1 Introduction With the development of the power industry, the power grid structure is becoming increasingly large and complex. The automatic power grid dispatching system (hereinafter referred to as the system) plays an increasingly important role in power grid dispatching and management, providing important technical means for the safe, high-quality, and economical operation of the power grid. The system consists of computers, RTUs, and control equipment. It is a highly integrated low-voltage device with low insulation level, extremely sensitive to external interference, and has very low tolerance to strong electromagnetic pulses such as lightning and overvoltages. Various external interferences are transmitted to the system in certain ways, which will affect the system's data acquisition, processing, and transmission, and thus affect the stable and reliable operation of the system. Therefore, it is necessary to take effective measures to strengthen and improve the system's anti-interference protection, reduce external interference to the system, and improve the system's anti-interference capability. 2 Formation of Interference 2.1 Main Sources of Interference 2.1.1 Interference from Lightning Lightning and its associated transient fields cause strong electromagnetic disturbances at the lightning strike point, and spread their influence to the surrounding area through the transient field, causing interference. Due to the implementation of certain lightning protection measures, the possibility of a direct lightning strike to the system is low. However, the strong electromagnetic pulse interference from lightning can not only intrude into the system through power lines, input/output lines, and grounding devices, but also generate strong induced overvoltages that can severely interfere with or even damage system equipment. 2.1.2 Interference from the power grid: Within the power grid, sudden changes in the operating status, such as the occurrence and resolution of transmission line accidents, the operation of various switching equipment, and changes in the impedance of distribution lines and the loads connected to them, such as the starting of high-power equipment and high-power motors, and the excitation impulse current of large transformers, can all cause instantaneous fluctuations in the supply voltage. These instantaneous fluctuations in the supply voltage can generate interference such as power outage overvoltage, current surges, and high-frequency oscillations. These interferences can intrude into the system through power lines or grounding networks, causing confusion in the logic circuits of system equipment, damaging programs and data in RAM, affecting the normal operation of the CPU, and leading to system paralysis. 2.1.3 Interference from electrical equipment: In the field, high-voltage, high-current, or electrical equipment with large voltage and current variations can couple through certain means to interfere with the system. In addition, power transmission lines and control lines also interfere with small signal lines (such as switch and analog signal acquisition lines, channel transmission lines, etc.). 2.2 Interference Propagation Paths The various interferences mentioned above are mainly propagated to the system through input/output lines, power lines, channel lines, equipment shielding shells, and grounding networks. 2.3 Interference Coupling Methods 2.3.1 Conductive Coupling External interference is directly conducted and coupled to the system through transmission lines (such as input/output lines, power lines, ground wires, etc.). 2.3.2 Coupling Through Common Impedance Due to the existence of common impedance, external interference current generates interference voltage through the common impedance and is conducted to the system; internal units of the system will also interfere with each other through the common impedance. 2.3.3 Electrostatic Coupling In high-voltage operation or situations with large voltage changes, due to the effect of power lines, the interference source transmits electrostatic changes to the system, forming electrostatic interference. The intensity of electrostatic interference is directly proportional to the interference voltage of the interference source and the system's resistance to ground, and inversely proportional to the distance between the interference source and the system equipment. 2.3.4 Electromagnetic coupling occurs in environments with high current operation or large current variations. Current changes cause changes in surrounding magnetic field lines, inducing electromotive force in various loops of the system, resulting in electromagnetic interference. The interference intensity is directly proportional to the area of ​​the magnetic field lines passing through the loop and the magnitude of the current change at the interference source. 3 Anti-interference Measures Based on the source, propagation path, and coupling mode of the interference, corresponding anti-interference measures can be taken to reduce external interference to the system. 3.1 Isolation ① Use a power isolation transformer of suitable capacity. The power supply is then supplied to the system via an isolation transformer to provide 220V power. The system is only magnetically coupled to the power grid, without direct electrical contact, achieving electrical isolation and enhancing the system's anti-interference capability against the power grid. ② Optocouplers or isolation transformers should be used for the input and output of switch and analog quantities, as well as the system's external communication ports. If possible, fiber optic communication should be used to strengthen interface isolation and improve signal transmission reliability. ③ The installation and positioning of automated equipment should be far away from electrical equipment operating with high current and high voltage to reduce electrostatic induction and electromagnetic induction. ④ When laying cables, care should be taken to avoid mixing different types of cables together. Small signal cables, control cables, and low-voltage power lines should be laid separately from high-voltage lines as much as possible to avoid interference between strong and weak signals. 3.2 Protection: Necessary protection, even multi-stage protection, should be installed at all input and output ports where overvoltages such as lightning may intrude, to suppress overvoltages that interfere with the system within permissible limits. ① Overvoltage protection devices should be installed before the isolation transformer, i.e., surge arresters and surge suppressors should be installed. Varistors can be used for surge arresters. The surge suppressor model should be selected based on the maximum value of surge voltage and current that the system may encounter in actual operation, with a margin. ② Varistors should be installed on the input and output lines between secondary equipment, transmitter panels, and RTUs, as well as at the input and output points of the system's signal transmission to suppress overvoltage surges on the signal lines. The capacitance of the varistor used should be as small as possible to prevent signal attenuation. 3.3 Filtering: ① Although the power supply uses an isolation transformer to achieve isolation between the system and the power grid, most common-mode noise and series-mode noise from the power grid can still couple into the system through the transformer. Using a power supply filter allows frequency components near the power supply frequency to pass through, quickly attenuating interference noise outside these frequency components. Installing a power supply filter as shown in Figure 1 after the isolation transformer can effectively suppress common-mode noise and cross-mode noise below 30 MHz from the power grid. ② For some frequently interfered communication channels (mostly carrier channels), a bandpass filter should be installed at the receiving end to filter out interference signals other than the remote signal frequency components in the channel, reducing interference from communication equipment and external factors to the remote signal. 