Abstract: In thermal power plants, the turbine protection system is crucial. This paper discusses the use of a PLC control system to implement various protection functions for the turbine, and describes the main functions, structure, and implementation of the system. Keywords: Steam turbine; PLC; Redundant system; Human-machine interface; Touch screen The turbine thermal monitoring and protection device, along with the signal alarm system and protection control system it comprises, are essential equipment for protecting the safe operation of the turbine. With the increase in unit capacity, turbine safety monitoring and protection become even more critical, placing higher demands on the accuracy and reliability of turbine safety monitoring and protection devices. Existing and early-designed protection systems mostly relied on relays and hardware logic connections, resulting in poor system reliability and high maintenance requirements. Therefore, using a programmable logic controller (PLC) to control the turbine thermal monitoring and protection device is extremely important and necessary. The following describes a turbine protection system using PLC control. 1. Overview of the Steam Turbine Protection System During normal operation, the steam turbine needs to adjust the load according to the user's electricity consumption. To prevent overspeed hazards caused by system malfunctions or sudden load shedding, and to ensure rapid shutdown in case of unit anomalies, the steam turbine must be equipped with protection for parameters such as speed, rotor axial displacement, bearing vibration, relative cylinder expansion, oil pressure and temperature of each turbine bearing, condenser vacuum, and generator main protection actions. If any of these parameters exceeds the specified allowable value, an automatic signal should be sent to activate the magnetic circuit breaker, release safety oil, cut off steam supply, and force a shutdown. Simultaneously, audible and visual alarms should be activated, the shutdown cause recorded, and interlock signals sent to the DCS control system and the regulating system to interlock the regulating valves, generator, and oil pump, allowing the entire system to quickly return to a safe state and achieve the purpose of protecting the steam turbine. 2. System Composition The steam turbine protection system consists of field equipment, a PLC control system, a human-machine interface (touch screen), and a peripheral communication system. The main field equipment includes: a turbine body monitoring system (detecting turbine speed, axial displacement, bearing vibration, cylinder relative expansion, etc.), a pressure detection system (detecting lubricating oil pressure, safety oil pressure, and condenser vacuum), an oil temperature detection system (detecting the temperature of return oil from each bearing), and a magnetic oil circuit shutdown system; the PLC control system includes: redundant PLC controllers, signal isolation relays, drive relays, and contactors; the peripheral communication system includes: a DCS control system and a turbine speed and load regulation system. The system structure is shown in Figure 1. The overall system constitutes field data acquisition and processing, monitoring, and field equipment control. 3 Hardware Design Due to the importance of the turbine protection system, malfunctions or failures to operate are not permitted at any time. Therefore, the PLC used as the logic control unit is generally selected from high-quality, stable products from domestic and international manufacturers, such as the AB-ControLogix series, GE-PAC7i series, and Schneider Electric's Modicon QUANTUM series. The following description uses the AB-ControLogix series PLC as an example to illustrate the system. The system employs a dual-machine hot-standby redundant configuration, with two sets of control core components (CPUs) installed in two independent racks. The CPUs in the two racks operate synchronously in hot standby mode via a fiber optic connection. If either CPU or related components fail or malfunction, the system seamlessly switches to the other CPU, ensuring reliable and stable operation. The PLC's I/O modules are installed in a separate I/O rack, and communication between the I/O and CPU is via a redundant ControlNet network. All PLC digital input/output points use relay isolation to completely prevent adverse effects from field interference signals, ensuring long-term reliable operation. The human-machine interface unit uses Allen-Bradley's PV1000 color industrial touchscreen. Monitoring, operation, and recording of operating parameters can be performed on the touchscreen. The touchscreen serves as the system's operator station, offering simple and convenient human-machine interaction, easy system configuration modification and expansion, and the advantage of long-term stable operation without crashes. Communication between the touchscreen and CPU is also via a redundant ControlNet network. A schematic diagram of the PLC structure of the turbine protection system is shown in Figure 2. As shown in the diagram, after the protection system is put into normal operation, a CPU failure or a communication medium malfunction will not affect the normal operation of the protection system. 4. Software Design The software design consists of two parts: PLC control and protection logic, and touchscreen monitoring and operation recording screen configuration. The PLC control and protection logic is written using ladder diagrams. The protection logic program selects continuous tasks to run throughout the entire time, ensuring that the protection logic program is always running. The protection logic program mainly includes several parts such as protection enabling/disabling, alarm and display, tripping action, event logging, and SOE logging. Due to the use of a touchscreen, protection enabling/disabling has been expanded from a single main protection switch to independent protection switches for each protection item, with each sub-protection switch configured on the touchscreen. The activation/deactivation of various protection devices is equivalent to selecting the open-loop or closed-loop operation of the turbine protection system. When a protection is deactivated, the turbine protection system operates in open-loop mode. If the corresponding operating parameters are abnormal, the system will only issue an alarm and will not trip. When a protection is activated, the turbine protection system operates in closed-loop mode. If the corresponding operating parameters are abnormal, the system will issue an alarm and trip, closing the main steam valve. The design of the individual protection switches makes the protection system more flexible. Field personnel can activate only the most important protection functions based on the operating conditions, while temporarily deactivating protections that do not meet the requirements. The turbine alarm and trip signals collected by the PLC are displayed on the touchscreen alarm and trip screens, respectively, replacing the previous method of using numerous indicator lights to display alarms and trips, significantly reducing daily maintenance. The trip action signal is recorded for the first trip, providing a good basis for analyzing the cause of turbine trips. The PLC protection logic and the touchscreen communicate in real time via the corresponding CortrolNet communication protocol, ensuring that turbine operating parameters are displayed on the touchscreen and events are recorded. Operations performed on the touchscreen are scanned by the PLC in real time and executed in the program. SOE recording requires millisecond-level accuracy, which needs to be implemented in the PLC through a program and recorded in the PLC. The recording results are then displayed on the touch screen. 5 Conclusions Based on the above ideas and methods, the PLC-controlled turbine protection system has been put into operation on the turbine equipment of dozens of user units, including Shenyang Xinbei Thermal Power Plant, Shouguang Juneng Thermal Power Plant, and Tongliao Shengfa Thermal Power Plant. The results show that the system design is reasonable, and the system operates stably and reliably on site. It not only improves the automation level of the equipment and reduces the maintenance workload of on-site personnel, but also extends the service life of the turbine, resulting in significant economic and social benefits. References: [1] Zhou Shiping, "300MW Thermal Power Unit Commissioning Technology", China Electric Power Press, 2002 [2] Li Jiangang, "Steam Turbine Equipment and Operation", China Electric Power Press, 2006 [3] Rockwell Automation, "ControlLogix Selection Guide" [4] Allen-Bradley, "System User Manual ControlLogix"