Abstract: This paper introduces a digital control system (PSR) for process control of aluminum electrolysis rectifier equipment. The system features good stability, high accuracy, and fast response. Practical application shows that the PSR system can fully meet the requirements of aluminum electrolysis production for silicon controlled rectifier equipment. Keywords: silicon controlled rectifier; control system; PSR The aluminum electrolysis rectifier system of China Aluminum Guangxi Branch (PGCA) has been upgraded and now consists of five 56KA, 1150VDC rectifier units connected in parallel. Each unit consists of an open-air switchgear, a step-down transformer, a rectifier transformer, a harmonic filter, and a rectifier. All rectifiers in the PGCA160 are three-phase fully controlled bridge rectifiers. Each rectifier has six bridge arms (A+, A-, B+, B-, C+, C-). Except for Unit 5, which has a special design due to its initial role in providing DC circuit follow current for the entire series of units during the initial stage of the renovation, each bridge arm of the other units consists of 12 thyristor elements connected in parallel, sharing a set of R and C commutation absorption circuits. The AC side of the rectifier has a central input line with parallel varistors for protection. The two bridge arms in phase (A+, A-) have temperature protection functions, and each fast fuse connected in series with the SCR can be monitored online. Effective monitoring of the rectifier equipment is mainly achieved through three control cabinets installed on-site. The control cabinets have AC power distribution, relay protection, analog panel operation, and system DI/DO and AI/AO input/output functions. The most critical component is the Programmable High-Speed ​​Processing Unit (PHSC), abbreviated as PSR. This article focuses on its hardware, software, system composition, and actual operation. [b]1 Main Functions of PSR System[/b] (1) Real-time monitoring of the working status and parameters of each part of the rectifier unit; and status monitoring of related equipment in the control loop. (2) Sampling, analysis and processing of analog quantities measured by AC and DC of the unit. Including AC feedback current for control, AC and DC voltage, and oil temperature of step-down transformer, oil temperature of rectifier transformer, pure water temperature, etc. (3) Judging the status of the unit to determine whether it can be put into operation; controlling the triggering of the thyristor element of the rectifier in normal state. (4) Analyzing the operating status of the unit, and having automatic protection function in fault state. In addition to traditional protection, there is also low synchronous voltage protection of the unit, fault protection of PSR system, etc. (5) Realizing the DC circuit freewheeling function to ensure stable and safe equipment. The freewheeling function can be started when the entire system trips due to common faults (such as reverse current protection, DC open circuit, emergency tripping and low grid voltage, low frequency, etc.). 2. PSR System Composition 2.1 PSR System Power Supply Configuration The system is supplied by three power sources: two 220VDC DC power supplies and one 220VAC AC power supply with a rectifier. Through a DC/DC power converter, 24VDC/48VDC power is supplied to the PSR control, triggering system, and peripheral input/output devices respectively. The power supply configuration is shown in Figure 1. The redundant power supply circuit ensures the safety and reliability of the PSR system and the rectifier trigger power supply. 2.2 PSR System Hardware Composition (1) Main plug-in modules of the standard control box. Processing unit PPC322AE; General-purpose processor PMA324 BE; Thyristor gate trigger control unit GD B021 BE; Digital monitoring device CS A465 AE; Comb I/O module UAC 326 AE; PSR power module KUC 321 bAE. (2) Peripheral devices connected via Arcnet bus. Control panel (AFC 094 AE); Relay output signal (ARC 093A); Rectifier unit signal measurement module (UAC 346 AE); Digital input module (UFC 092 AE); Fieldbus I/O coupler (UPC 090 AE); Fuse monitoring device (USB 030 AE). ⑶ PSR system hardware assembly. The system hardware mainly consists of a standard control box and its plug-ins, and peripheral devices connected via the ArcNet bus. The thyristor gate trigger control unit (GDB021 BE) is connected to the rectifier via optical fiber, thereby enabling its control. [align=left]2.3 Characteristics of the PSR System ⑴ Very high processing speed; ⑵ Allows a project's program to be divided into several blocks, which can run independently at different processing speeds to adapt to different permission requirements; ⑶ Very fast switching speed between different user program segments; ⑷ Has special function blocks specifically for high-power rectifiers; ⑸ Easy to add processing units to improve system processing capacity; ⑹ Input/output channels can be expanded by inserting corresponding modules; ⑺ Uses remote peripheral devices, resulting in relatively low system equipment prices; ⑻ Employs a redundant system, improving system safety and reliability; ⑼ Has system self-diagnostic capabilities. 