**1 Overview** The Tongjiezi Hydropower Station is located in Shawan District, Leshan City. It is the final stage of the Dadu River cascade development and is under the jurisdiction of the Gongzui Hydropower Plant. It is approximately 33 km from the upstream Gongzui Hydropower Station and approximately 32 km from the Shawan Plant base. The station has an installed capacity of 4×150 MW, with a dam-type open-air powerhouse. The generating units and transformers are typically unit-connected. The main transformer has a capacity of 180,000 kVA and uses a forced oil circulation air-cooled cooling system with a total of 5 coolers. Due to its early design, the automatic control system for the main transformer coolers has long suffered from numerous defects in terms of measurement and control methods, control components, and peripheral measuring components. In previous years of operation, there have been several instances of automatic control device malfunctions causing the coolers to fail to start automatically; large amounts of oil leakage from the main transformer resulted in no signal output, forcing the entire generating unit to shut down; and the measurement system was technologically outdated and severely damaged, preventing it from performing protection functions and automatically starting the coolers based on load and temperature when the main transformer temperature rose. This posed a significant safety hazard to production. Furthermore, the main transformer oil level and temperature required manual, timed inspections and recordings, as well as periodic cooler switching operations, which could not meet the requirements of unmanned (or minimally staffed) operation in hydropower plants. To address these issues and enable the system to operate under unmanned (or minimally staffed) conditions, our plant decided to upgrade the main transformer cooler automatic monitoring and control system. After more than six months of research and data collection, and following thorough analysis and demonstration, we collaborated with Sichuan Zhongding Electric Control Co., Ltd. and Sichuan Electric Power Research Institute to develop a new type of automatic monitoring and control system for power air-cooled transformer coolers (TCPC-F automatic control device) using a programmable logic controller (PLC). After more than six months, the device was officially put into use at the No. 11 main transformer of the power station in November 1997. Actual production operation tests showed that the device fully met the pre-design requirements, fundamentally solving the various problems existing in the original device and satisfying the requirements of unmanned (or minimally staffed) operation and computer monitoring. **2. Development Principles of the Measurement and Control System** To adapt to the technical requirements of the new situation, the developed device must adopt new technologies, methods, and components. Furthermore, the new system should completely change the traditional measurement and control methods, requiring high automation, reliable operation, complete functions, and relatively comprehensive self-control, protection, and information transmission capabilities. It should be able to adapt to various operating conditions of the main transformer, achieving the goal of maintenance-free and operation-free operation. **3. Technical Challenges in the Development of the Measurement and Control System** First, the measurement and control device must be able to start and stop the coolers based on the temperature values ​​sent by the temperature switches and temperature sensors, while ensuring that the start and stop of each cooler meets the manufacturer's requirements for symmetrical heat dissipation of the transformer. Second, to ensure reliable operation when the main transformer oil temperature rises, avoiding false tripping of switches due to malfunctions of measuring elements, and to automatically send a signal when any temperature sensor or temperature switch malfunctions. [b][b]4 System Hardware Configuration and Technical Specifications[/b] The main transformer cooler measurement and control system consists of 6 main parts: (1) TCPC-F Main Transformer Cooler Automatic Measurement and Control Device This measurement and control device includes: FX2-80 MT programmable controller, 4 AD modules, switching power supply, relays and other peripheral parts. PLC input: 4 analog quantities, 40 digital quantities; PLC output: 40 digital quantities; Uploaded LCU: 12 digital quantities; Contact capacity: 220 VDC/5A; Measurement accuracy: 0.5%; Power supply: three 220±10% VAC; Working mode: continuous; Working environment: temperature 0°～55℃, relative humidity: 95%. (2) Temperature transmitter model: JUMO-P t100 (Germany); Working pressure: 300 kPa; Adjustable temperature range: 0°～100℃; Maximum allowable temperature: 100℃; Output C4～20 mA; Accuracy: 0.2%FS; Protection class: IP65. (3) Temperature switch model: B403-120 (USA); Working pressure: 600 kPa; Adjustable temperature range: -15°～105℃; Maximum allowable temperature: 0°～135℃; Output: 2 SPDT, 3 SPDT; Protection class: IP65. (4) Flow switch model: FCS-G 1/2 AN8 X (Germany); Pressure: 6 MPa; Power supply: 24 VDC; Working range: H201～150 cm/s; Protection class: IP67. (5) Circuit breaker, contactor, TK (including phase loss protection) Power supply: 3-phase AC; Working voltage: 400 V; Working current: 100 A, 15 A. (6) Differential pressure transmitter model: FOXBORO-841 (Shanghai); Range: 0～4 m water column; Accuracy: 0.2%FS; Power supply: 24 VDC±20%; Output C4～20 mA. [b]5 Main control functions of the device[/b] The core of the TCPC system is the PLC, which is responsible for the acquisition and processing of all information. When all monitored switch quantities are sent to the PLC input port, and the temperature transmitter sends the measured temperature signal to the PLC's dedicated A/D conversion module for data processing, the device completes the automatic measurement of the main transformer oil temperature and the automatic control of the cooler according to the pre-programmed procedure. In addition to automatically adjusting according to the main transformer oil temperature, its operation also