 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. [align=center] Figure 3 Input waveform of unit series multilevel inverter Figure 4 Output waveform of unit series multilevel inverter [/align] Compared with ordinary current source inverters and three-level voltage source inverters that use high-voltage devices directly in series, since power units are connected in series, the highest voltage that the devices withstand is the voltage of the DC bus within the unit. The devices do not need to be connected in series, and there is no voltage equalization problem caused by device series connection. Conventional IGBT power modules are used in the power units, the drive circuit is simple, and the technology is mature and reliable. The power units adopt a modular structure, allowing all power units within the same inverter to be interchangeable, and maintenance is very convenient. Due to the series connection of the power units, a power unit bypass option can be used. When a power unit fails, the control system can automatically bypass the faulty unit, and the inverter can still continue to operate at a reduced rating, greatly improving system reliability. Figure 5 shows the external appearance of the 6kV/1000kVA high-voltage inverter and photos of all internal power units. [align=center] [B]External appearance photos of the lnnovert series high-voltage inverter a) Overall appearance b) Power unit appearance[/B][/ALIGN] 5. Application Examples In May 2006, a cement plant in southern China adopted the lnnovert series high-voltage variable frequency speed control device manufactured by Shanghai Aipa Power Electronics Co., Ltd. for its kiln tail exhaust gas treatment fan. So far, 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. The motor parameters are as follows: wound-rotor asynchronous motor, model YRKK-630-10, rated power 800kW, frequency 50Hz, rated voltage 10000V, wiring Y/Y, rated current 62.21A, rotor open circuit voltage 1591V, rated rotor current 307A, speed 594r/min, power factor 0.781, insulation class F, protection class IP54. It was manufactured by Lanzhou Electric Motor Factory in 2004. The frequency converter is model Innovertl0/10-70, rated voltage 10kV, rated current 70A, capacity 1050kVA. In the original production process, the exhaust fan's airflow was adjusted by changing the opening of the damper according to 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 dampers 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. Before and after the frequency converter upgrade, relevant operating data were statistically analyzed, and some data are presented below. Before the upgrade, the average motor power consumption 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 x 24 x 330 = 2,138,400 kWh. At a price of 0.5 yuan per kWh, the annual energy saving benefit is 2,138,400 x 0.5 = 1,069,200 yuan, with a power saving rate of 270/680 = 39.7%. A comparison table of fan performance before and after the upgrade is attached. This inverter is also designed with a power frequency bypass circuit (see Figure 6). In case of inverter failure, the motor can be automatically switched to the original power frequency power supply circuit, and the original water resistor can be started to ensure the normal operation of the motor without affecting production. When running at power frequency, 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] Appendix [/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 Innnovert series adopts the mainstream power unit series technology scheme, instead of direct series connection of power devices, avoiding the voltage equalization problem caused by direct series connection of devices, and essentially ensuring the reliability of the system. At the same time, the product's unique sensorless undervoltage 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) Impact of Inverter Input Harmonics on the Power Grid: High input current harmonics in the inverter can cause the following problems: Malfunctions in the power supply system's relay protection devices may lead to widespread power outages. Increased errors in measuring instruments affect metering accuracy and control performance. Affecting the normal operation of other power electronic devices, computer systems, and communication equipment. Harmonics increase losses in electrical equipment such as motors, transformers, and capacitors, potentially causing overheating or burnout. Innovert series high-voltage inverters have extremely low input current harmonic distortion, producing virtually no harmonic pollution to the power grid, and meet IEEE 519-1992 and GB/T 14549-93 standards. Large and medium-sized cement plants have high levels of automation, mostly using automated instruments and computer control systems, which place high demands on the harmonic control of the power system. Innovert inverters have a significant advantage in this regard. [ALIGN=CENTER] Figure 6 Main Circuit Single-Line Diagram [/ALIGN] 3) The Influence of Inverter Output Waveform on Motors Since a large portion of the application of frequency converter speed regulation in cement plants involves 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. If the quality of the inverter output waveform is poor, it will have an adverse effect on the motor. Inverter output harmonics will cause additional heating and torque pulsation in the motor, and increased noise and common-mode voltage will affect the motor's insulation. Because the Innovert series high-voltage frequency converter has good output waveform quality, it is not necessary to set up an output filter, and the original ordinary asynchronous motors can be used. 6. Conclusion The Innovert series high-voltage frequency converter has high reliability and good input and output waveform quality, making it suitable for frequency conversion speed regulation of cement plant fans. It can improve the reliability of equipment operation, save a lot of energy, and bring significant economic and social benefits to cement plants, making it highly valuable for promotion. (Article source: "Energy Saving Innovation 2006 - Proceedings of the First National Electrical Energy Saving Competition")