1. Project Introduction Addressing the severe energy waste in equipment operation is not only a requirement for enhancing enterprise competitiveness but also a requirement of the national policy of creating a resource-saving society. Variable frequency speed control technology is the preferred speed control technology for equipment such as fans and pumps, and it is also the inevitable direction of this technology's development. After retrofitting equipment with variable frequency technology, it can significantly reduce the enterprise's production costs, reduce the failure rate of production equipment, extend the service life of equipment, generate significant economic, social, and environmental benefits, and improve the enterprise's overall competitiveness and development potential. Replacing the traditional damper airflow regulation technology with direct variable frequency speed control technology, changing the fan speed to regulate the exhaust volume, provides better speed and accuracy of airflow regulation, achieving the goal of saving electricity. Due to historical reasons, most fans in the boiler and clinker kiln systems of Zhongzhou Branch have a large design margin, and the rated flow of the fans is far greater than the actual airflow required by the production system. In order to meet normal production needs, it is necessary to adjust the opening of the dampers to control the airflow according to the working conditions of the production system. Because this type of regulation method actually involves the motor running at its rated speed, controlling the air volume by increasing pipe resistance, the fan operates under high pipe resistance for extended periods. This significantly increases the system's pipe resistance, causing the fan's actual operating point to deviate from its rated operating point, reducing system efficiency. Furthermore, the design has a larger power reserve than the actual operating conditions, resulting in a "large horse pulling a small cart" phenomenon and substantial energy waste. This not only affects the reduction of unit production costs but also lowers the competitiveness of the company's products, limiting its long-term development. Therefore, the Zhongzhou Branch decided to first implement frequency conversion upgrades for the No. 5 and No. 6 exhaust fans of the clinker kiln and the No. 5 and No. 6 induced draft fans of the boiler. 2. Feasibility Analysis Our branch's fan and pump equipment mainly uses speed control to adjust air volume and valve opening to meet process requirements. Using frequency converters to control fan speed is an effective energy-saving method, showing significant energy-saving effects compared to the commonly used method of adjusting dampers to control air volume. Variable frequency speed control technology has been widely used in low-pressure fans and pumps in recent years, and AC variable frequency speed control technology is also developing towards larger capacity, higher voltage, and higher performance. In the alumina industry, many high-power fans and pumps still rely on methods such as baffles and valves to adjust the flow rate. Since these methods reduce flow by increasing pipe resistance, they result in significant energy waste, which is no longer sufficient for modern industrial development and is a key factor affecting enterprise competitiveness. Replacing traditional damper-based airflow regulation (valve-controlled flow) technology with high-voltage variable frequency speed control technology, which adjusts airflow by changing the fan (pump) speed, offers better speed and precision, achieving energy savings. The parameters of the relevant equipment for this upgrade are shown in Table 1. 3. Equipment Selection and Control Principle Diagram of the Wind Turbine Energy-Saving Retrofit System High-voltage frequency converters generally have the following main options: One is direct high-voltage control (high cost), currently using harmonic-free frequency conversion technology similar to Robicon in the US, where low-voltage modules are connected in series to form a high-voltage output. Its advantage is extremely low harmonics, but it requires a dedicated input transformer, resulting in the highest investment cost. At low frequencies, the IGBT saturation voltage drop in series leads to low efficiency. The second option is three-level (four-level) control (medium cost). Due to the low voltage withstand capability of power semiconductors, a series scheme is used to increase the output voltage, similar to low-voltage frequency converter technology. The difference lies in doubling the output voltage and lower output current harmonics, making it more suitable for medium-voltage applications (690～3300 V). The capacity is also medium. Since IGCT high-voltage power modules are also used, the voltage can be increased to 6000 V, but its market application is currently limited. Three-level frequency converters have a simple structure, but the increased number of diodes and circuits, coupled with the inconsistent drive waveforms of each IGBT, will inevitably lead to inconsistent clamping and switching performance. The switching on and off of power components is ensured by clamping diodes. Clamping diodes require high voltage withstand capability, good fast recovery performance, and a large number of main components, resulting in a relatively complex system structure and limited expansion capabilities. The third type is high-low voltage control (low-cost). High-low voltage control frequency converters refer to using transformers to reduce high voltage to low voltage, and then purchasing low-voltage frequency converters to drive low-voltage motors. This system has the most mature technology, the highest reliability, the highest operating efficiency, the lowest investment cost, and convenient maintenance services. It is most suitable for medium and low capacity motors (generally referring to those below 2000kW). Transformer + 690V frequency converter + dedicated frequency converter motor belongs to this category. There is also a high-low-high configuration, which uses a step-down transformer + low-voltage frequency converter + special step-up transformer + motor configuration. Because the output transformer needs to be specially manufactured, the cost is high, the power factor is low, the efficiency is low, the self-loss is large, and it is bulky. The