Abstract: With the continuous development of computer industrial control technology and the increasing sophistication of computer monitoring technology, PLC (Programmable Logic Controller) industrial control systems provide highly reliable control applications for various automated control equipment. This is mainly because they offer safe, reliable, and relatively complete solutions for automated control applications. Based on the unique characteristics of automatic control and logical sequence control in hydropower plants, PLCs began to be gradually used in the power industry in the early 1990s. This paper discusses some experiences regarding the successful application of PLCs in our plant's computer monitoring system for reference. Keywords: PLC; computer; monitoring; application 1 Overview Our plant was built in the early 1980s, located in northern Guizhou Province, 105 kilometers from the provincial capital Guiyang and 45 kilometers from Zunyi. The original power station was designed with an installed capacity of 630MW, installing three 210MW hydro-turbine generator units. The main functions of the power station in the system are peak shaving, frequency regulation, and system emergency backup; it is a comprehensive utilization project primarily for power generation. Early unit automation and auxiliary equipment relied entirely on electromagnetic relays for control, resulting in bulky and cumbersome control panels with complex wiring. This led to low safety and stability, poor anti-interference capabilities, and frequent equipment failures and accidents. Our plant's PLC upgrade of the units and auxiliary equipment can be roughly divided into three phases: The upgrade of the units and auxiliary control equipment began in the mid-1990s, culminating in the successful completion of the PLC upgrade for the unit's intake gate at the end of 1996—a successful attempt for our plant; Automation components were upgraded for Unit #3 in early 1998, and the installation and commissioning of PLCs for all auxiliary equipment in the plant was completed in 1999; The installation and commissioning of the computer monitoring system for the three units and the common systems was completed from the end of 2000 to the beginning of 2001, bringing our plant's computer monitoring system to a more complete and mature stage; and cultivating a group of highly skilled unit computer monitoring technicians for Wujiang Company. 2. Application Scope of PLC 2.1 Sequential Control of Generator Units The automatic operation of our plant includes the conversion of various operating conditions of the turbine generator units, the adjustment of auxiliary equipment, and the automated control of all public equipment in the plant. This type of control belongs to the sequential control system within the scope of automatic control. Each sequential control is set according to the requirements of the production process and the characteristics of the production equipment. 2.2 Based on the Operation Object, it can be divided into: 2.2.1 Automatic Operation of Generator Units The automatic operation requires completing the following operations in a predetermined sequence with a single pulse: automatic start-up to no-load, start-up to no-load, generator to no-load, generator to no-load, generator to shutdown, generator to phase adjustment, phase adjustment to power generation, phase adjustment to shutdown, etc. The operation objects include the turbine generator and its governor, excitation system, unit cooling system, and other auxiliary equipment. 2.2.2 Operation of Public Equipment Public equipment includes the entire plant drainage system, oil supply and drainage system, high and low pressure compressed air system, intelligent DC module rectifier power supply and battery float charging system, plant power system, etc. For remote control, it also includes automatic quasi-synchronous grid connection devices. 2.2.3 Plant-wide Operations Plant-wide operations include the operation of fire alarm systems, communication systems, switch stations, and disconnectors. The overall requirements for these automated operations are safe and reliable operation, convenient maintenance, clear and concise operation, and economic efficiency. 3. Application of PLC in Sequential Control Sequential control refers to the automatic control of production equipment and processes, based on process requirements and following rules such as logical operations, sequential operations, timing, and calculations. Through a pre-programmed sequence, and under the influence of field input signals (including digital and analog signals), the actuators act according to a predetermined program, achieving automatic control primarily based on digital signals. The design and installation of our plant's PLC is based on this principle. Its inputs mainly consist of control signals, primarily digital signals, such as buttons, limit switches, and normally closed contacts. Outputs are driven by relays, solenoid valves, and other drive components. The PLC's internal control section includes timers, counters, intermediate relays, and numerous normally open and normally closed contacts. Traditional sequential control is