Abstract: This paper mainly studies the application of PLC in the relay protection and control system of 10kV high-voltage circuit. Keywords: relay protection, control system, programmable logic controller (PLC) 1. Introduction With the development of social economy and the rapid rise of industry, the number of 10kV power distribution systems has increased significantly. The simplicity, reliability, safety, energy saving and cost-effectiveness of power distribution systems are particularly important. At present, traditional 10kV power distribution systems still use relay systems and distributed monitoring and metering, while adopting PLC (programmable logic controller) systems for centralized control and centralized monitoring and metering is beneficial to improving the automation level of power distribution system operation and management, ensuring the safety and stability of power distribution, and reducing the workload of operators. 2. Comparison of Relay Systems and PLC Systems PLC (programmable logic controller) is a new type of industrial controller developed in recent decades. Due to its flexible programming, complete functions, wide application, simpler control than relay systems, convenient use, strong anti-interference ability, high cost performance, and long service life, it also has the characteristics of small size, light weight and low power consumption. Relay systems have significant drawbacks: large size, low reliability, short service life, and difficulty in troubleshooting. In particular, because they rely on hard-wired logic, wiring is complex, adaptability to changes in production processes is poor, and centralized control is inconvenient. PLCs, on the other hand, are easy to install and wire in the field. Their internal soft relays can simplify the complex intermediate steps of relay systems, enabling soft-wired logic systems for convenient centralized control. Furthermore, PLCs have self-diagnostic capabilities, fault alarms, fault alarm type display, and network communication functions, facilitating inspection by operators and maintenance personnel. 3. Example of Centralized Control and Centralized Monitoring Metering in a 10kV Primary Distribution System: In a 10kV primary distribution system, two 1000kVA transformers operate in parallel. Figure 1 shows the schematic diagram of this primary distribution system. Figure 1. Schematic diagram of a 10kV primary distribution system. 3.1 The role of PLC in centralized control. In the primary distribution system, relay systems are mainly concentrated in the main receiving cabinet and transformer distribution cabinet. Using a PLC system to replace the relay system can reduce hard wiring between cabinets, save many relays, simplify the process, reduce system manufacturing costs, and improve the reliability, safety, and energy efficiency of the distribution system. The PLC system block diagram is shown in Figure 2. Figure 2. PLC system block diagram. The PLC is the nerve center of the entire system. All control, protection, and working status indications are completed through virtual relays inside the PLC via soft wiring in conjunction with external given switching quantities and signals. Control voltage is below the safe voltage, which can improve the safety of operation. Operation away from high-voltage rooms can avoid operator errors. One-stop control can improve work efficiency and reduce the labor intensity of operators. Signal transmission of the entire system can be realized using two fieldbuses. The working status and alarm indications of the PLC facilitate troubleshooting by operators and maintenance personnel. In addition, compared with relays, PLCs have high maintenance-free operation and long service life. 3.2 PLC I/O Allocation In a 10kV primary power distribution system, besides power-on/power-off control, there are also overcurrent, undervoltage, and gas protection for the transformer. We take an Omron CAMP2AH 40-point PLC as an example for I/O allocation, as shown in Table 1. Power-on/power-off control is a switching quantity, which can be achieved using control buttons. Overcurrent, undervoltage, and gas protection involve automatic detection technology, which is implemented using intelligent sensors to improve the reliability of the protection. Table 1 PLC I/O Allocation Table 3.3 Design of Centralized Control and Centralized Monitoring Metering for 10kV Primary Power Distribution System The power distribution system is the nerve center of the power grid. The normal operation of the power distribution system is inseparable from our life security and work order, which requires higher reliability. The intelligence, energy saving, ease of operation, and convenient maintenance of the power distribution system are necessities for rapid economic development. Users desire higher safety requirements, lower labor intensity, and higher cost-effectiveness for equipment during the operation and maintenance of the power distribution system. Based on the above points, we have made the following improvements to the 10kV primary distribution system: We have implemented centralized control of the main receiving cabinet and distribution cabinets using PLC, and centralized monitoring and metering of the system using digital instruments. The improved 10kV primary distribution system block diagram is shown in Figure 3. After the improvement, with the integrated cabinet as the working platform, staff in the control room can control the operating status of the high-voltage room, which is both convenient and safe. Staff can also record and inspect the monitoring and metering instruments, as well as their working or alarm status at any time, which is convenient, timely, and clear, and also reduces labor intensity. In conclusion, using a microcomputer PLC to operate the relay protection and control system greatly improves the system's automation level and reliability, while also facilitating centralized control and monitoring, simplifying the system's information management, significantly reducing costs, and improving work efficiency, thus having certain promotional significance.