1. Introduction As is well known, substations are an indispensable and crucial link in the power system. They bear the heavy responsibility of power conversion and redistribution, playing a vital role in the safe and economical operation of the power grid. However, many existing old-style substations suffer from a series of shortcomings, such as inadequate safety and reliability for real-time power system control, and thus cannot meet the requirements of modern power systems. Therefore, proposing a safe, reliable, and efficient substation integrated automation design scheme that improves the operation and management of the power system has become an urgent task. Currently, integrated automatic control systems already in operation include LAS systems and distributed substation control systems based on CAN/LON networks. These systems have achieved good results in practical applications, but they also have various technical and economic drawbacks. Based on the development of an intelligent on-load tap-changing transformer monitoring system, this paper proposes a novel substation integrated automatic control system structural design scheme, starting from the general direction of substation integrated automation development (i.e., from centralized control to distributed (layered) network; from dedicated equipment to platform; and from traditional control to integrated intelligence). This scheme has broad application prospects in substation integrated automatic control systems. Substation integrated automation encompasses many aspects. It is a comprehensive automation system that utilizes computer technology and modern communication technology to combine and optimize the secondary equipment (control signals, measurement and protection, automatic devices, and remote control devices) of the substation, performing automatic monitoring, control, and coordination. The following example, using a transformer on-load tap changer monitoring system, illustrates the effectiveness and feasibility of the PLC hierarchical control structure in substation integrated automatic control. [b]2. Structure of PLC Hierarchical Control System[/b] Programmable Logic Controllers (PLCs) are considered one of the three pillars of modern industrial control (PLC, robots, and CAD/CAM). They are characterized by high reliability, ease of control, simple programming, high cost-effectiveness, and strong environmental adaptability, and have been widely used in the control field, including in substation integrated automatic control. However, PLCs still fall short in data and information processing and image display, and cannot yet compare with computers, thus failing to fully utilize their powerful functions. Generally, PLCs are only used to control switching quantities. However, in recent years, with the continuous enhancement of PLC communication network functions, it has become convenient to connect PLCs to computers. Leveraging the advantages of high computing speed, convenient information processing, and high display performance, the computer is used as the host computer to perform management functions, forming a complementary hierarchical control system with the PLC. In this way, the PLC can execute complex control functions, enabling optimal integrated control of the substation. The essence of hierarchical control is to allocate a large control system hierarchically according to function or structure, assigning monitoring and control functions of the entire system to different levels. Each level completes its assigned function and transmits relevant information to the next higher level for management. The integrated control function is executed by the highest level, with each level coordinating its work to strive for optimal performance of the entire control system. Hierarchical control is designed based on the anthropomorphic principle that "the higher the level, the higher the intelligence, and the lower the control precision; the lower the level, the lower the intelligence, and the higher the control precision." The PLC-based hierarchical control system is divided into three levels: the organization level, the monitoring/coordination level, and the execution level. Its system structure diagram is shown in Figure 1. (1) Organization Level: This is the highest level of the entire system, with the highest level of intelligence. It executes organizational management decisions and provides guidance and monitoring to lower levels. This level communicates with managers through a human-machine interface to perform management decision-making functions. It monitors, guides, and coordinates all behaviors of the lower level. It has the highest level of intelligence, but its precision is not high. It is better to provide macro-level guidance than micro-level guidance. This level can also automatically or semi-automatically propose reasonable control targets or indicators based on information such as actual production processes and environment, and form corresponding commands or tasks to be issued to lower levels. This part is usually completed by a high-performance computer. (2) Coordination Level: This level mainly coordinates the operation of lower-level PLCs according to the commands of the organization level, avoids conflicts between lower-level PLCs, and transmits the information of lower-level PLCs to the host computer. The monitoring/coordination machine can be an industrial control computer, a main PLC, or a PLC terminal, which can be selected according to control requirements. (3) Executive Level: This is the lowest level of the control system, executing field control functions and is the key level of control in an automatic control system. This level has the lowest intelligence but the highest requirements for reliability, control accuracy, and real-time performance, making a PLC the optimal choice. Simultaneously, the PLC at this level can connect to the upper-level