automatically performs timed, cyclical switching between coolers. When a cooler malfunctions during operation, the monitoring and control device can automatically switch between "working" and "standby" states; simultaneously, it sends an alarm signal. The entire system, according to its design, completes the automatic control of the main transformer cooling system, realizing the automatic start and stop of the oil pump and coolers, and maintaining the main transformer oil temperature within the normal operating range. Based on actual operating conditions, once the main transformer is put into operation, three coolers will be put into use as the basic cooling load, with the remaining two for automatic adjustment. Simultaneously, the coolers are activated according to symmetrical heat dissipation conditions: when only three coolers are running, the combination is ①-③④, ②-④⑤, ①, or ②-③⑤. When four coolers are running, the combination is ①②-④⑤, ①②-③⑤, or ①②-③④, and automatic timed switching is possible. 5.1 Power Regulation Function: The PLC uses the setpoints from the power and temperature transmitters to regulate the startup of the remaining two coolers. When the main transformer load increases rapidly (generator carrying 50%–100% load), the PLC prioritizes the power transmitter information, supplemented by the oil temperature rise gradient value, to start one or two coolers involved in the regulation. 5.2 Main Transformer Oil Temperature Regulation Function: The PLC uses the setpoint temperature value from the temperature transmitter to regulate the startup and shutdown of the remaining two coolers, and can automatically switch them at set intervals. When the main transformer oil temperature is between 50°C and 55°C, the coolers do not need to be started. If the oil temperature rises to 60°C, the PLC commands one cooler to start; when the oil temperature continues to rise to 65°C, the PLC commands another cooler to start; when all coolers are in operation, if the oil temperature continues to rise above 75°C, the PLC will issue a delayed, segmented alarm and send the alarm signal to the computer monitoring system. Conversely, during operation, when the oil temperature drops from 65℃ to 55℃, the PLC will shut down the two coolers one by one. 5.3 Cooler Complete Shutdown Protection Function For a normal complete shutdown of the coolers, the following conditions must be met: the unit oil switch trips, the main transformer oil temperature drops to the temperature switch reset contact (50℃), and the PLC simultaneously determines that the temperature switch action sequence (from high to low) is consistent with the information sent by the power transmitter (power is zero). Otherwise, the coolers will continue to operate. For a complete cooler failure to stop the main transformer operation, the following conditions must be met: ① All coolers fail to shut down, with a delay of 60 minutes; ② All coolers fail to shut down, and simultaneously the main transformer oil temperature rises to the temperature sensor and temperature switch linkage (75℃), with a delay of 10 minutes. In these cases, the PLC output will trip and send a trip signal. When the oil temperature approaches the trip temperature, the PLC can first send an alarm signal. 5.4 Main Transformer Oil Temperature Rise Protection Function To ensure reliable emergency shutdown operation when the main transformer oil temperature rises, the monitoring and control device, in addition to self-checking and mutual checking the temperature sensing elements, also performs a two-out-of-three correlation action between the two temperature switches and one temperature sensor to verify the correct operation of the three temperature switches and one temperature sensor. (That is, the device will only output an emergency action signal to the trip circuit when at least two of the outputs of the two temperature switches and one temperature sensor reach the emergency temperature set value.) ① When the oil temperature ≥ 75℃, the monitoring and control device will send a signal to the main transformer trip circuit after a 1-hour delay; ② When the main transformer oil temperature ≥ 75℃, but the main transformer cooler experiences a complete cooler failure or both power supply stages disappear, a signal will be sent to the main transformer trip circuit after a 10-minute delay; ③ When the oil temperature reaches 80℃, the monitoring and control device will immediately send a signal to the main transformer trip circuit. 5.5 The automatic monitoring and control device can monitor the power supply of the main transformer cooler and automatically control the switching. [b]6 Conclusion[/b] With the joint efforts of Gongzui Hydropower Plant, Sichuan Zhongding Electric Control Co., Ltd., and Sichuan Electric Power Research Institute, the automatic monitoring and control system has completed the functional research and development of the entire automatic monitoring and control system and has been successfully put into use at Tongjiezi Power Station. After more than a year of operation, it has been proven that the equipment is safe, stable, and reliable, eliminating the previous high failure rate and unstable operation of this part of the plant, and improving the automatic control functions that could not be realized before. It has improved the safe operation rate of the main transformer, reduced the failure rate of the equipment, greatly reduced the maintenance work of this part of the equipment, and extended the service life of the equipment. The performance and monitoring and control functions of the whole system are perfect. All components of the system are well selected, achieving the technical transformation goal of maintenance-free and unattended operation. The whole system and peripheral automation components have strong outdoor protection capabilities and can fully meet the requirements of "unmanned operation (fewer staff)". In January 1999, an expert appraisal committee organized by Sichuan Electric Power Company gave the device a high evaluation: it has significant economic and social benefits, has broad application value, and plays a positive role in promoting the automation process of hydropower plants.