Abstract: This paper analyzes and discusses the problems existing in the application of PLC control systems in thermal power plants, details the preventive measures for electromagnetic interference, and explains that by optimizing the power supply system and ensuring reliable grounding of the PLC system, the stable operation of the PLC control system can be ensured. Keywords: PLC control system, fault analysis 0 Introduction With the development of computer and network communication technology, PLC (Programmable Logic Controller) has been widely used in thermal power plant control systems due to its powerful functions and high reliability. Its reliability is related to the safe operation of various systems in thermal power plants, and even affects the safety and economy of unit and power grid operation. With the increase of service life, among the various accidents that occur during unit operation, unit accidents caused by PLC system failures have accounted for a certain proportion. Therefore, PLC control system failures and their prevention have become problems that need to be considered and solved. 1 Existing Problems The environment of power plants contains extremely strong electromagnetic fields. The voltage of generators reaches thousands of volts and the current reaches hundreds of amperes. The output voltage of switch stations reaches tens or hundreds of kilovolts. Due to site limitations, sometimes a section of high-voltage power cable and signal line, hundreds of meters long, cannot be effectively separated, and may even have to be kept in the same cable trench. This can lead to strong electrical interference generated during high-voltage, high-current switching, which may induce voltage and current on the PLC input lines. This interference can cause inaccurate measurement data, or even cause the LEDs in the PLC's optocoupler to light up, leading to malfunctions. This phenomenon frequently occurs in the field. For example, the Shaanxi Jintai Chlor-Alkali Chemical self-owned power plant consists of 3×130t/h + 2×25MW thermal power units. The coal conveying system, chemical water treatment system, and water source well system all utilize PLC control systems with host computers, while smaller PLC control systems are used in the boiler soot blowing system, ash removal, electrostatic precipitator, coal mill thin oil station, and turbine ball cleaning system. The coal conveying PLC control system repeatedly experienced the problem of belt 2A starting in vain. Inspection revealed induced voltages as high as 57V in both its input and output circuits, causing the input opto-isolator (DC24V driven) to activate, which in turn caused the contactor to engage and start belt 2A. Subsequently, the power plant implemented anti-interference measures, connecting RC surge absorbers in parallel across the load terminals. Since then, similar phenomena have not occurred. 2. Preventive Measures 2.1 Interference Prevention Measures The PLC internally uses optocouplers, small relays, and opto-thyristors to isolate switching signals. The PLC's analog modules also generally employ optocoupler isolation. These measures not only reduce or eliminate the impact of external interference on the system but also protect the CPU module from external high voltage damage. Therefore, it is generally unnecessary to install interference isolation devices externally to the PLC. For details, please click: Problems and Preventive Measures of PLC Control Systems in Thermal Power Plants.