system performance is poor and it can be used for general process speed regulation, but it is not suitable for speed regulation and energy-saving applications. During the implementation of this project, a comprehensive evaluation was conducted based on multiple indicators, including energy-saving effect, short payback period, daily maintenance, and economic benefits. Based on this comprehensive evaluation, a configuration of step-down transformer + 690V inverter + dedicated 690V variable frequency motor was adopted. This configuration offers high operating efficiency, low investment cost, and convenient maintenance services, making it particularly suitable for retrofit projects. The ABB ACS800-04 series inverter was selected through a bidding process. Considering that the exhaust fan and induced draft fan are crucial production equipment, a failure of the variable frequency speed control device would cause the entire clinker kiln and boiler system to shut down, resulting in significant economic losses. Therefore, the design also included a power frequency to variable frequency switching circuit to ensure that the equipment can be started at power frequency in case of inverter failure. 4. Project Implementation and Operation Status: On July 15, 2006, commissioning and trial operation of the high-voltage variable frequency retrofit began. The inverter has been operating well to date. 4.1 Energy Saving Analysis The rated power of the No. 5 exhaust fan motor is 475kW, the rated current is 58.2A, the average operating current is 33A, the average load rate is 54%, and the average valve opening is 40%. Based on the system operation, considering the speed range and flow adjustment range of the exhaust fan, the energy saving calculation analysis for the frequency conversion modification is as follows: Since the fan and water pump are square torque loads, that is, the motor load torque is proportional to the square of the speed, and the flow rate is proportional to the motor speed. The general expression for the output power of the motor is that the shaft power of the motor is proportional to the cube of the motor speed. The power consumption of the exhaust fan after the frequency conversion modification is calculated as follows: The actual air volume provided by the frequency conversion exhaust fan is Q₂ = 40%Q₀, and the actual speed of the exhaust fan is N₂ = 40%N₀. Considering the efficiency of the frequency converter, the exhaust fan, and the air supply pressure, this value will be higher than the theoretical calculation value. At low speeds, the difference is even greater. Empirical data shows that after frequency conversion modification at 40% of the rated airflow, the actual power is approximately 25% of the rated power, i.e., P₂ = P₀ × 0.25 = 119 kW. The annual electricity consumption is W₂ = P₂ × 24 × 300 = 870,000 kWh. From the above analysis, it can be seen that after frequency conversion modification, the annual electricity saving for this exhaust fan is approximately W = W₁ - W₂ = 194 - 87 = 1,070,000 kWh. 4.2 Actual Energy Saving Effect Table 2 compares the actual daily electricity consumption of exhaust fans 5 and 6 of clinker kiln #3 after modification with the daily electricity consumption of exhaust fans 3 and 4 of clinker kiln #2 without modification. The actual energy saving effect is very significant, with each kiln saving 6173 kWh per day. The annual electricity saving per kiln is approximately 2.25 million kW•h, which is 6173 × 365 = 2253 145 kW•h. Since it is still in the experimental stage, the correspondence between the fan speed and the air volume required for different operating conditions of the clinker kiln is still being explored. Currently, the speed is 90% of the rated value, and there is still considerable room for reduction in the actual speed required to meet process requirements. Therefore, the electricity saving effect should be more significant. 5. Effects and Analysis after the Modification 1) Significant Electricity Saving Effect: The actual electricity saving effect is very significant, with each kiln saving 6173 kW•h per day. The Zhongzhou branch currently has 7 clinker kilns of the same specifications. If all of them are modified, the total electricity saving will be 7 × 6173 = 43211 kW•h per day. 2) Fast and Precise Air Volume Adjustment: Direct frequency conversion speed regulation technology replaces the traditional damper air volume adjustment technology, changing the fan speed to adjust the exhaust volume, resulting in better speed and precision in air volume adjustment. 3) No Impact on Equipment, Improving Operational Safety: After the modification, the fan operates smoothly and vibration is reduced. 4) The soft-start function reduces the impact on the power grid during startup and extends the motor's lifespan. 5) Harmonic interference meets standards, with excellent harmonic suppression. The built-in filter ensures that the voltage harmonic content is less than 3%, complying with IEEE 519-1992 and GB/T 14549-1993 standards. The ABB Duldt filter added in this upgrade protects the motor insulation and reduces bearing current. Cable selection and installation strictly adhere to specifications, such as using symmetrically shielded power cables, and laying control cables separately from power cables with proper grounding. 6) Comprehensive protection functions, including reliable overcurrent, overvoltage, undervoltage, and fault protection. 7) Complete indication functions, including input and output current and voltage, operating frequency, fault display, and operating status indication. 8) Simple operation, similar to ordinary ABB low-voltage frequency converters, with simple and convenient function settings and adjustments. Speed ​​adjustment at the host computer level is simple and reliable. 9) The operating patterns of the exhaust fans and induced draft fans were explored, and the optimal correspondence between fan speed and process parameters was gradually established. The corresponding equipment operating procedures were rebuilt. In summary, after the project implementation, significant energy savings were achieved, while precise airflow adjustment was realized. The operation is very simple and clear, and the system can provide feedback on a large amount of on-site operating data, greatly reducing the workload of on-site operators and improving operational comfort.