1. Introduction Qingtongxia Aluminum Industry Group Co., Ltd. is a large-scale enterprise with over 40 years of history, producing electrolytic aluminum. Aluminum electrolysis is a high-energy-consuming industry, often referred to as an "electricity guzzler." Producing 1 ton of aluminum consumes 15,000–18,000 kW·h of DC electricity, accounting for approximately one-third of the cost. Reducing energy consumption in aluminum electrolysis production is not only crucial for lowering aluminum costs, but also of paramount importance for energy conservation given the increasing electricity demand in China. With the continuous development of electrolytic aluminum technology, our company's electrolytic aluminum production has undergone a rapid transformation, increasing from 32,000 tons per year at the plant's inception to the current 430,000 tons per year. The DC current system used in electrolytic aluminum production has also expanded from one 80 kA series to three series: 120 kA, 150 kA, and 350 kA. Therefore, reducing energy consumption, lowering production costs, and improving economic efficiency have become long-term research topics for our company's engineering and technical personnel. One of the most effective measures is rational power use, economical equipment operation, and the implementation of a silicon rectifier current stabilization system, which meets the DC current requirements of electrolytic aluminum production. 2. Discussion The DC current used in electrolytic aluminum production involves receiving 110 kV AC power, which is then stepped down by a transformer and converted to 1 kV DC power by a silicon rectifier to supply power for electrolytic production. The stability of the DC current directly affects the quality and yield of electrolytic aluminum. The ways to improve the quality and yield of electrolytic aluminum are twofold: first, to increase the average current intensity; and second, to improve current efficiency. 2.1 Improving Average Current Intensity Under normal circumstances, a complete power outage is not permitted, as this would reduce the average current intensity and decrease aluminum production. When a power outage is unavoidable, the duration and frequency should be minimized to avoid excessive reduction in current intensity and consequently, production decrease. Under a current condition of 80 kA, an increase of 1000A in current, calculated for 160 electrolytic cells, can increase annual aluminum ingot production by 400 tons. 2.2 Improving Current Efficiency Under a current condition of 80 kA, for 160 electrolytic cells, a 1% increase in current efficiency is equivalent to increasing the production of two more electrolytic cells. Current efficiency is a crucial factor determining aluminum production. Therefore, electrolytic aluminum production places high demands on the power supply quality, reliability, and stability of the power supply system. The silicon rectifier cabinet's current stabilization control system plays a vital role in stabilizing the current and achieving stable power supply. To this end, our company's technical personnel have continuously researched and analyzed the system, making the DC power supply more stable and economical. The current stabilization principle block diagram is shown in the attached figure. [ALIGN=CENTER] Attached diagram: Current Stabilization Principle Block Diagram [/ALIGN] When the rectifier is operating normally, if the grid voltage or load changes, the output current of the rectifier will change accordingly. The feedback voltage signal extracted from the DC current measurement circuit is compared with the given voltage of the current stabilization device. The difference is amplified by the regulating amplifier, limited, and then amplified by the buffer stage before being sent to the integrated circuit. The pulse transformer sends a pulse signal to change the control conduction angle of the saturated reactor, adjusting the control winding current of the saturated reactor, causing a change in the inductive reactance of the saturated reactor, and changing the output current of the rectifier, thereby achieving the purpose of current stabilization. 3. Modification Analysis 3.1. First Modification In 1995, a transistor current stabilization device was used. By 2003, many problems had arisen. 1) Component aging: Long operating time led to decreased transistor performance, parameter changes, mismatch between components, and decreased current stabilization effect, frequently resulting in current stabilization mismatch. 2) Low reliability: High transistor component damage rate, frequent component failures causing the current stabilization system to shut down, resulting in serious losses in electrolysis production. 3) Low current stabilization accuracy and poor sensitivity: Due to the nonlinear control characteristics of saturated reactors, analog circuit control involves multiple components, complex adjustment, and limited response speed. Synchronization signals cannot be fed back and compared in a timely manner, resulting in low control accuracy. During system fluctuations, the response speed is slow, adjustment sensitivity is poor, current return speed is long, and stabilization time is as long as 2-3 minutes. For these reasons, we modified the current stabilization system, selecting a Siemens S7-200 PLC current stabilization control system. Using a comprehensive control method, all reactor control windings are connected in series. By changing the current in the control windings, the reactance value of the reactor is adjusted to achieve current stabilization. The modified system has the following characteristics: 1) Flexible system configuration and high reliability: The PLC programmable controller is a new type of general-purpose automatic control device that integrates traditional relay technology, computer technology, and communication technology. It adopts a modular structure, has a large number of I/O points, fast sampling speed, strong analysis capabilities, and flexible response. 2) High current stabilization accuracy and high speed: The programmable controller can provide thousands of instructions to directly complete data calculation functions. The PLC automatically identifies the nonlinear operating range of the saturated reactor, with a fast execution speed of only 0.1μs, which correspondingly improves the return speed of the system current, thereby improving control accuracy. Economic benefits: 1) This upgrade greatly improved current efficiency from 91% to 97%. Since every 1% increase in current efficiency is equivalent to adding the output of 2 electrolytic cells, it is calculated that this is equivalent to adding 12 electrolytic cells, increasing annual aluminum production by 2400 tons. Calculated at a price of 16,000 yuan/ton, the annual increase in efficiency is 38.4 million yuan. 2) Due to the improvement in current efficiency, AC power consumption is reduced by 41 million kWh. Calculated at 0.31 kWh/kWh, this translates to a saving of 12.71 million yuan in electricity costs. 3.2 Second Upgrade In March 2005, the company expanded its Phase I scale, increasing the current from 80 kA to 120 kA and adding one rectifier unit. Because the capacity of the new rectifier unit was 1.5 times larger than the original four rectifier units, the original current stabilization device could not effectively regulate it. Therefore, a second upgrade of the current stabilization system was carried out. A 5+1 structure was adopted, realizing small closed-loop current stabilization control for each of the five individual units and a large closed-loop control for the overall current stabilization. The small closed-loop control converts the DC output current of each unit and feeds it back to the PLC. This is compared with the signal given by the host computer. After calculation, the PLC controls the duty cycle of the IGBTs, changing the displacement winding current of the saturated reactor to achieve current stabilization. The large closed-loop regulation involves the host computer setting the total series current. The main PLC compares the current fed back from the total current transformer with the given current, performs PID regulation, and distributes it evenly to each unit, ultimately achieving a constant series DC current. Economic Benefits: 1) The DC current increased from 80 kA to 120 kA, an increase of 40 kA. Based on the production of 160 electrolytic cells in the first phase, this can increase the annual production of aluminum ingots by 16,000 tons. At a price of 16,000 yuan/ton, this translates to an annual increase in efficiency of 256 million yuan. 2) The current efficiency increased from 97% to 98%, a 1% improvement. Since each 1% increase in current efficiency is equivalent to adding the output of two more electrolytic cells, this results in an annual increase of 400 tons of aluminum. At a price of 16,000 yuan/ton, this translates to an annual increase in efficiency of 6.4 million yuan. 4. Conclusion The improvement in the current stabilization system technology reduced the number of unit power outages, stabilized the electrolytic power supply, saved AC power, and resulted in a considerable increase in electrolytic aluminum production. This achieved the goals of energy conservation, consumption reduction, and improved economic efficiency. (Article sourced from "Energy Saving Innovation 2006—Proceedings of the First National Electrical Energy Saving Competition")