Abstract: This paper analyzes the DC load characteristics of the electrolytic cell system and introduces the positive role of the freewheeling and ramping functions of the thyristor rectifier system in the overall system coordination control and safety stability. Keywords: thyristor; rectifier system; freewheeling operation 1 Problem Statement The PGCA aluminum plant electrolytic I-series rectifier system consists of 5×56kA, 1150VDC rectifiers. This equipment has been in operation for many years and its production has been relatively stable. However, in the event of a power supply loss, due to the dispersion of trigger pulses and rectifier component parameters, overload and overheating of the rectifier bridge arm components may occur. If the breaking capacity of the fast fuse itself is insufficient, and there may be undiscovered fatal faults, an explosion may occur. In severe cases, the thyristor may be broken down or the external porcelain tank may be ruptured, and the AC and DC buses may be burned by short-circuit current. [b]2 Solution Methods[/b] The faults of the entire rectifier system include: emergency tripping, reverse current relay operation, DC circuit open circuit, grid-side low voltage protection, power system low frequency protection, and unit synchronous low voltage protection. After the rectifier trigger circuit fails and stops triggering, due to the special load characteristics of the electrolytic cell series, a large amount of unreleased energy remains in the DC circuit. This energy must be gradually released along a specific circuit through artificial means, rather than being allowed to freely impact. We know that the electrolytic cell series is a special load with a certain back electromotive force E, equivalent to an infinite inductance, where its equivalent inductance L >> the equivalent resistance R, and the time constant τ = R/ωL. The values ​​of L and R in a typical electrolytic cell series can be measured, and the time constant τ is approximately between 0.5 and 1 second, meaning that it takes about 0.5 to 1 second for the energy in the electrolytic cell series to be completely released. Based on this understanding, the following two solutions can be proposed: (1) Inclined tripping method. This method is mainly designed for tripping due to a fault in a single unit. Its purpose is to ensure zero-current tripping of the unit under fault tripping conditions without damaging unit components. This method primarily targets the following faults: high temperature (≥105℃) and high pressure of the main oil tank in the step-down transformer, as well as heavy gas; high pressure and heavy gas in the on-load tap changer; heavy gas, high pressure, and high oil temperature (≥105℃) in the rectifier transformer; high temperature of deionized water and low flow rate (≤420L/Min) of cooling water in the pure water system, and low liquid level in the expansion tank; two fast-acting fuses blowing on the same bridge arm of the rectifier or the bridge arm temperature being too high; PSR system fault tripping; and low synchronous voltage tripping of the unit. When the above faults occur, the PSR of the faulty unit will issue a ramp-down command, causing the current of the unit to smoothly decrease from the operating value to zero. (2) Freewheeling method. This is the most important method among the countermeasures taken by PGAC. This method is designed to solve common system faults. It mainly targets the following faults: DC bus open circuit, unit reverse current protection relay operation, emergency fault tripping, high voltage side power failure of the rectifier system (U≤set value), and low frequency protection of the power system (f≤set value). If any of the above faults occur, the rectifier system will perform a follow-up operation. If a common trip signal occurs, the common trip and follow-up start circuit will automatically send a trip command and a follow-up start signal to each unit. Therefore, improvements have been made to the design of the dual-tube leaf filter, as follows: (1) Reduce the contact area and improve the sealing effect between the filter cartridge and the filter head. The contact between the filter head and the filter cartridge is changed from surface contact to ring contact, reducing the contact area and enhancing the pressure resistance (see Figure 2). Based on this solution, the pressure resistance of the improved sealing method and the previous sealing method was further compared through calculation. The calculation results show that the pressure resistance of the improved sealing method is greater than that of the previous sealing method. (2) Adopt a new idler wheel type. Addressing the unreliability of cylindrical idler wheels, the idler wheel type is changed to a sprocket type. The sprocket type idler wheel not only supports the chain but also has a limiting function (see Figure 3). Under normal circumstances, the chain only moves within a reasonable range and will not detach from the sprocket, thus ensuring the normal operation of the transmission device. [align=left]⑶ The pressure gauge and safety valve pipelines are in indirect contact with the cylinder. A cylindrical device is added to the cylinder, and the pressure gauge and safety valve pipelines are installed on the cylindrical device. A diaphragm pressure gauge is used (see Figure 4). ⑷ Add a sealing ring to enhance the reliability of the filter plate and the outlet pipe. A conical sealing ring is added between the filter plate and the outlet pipe, which not only makes the connection between the filter plate and the outlet pipe convenient and enhances the integrity of the filter plate and the outlet pipe, but also provides a good sealing effect (see Figure 5). [align=left][b]3 Conclusion[/b] ⑴ Our 385m2 leaf filter adopts a form that reduces the contact area between the filter head and the filter cylinder, which has a good effect. The 75 square meter double-cylinder leaf filter has similar working conditions to the 385m2 leaf filter, but its efficiency is better than that of the 42 and 50 square meter double-cylinder leaf filters. ⑵ The sprocket-type idler wheel is used in the actual production of the second alumina plant. Its function is better than that of the cylindrical idler wheel, and the feedback on its use is good. [align=left]⑶ In 2001, a 50-square-meter double-cylinder leaf filter I manufactured for a user in Henan experienced material leakage between the outlet pipe and the filter discs. After adding a conical sealing ring between the outlet pipe and the filter discs, the leakage improved significantly. ⑷ The pipeline design scheme for the pressure gauge and safety valve was based on on-site investigation. After review, the safety accessories functioned normally. Furthermore, they performed well in actual production. ⑸ The four 75-square-meter double-cylinder leaf filters designed and manufactured according to the above scheme met the user's requirements. This not only increased the variety of products in our factory but also generated 320,000 yuan in economic benefits for our factory. [/align] Click to download: Application of the freewheeling function in a thyristor rectifier system. Editor: Chen Dong