2.4 Characteristics of the PSR System Software[/align] Strong adaptability and low engineering workload are the two biggest characteristics of the PSR control system software. The features of the PH SC system function block programming language are as follows: It is particularly suitable for system open-loop, closed-loop control, monitoring and protection; the program flow corresponds one-to-one with the function blocks, and all program blocks are completely consistent in form, making the application interface very user-friendly; the language structure is simple and systematic, and users can allocate storage units for signals in different memory areas; the controller memory allocation corresponds one-to-one with the physical address of each device; user application programs can be written directly on a PC using the FUPLA software tool; online program modification during debugging can also be completed on a PC; the function block library can be expanded as needed. 3 Characteristics and operating effects of the rectifier system 3.1 Characteristics of the rectifier system (1) Freewheeling operation function. When the 220KV system loses voltage, the PSR system will start the freewheeling function, causing all rectifier unit main switches to trip and stop the system DC output; at the same time, an unobstructed channel is opened to naturally release the energy stored in the electrolytic series. This avoids the adverse consequences of impact damage to the rectifier equipment under the same conditions before the modification. (2) The asymmetry of the thyristor trigger pulse is almost zero. The pulse output pulse of the LTC743 pulse amplifier was tested. The pulse asymmetry of the two components transmitting control signals through different optical fibers in the same bridge arm was only 0.025° electrical angle, far less than the technical requirement of 1° electrical angle. In contrast, the pulse asymmetry of the original analog control and triggering system was generally 2°–3° electrical angle, and sometimes even above 4° electrical angle. ⑶ Fast system dynamic response. When the simulated current setpoint in the PSR system debugging program is disturbed, the control system should respond and control quickly. Actual measurements show that the dynamic response time of the PSR control system in operation is 60ms, better than the required value of 100ms. This is comparable to the analog control and triggering system, and the dynamic effect is excellent. ⑷ Significantly improved non-characteristic harmonics in the rectifier unit. Actual measurements show that the non-characteristic harmonics generated by the modified system are significantly improved; compared with the non-characteristic harmonics generated by the original analog control rectifier system, the values ​​of the 3rd, 4th, and 6th non-characteristic harmonics have decreased to 20%–30% of their original values, a very significant effect. Small static deviation of DC current. (5) Due to the high system control precision and good current stabilization effect, the current deviation is less than 1%, meeting the operating requirements. (6) Due to the high symmetry of the thyristor trigger pulse, a high-tech current sharing coefficient is obtained. (7) If the main feedback DC current signal is lost, the backup feedback signal will automatically and quickly switch. (8) When the communication between the main PSR system and the unit PSR is interrupted, the function can operate normally independently. (9) The harmonic compensation device can be automatically switched on and off. (10) Series KAH regulation mode. (11) Rectifier system power MW regulation mode (with time limit). (12) Series DC circuit open circuit protection. (13) Series DC overcurrent protection. (14) DC voltage grounding alarm. 3.2 Operational effect Since the system was put into operation, it has maintained stable and safe operation, and its actions are sensitive, accurate, and reliable. In particular, the follow current function designed for common system faults has played a great role. In several cases where power system faults caused all PGCA rectifier equipment to trip and lose power, none of the rectifier equipment or components were damaged. Meanwhile, the modified rectifier equipment operates stably with significantly reduced noise, thus greatly improving the working environment for operators. The S system is operating successfully.