implemented using relay control panels. However, these systems suffer from poor reliability due to their large size, high power consumption, slow operation, complex wiring, poor versatility and flexibility, high maintenance workload, and high failure rate. They also lack computational and storage capabilities. PLC control systems overcome these weaknesses by organically combining computer technology with relay control, providing a perfect modern control device for industrial automation. Their advantages are mainly reflected in: 3.1 A PLC is a new type of controller developed from relays, contactors, sequential controllers, and complex control systems composed of small- and medium-scale integrated circuits and other electrical components. It uses microcomputer technology (large-scale integrated circuits) to replace the logic controllers that previously relied on hard-wired wiring, resulting in low cost, low power consumption, small size, and light weight. 3.2 A complete PLC configuration mainly consists of six modules: a power supply module, a CPU module, a hot standby module (optional), an input/output (I/O) module, an A/O module, and a communication module. The power supply module provides the PLC with a 24V DC operating power supply; the CPU is the microprocessor (the core component of the PLC); the hot standby module is used for mutual hot standby when both PLCs are online, and can be manually switched between working and standby states to improve safety and reliability; the input/output (I/O) module refers to the external circuits inputting high-level (or low-level) signals to the PLC and the PLC outputting high-level (or low-level) signals to the external circuits; the analog-to-digital converter (A/O) module converts the electrical signals input from the external devices into digital signals for the PLC to perform calculations, judgments, comparisons, and transmissions (configuration shown in Figure 1). 3.3 Because PLCs are easy to integrate with industrial control systems, easy to expand functions, and have advantages such as simple and quick interfaces, low workload, suitability for harsh operating environments, low failure rate, high reliability, strong resistance to electrical interference, and convenient maintenance. 3.4 PLCs adopt a scanning working mode, which is particularly suitable for sequential control with high logic control requirements. 3.5 The sequential control system of our plant is relatively complex. In the early 1990s, the conventional relay hard-wired control method was large-scale, difficult to maintain, and had a high probability of failures and accidents. After a fault occurred, it was difficult to find and eliminate the fault, resulting in many hidden equipment hazards. Our plant's safety production record was frequently broken when using the traditional relay sequential control method. After adopting PLC control, our plant not only improved equipment reliability but also made equipment inspection and maintenance much more convenient and quick, without needing to shut down the unit and auxiliary equipment, greatly improving work efficiency. The safety production record has now reached nearly 3000 days, setting a new historical record. 4. Design of the Unit Sequential Control Program Through the modification of the existing PLC control system for the intake working gate and unit auxiliary equipment, and combining the successful experience of other power plants, strictly adhering to the principles that power production must be safe, reliable, economical, and suitable for development, our plant began the research and design work for the computer monitoring system of Unit 2 in early 2000, and continuously dispatched engineering and technical personnel to collaborate with research units. First, the program is divided into blocks according to the requirements of the entire production control process of the unit. Second, instructions are used rationally, signal name definitions are strictly defined, and various methods are used to correctly write the programs for each program block. Then, after unit debugging, hardware and software integration debugging, and system overall debugging, the program is continuously modified and improved. After continuous simulation tests, it is put into actual field work. The block structure program is based on the characteristics of the project, dividing a control project into multiple simple and small-scale control tasks. Then, these control tasks are assigned to a subroutine block, and the control program for the specific task is written in the subroutine. Finally, a main program manages them uniformly, so that they can be called in time when the process needs to be. For example, automatic start-up to idle, start-up to no-load, generator to no-load, generator to idle, generator to shutdown, generator to phase adjustment, phase adjustment to generator, phase adjustment to shutdown, etc. are different subroutine blocks. The program logic of the shutdown to generator program block is shown in Figure 2. References: [1] Quantem PLC User Manual [2] Wujiangdu Power Plant Unit Program Control Diagram Atlas [3] Wujiangdu Power Plant Computer Monitoring PLC Secondary Diagram Atlas