monitoring and coordination level via a fieldbus for real-time online control and coordination. Fieldbus technology generally adopts a collapsed structure, using a low-level protocol of the Open Systems Interconnection (OSI) reference model, resulting in a simple structure and strong real-time performance. This structure leverages the advantages of a computer's fast processing speed and powerful information processing capabilities, enabling the computer to centrally manage various control subsystems, comprehensively process field information, and provide optimal solutions. Furthermore, the control-level computer can connect to other computers via a local area network, achieving resource sharing and allowing different systems to work in coordination under unified scheduling, reducing resource waste. After the lower-level PLCs or remote workstations are distributed and connected to the network, the controllers at the execution level can achieve distributed control on site and transmit information to the upper-level controller through the network, so that the upper-level controller can perform centralized management. Even if individual devices of the lower-level PLCs or remote workstations fail, it will not cause the entire system to be paralyzed. The overall performance is good and the operation is reliable. 3 Application of PLC hierarchical control system in substation integrated control system Currently, some substations have introduced PLCs into the control system, but they only use PLCs to control switching quantities, such as adjusting the tap changer of the on-load tap changer and switching the parallel compensation capacitor. The powerful functions of PLCs are not fully utilized. 3.1 Structure of PLC hierarchical control system in substation integrated control system Using the hierarchical control structure mentioned above, we can design the substation integrated control system according to the three-level mechanism. (1) Design of organization level The organization level is the highest level of this system and undertakes the function of optimal decision-making. Currently, most substation integrated control still follows the traditional nine-zone control method, using voltage and reactive power as dual parameters to divide the substation's operating state into nine zones, and adjusting according to the control scheme corresponding to each zone. However, in this control system, the reactive power adjustment criterion is a fixed boundary parallel to the voltage coordinate axis, independent of voltage, without fully considering the coordination relationship between reactive power adjustment and voltage adjustment. Based on the fundamental principle of substation voltage and reactive power integrated regulation—"ensuring qualified voltage, basic reactive power balance, and minimizing adjustment times"—the reactive power adjustment boundary should be a fuzzy boundary influenced by voltage status and serving voltage regulation within a certain range. Therefore, we have improved the traditional control strategy by introducing a reactive power adjustment criterion and proposing a fuzzy boundary reactive power adjustment. Based on the mutual influence of voltage and reactive power, the following mathematical model is established for the capacitor bank switching criterion: [align=left] Where: U0 is the standard voltage; Q0 is the capacity of each capacitor bank; U is the real-time voltage value; Q is the real-time power value; α1 and α2 are weighting coefficients. Based on the mathematical model derived above, the fuzzy boundary of the corrected voltage-reactive power dual-parameter regulation can be obtained, as shown in Figure 2. We use a computer for fuzzy reasoning to obtain the optimal control strategy, form a control rule table, and transmit it to the lower level for coordinated control. Simultaneously, this level provides operators with a user-friendly interface, displaying voltage, current, active power, and reactive power information in real-time using graphs and bar charts. Furthermore, it provides audible and visual alarms in case of abnormalities, allowing operators to promptly and comprehensively understand the system's operating status and adjust and control the production process. This level's computer is equipped with an expert knowledge base; when a fault occurs within the substation, it can be quickly resolved under the guidance of the expert system. Timed call printing and unattended meter reading functions facilitate unattended operation of the substation's integrated control. Based on the actual operating conditions of each substation and voltage and reactive power fluctuations at different times, voltage setting values ​​and sensitivity parameters can be set through the control-level computer. Moreover, according to control requirements, the tap changers and compensation capacitors of the on-load tap-changing transformer can be directly controlled via function buttons to further increase control flexibility. The computer at this level can also be networked through local area networks such as Ethernet and ARCNET to achieve information sharing and comprehensive control of a certain area. This not only allows for overall control but also improves the power supply quality of the entire area and reduces resource waste. (2) Design of the monitoring/coordination level The main function of this level is to complete the commands issued by the organization level and to coordinate the work of the execution level PLC. This level can be composed of a computer or a main PLC. With the continuous improvement of the performance-price ratio of PLCs, the monitoring/coordination level of a general substation can be undertaken by the main PLC. In a substation, the synchronous regulation of multiple transformers is mainly the responsibility of this level. At the same time, it is also responsible for the transmission of field information of the