3.4 Proper Grounding Proper grounding is an important anti-interference measure. Incorrect grounding methods can actually become a cause of interference. 3.4.1 Grounding Resistance Requirements System grounding is divided into power grounding, safety grounding, and logic grounding. ① Power grounding mainly carries the loop current of the load equipment, as well as unbalanced current under normal conditions and grounding current under abnormal conditions. These current fluctuations constantly cause potential changes between devices, creating interference. Experiments show that when the power grounding resistance is greater than 4 Ω, the above interference may affect the normal operation of the load equipment. Therefore, the power grounding resistance should not exceed 4 Ω. ② Safety grounding refers to the grounding of the equipment's metal casing. Its function is to shield high-frequency interference to the system equipment and prevent voltage rise or leakage caused by charge accumulation on the casing, which could threaten personnel handling the casing. Experiments show that the grounding resistance of safety grounding should not exceed 4 Ω. ③ Logic grounding is used to provide a unified reference voltage for low potentials in signal loops. Since the logic "1" and logic "0" of commonly used TTL and C-MOS circuits in system equipment differ by only a few volts, voltage drop fluctuations on logic grounding can easily cause confusion in the logic ground potential, affecting the normal operation of the system equipment. Therefore, the grounding resistance of logic grounding must be sufficiently small. Experiments show that the grounding resistance of logic grounding should be less than 1 Ω to ensure the stability of the logic ground potential. 3.4.2 Equipotential Grounding Equipotential grounding can effectively prevent accidents caused by potential differences between equipment and improve the lightning protection capability of system equipment. The specific method is to connect the grounding systems of secondary equipment such as computers, RTUs, communication equipment, and relay protection to the nearest closed-loop grounding busbar and ensure a secure connection to maintain equipotential at each grounding point. 3.4.3 Hybrid Grounding System The grounding within the equipment is generally designed by the manufacturer as a hybrid grounding system. This means that the logic ground, power ground, and cabinet outer casing are connected to a single terminal inside the cabinet, and then a main grounding wire is led out and connected to the grounding device during installation. Its advantage is simplicity and ease of installation. However, its anti-interference capability is limited, mainly because the logic ground and cabinet outer casing ground are directly connected. Interference from the outer casing grounding potential can cause confusion in the logic grounding potential, affecting the normal operation of the system equipment. To improve the anti-interference capability of the system equipment, the outer casing grounding should be separated from the logic grounding and connected separately to the safety grounding in the computer room. 3.4.4 Grounding Wire Connection Techniques Proper grounding wire connection can reduce the contact resistance of the grounding wire, providing a long-term reliable low-resistance path for the grounding system. The system grounding generally uses a riveting method. Key points for riveting: ① Treat the metal contact surfaces before riveting, removing any covering layers; ② Rivet tightly; ③ It is best to use the same metal for riveting, as long-term contact between different metals can cause corrosion and alloying, affecting the mechanical strength and contact resistance of the connection. 3.5 Shielding Shielding is a very effective method to prevent static electricity and electromagnetic interference. ① The conductors and equipment around the plant automation system carry high voltage and large current, and the system is in a strong alternating electromagnetic field. Experiments show that when the spatial magnetic induction intensity is greater than 3×10-6 T, the computer will malfunction and miscalculate. Using this as a reference standard, when the environmental magnetic induction intensity is greater than 3×10-6 T, electromagnetic shielding should be considered for the system equipment, such as using shielding nets, shielding cages, etc., to block and attenuate electromagnetic interference. The wall thickness, geometric dimensions, conductivity, and permeability of the shielding body should be selected according to the frequency of electromagnetic interference. These can all be found in relevant materials; it is important to note that the shielding body must be effectively grounded. ② The power distribution lines of the system should use power cables with metal shielding layers, and the shielding layers at both ends of the cable should be effectively grounded to prevent lightning from inducing overvoltage on the power lines. Small signal lines are best to use multi-shielded cables, and each shielding layer must be reliably grounded. ③ The magnetic shielding effect of twisted pairs. In the presence of external interference magnetic flux, each wire of a twisted pair cable is induced with an interference current. The induced currents flowing through the two segments of adjacent loops of the same wire are equal in magnitude and opposite in direction, thus canceling each other out. Therefore, in the overall effect, the wires are not induced with interference current. Thus, twisted pairs have excellent shielding against external magnetic field interference. Adding an external shielding layer to twisted pairs can overcome their susceptibility to electrostatic induction, providing excellent electromagnetic shielding for signal lines. Shielded twisted pairs should be used for signal lines, especially small signal lines. 3.6 Reducing Common Impedance The internal resistance of the power supply, power lines, and grounding wires can all become common impedances. It is essential to reduce the internal resistance of the power supply and the impedance contained in the power lines and grounding wires. ① The system should not share power lines with other equipment; a dedicated line should be used, and the power line should be sufficiently thick and as short as possible. ② Use regulated power supplies and isolation amplifiers whenever possible to reduce the internal resistance of the power supply. ③ The system's grounding wire should be made of short, thick copper wire and reliably connected to reduce the impedance of the grounding wire. 4. Conclusion ① Scientific management of the power grid relies on a stable and reliable power grid dispatch automation system; and comprehensive anti-interference measures are a crucial guarantee for the stable and reliable operation of the power grid dispatch automation system. Therefore, anti-interference issues must be given high priority during the system's design, installation, and commissioning. ② Anti-interference measures should be comprehensively utilized based on actual conditions; taking measures from only one aspect is unlikely to achieve the desired results.