execution level and plays a bridging role in the entire hierarchical control. In small substations, in order to save investment, the organization level and the monitoring/coordination level can also be integrated into a high-performance computer. (3) Design of the execution level The execution level has the lowest level of intelligence but the highest requirements for control accuracy and real-time performance. Due to the severe electromagnetic interference in substations, conventional control devices are difficult to achieve precise control. Therefore, PLCs with high reliability, good real-time performance, and high performance-price ratio are the best choice. Because PLCs are networked with computers, optimal control results can be downloaded to the PLC, enabling various optimal control methods. For key components such as the main transformer, PLC redundancy technology can further improve reliability. PLC redundancy technology means that during normal operation, one PLC acts as the master PLC for control, while other PLCs serve as backups, monitoring system operation. When the master PLC fails, a PLC coordinating device designates another PLC as the master PLC to control system operation, and the faulty PLC is replaced for repair. Since the probability of PLC failure is extremely low, the failure rate after adopting redundancy technology is almost zero. Most modern PLCs offer fieldbus technology, allowing multiple PLCs in the field to be easily connected into a fieldbus local area network using configuration software. Fieldbus uses an open standard bus structure, which can easily connect distributed intelligent devices, facilitating thorough distributed control and improving the coordinated operation of each PLC, thus enhancing system reliability. 3.2 Communication Port Design The C200H of the C series is equipped with a HOST LINK communication module, which can communicate with a computer for network connection; and can achieve short-range or long-range communication via RS-232, RS-485, etc., to monitor various monitoring points on the production line. This system connects only a few PLCs, so a "polling" working mode can be adopted to transmit data to the connected PLCs sequentially. The host computer identifies and analyzes the data from the field, and then issues commands to the lower level through the communication port according to different states. Operators can also visually monitor the operation of the PLCs through the PLC terminal and issue commands via the touch screen. Therefore, the normal operation of the entire system is greatly related to the design of the communication port. To ensure smooth and reliable communication, the following points should be noted when programming: (1) The baud rate setting should be consistent with the setting of SW3 in the HOST LINK unit; (2) To ensure reliable transmission, perform an XOR operation on each character of the instruction frame to form the FCS code for communication instruction verification; (3) Process the response frame returned by the HOST LINK unit after judging that its corresponding bit is "00". If the FCS check is wrong or the corresponding bit of the response frame is not "00", display the error message and resend the instruction. The substation integrated automatic control system based on PLC hierarchical control not only absorbs the advantages of "centralized information and decentralized control" of distributed control system, but also retains the inherent advantages of PLC in terms of reliability, flexibility and high performance-price ratio. At the same time, it greatly reduces the cost of traditional distributed control system and improves system performance, achieving high-tech automation at the lowest cost. There is both division of labor and connection between the various levels of the control system, and they work in coordination. Meanwhile, according to the actual control needs on site, the execution-level PLC adopts a distributed control structure. Each PLC is distributed and networked. On the one hand, all information from the substation can be transmitted to the organizational-level computer via the network for centralized information management; on the other hand, it avoids the paralysis of the entire system due to the failure of individual devices, thus improving reliability. Because the control system adopts a modular structure, each substation can select different numbers and specifications of PLC modules according to its needs. The entire system adopts a hierarchical and distributed network structure, so adding or removing certain units will not affect the overall system function. At the same time, the PLC can be programmed online, allowing for the adjustment of setpoints according to different needs without changing the entire system structure, thus greatly improving the system's flexibility. [b]4 Conclusion[/b] This paper introduces the advanced concept of hierarchical PLC control into the integrated control of substations, proposing an advanced, reliable, and cost-effective integrated automatic control system structure design scheme for substations. Its basic idea is to combine the advantages of high reliability, flexibility, and cost-effectiveness of PLCs with the advantages of fast information processing and strong display performance of computers through a communication network, realizing a distributed control system of "centralized management and decentralized control" for substations. Meanwhile, introducing intelligent technologies such as fuzzy control and expert systems into substation integrated control can effectively reduce the number of tap changer and compensation capacitor operations, reduce transformer failures, and improve voltage quality. Field experiments have proven that this design has high reliability, low cost, and strong practicality, aligning with the trend of substation integrated control towards networking and intelligence.