Abstract: This paper introduces the composition and functions of the computer monitoring system of Qingxi Hydropower Plant and proposes solutions to the problems existing in the old control system. This paper can serve as a reference for the design of computer monitoring systems in large and medium-sized hydropower plants in China. Keywords: Hydropower plant, computer monitoring system, renovation design Qingxi Hydropower Plant, with an installed capacity of 4×36MW, is located on the Tingjiang River in eastern Guangdong Province and was completed and put into operation in 1993. When the plant was put into operation, the unit control system adopted a design scheme with conventional control as the main method and computer control as the auxiliary method. Due to the limitations of computer technology at that time, an 80286 microcomputer was used as the host computer and Bitbus was used as the control network, which resulted in problems such as low network communication rate and poor reliability. In addition, the host computer used the IRMX operating system, which had poor versatility and made it difficult to realize communication functions with the provincial power dispatch center. In order to meet the requirements of "unmanned operation" (minimal staffing) of the hydropower plant and realize direct remote start-up and shutdown by the superior power dispatch center, in 2001 we adopted the fully distributed open control system of Nanjing NARI Group Corporation to renovate the plant's computer monitoring system, completely eliminating conventional control and realizing full computer control. The renovated monitoring system represents the latest level of automatic control in hydropower plants in China. I. System Composition and Configuration 1. Host/Operator Workstation: Two Compaq Alpha Station XP1000 workstations (500MHz 64bit CPU; 256MB RAM; 9.1GB hard drive) are configured for redundancy, serving as hot standby for each other. They run a UNIX operating system, with a WINDOWS/MOTIF human-machine interface. The host can implement all functions of the plant-level control system. 2. Engineer Workstation: One Compaq Alpha Station XP1000 workstation is configured for system management and maintenance functions. 3. Communication Workstation: The communication workstation is mainly used for data communication between this computer monitoring system and the provincial power dispatch center, regional dispatch center, and other systems within the plant. The communication workstation uses one American ICS industrial control computer (800MHz PII CPU, 128MB RAM, 15GB hard drive, and equipped with an 8-serial-port interface board). 4. Local Control Unit (LCU): The entire control system is configured with 6 LCUs: one LCU per generator unit, one common LCU, and one switchyard LCU. These units are responsible for data acquisition and control of the hydro-generator unit, auxiliary equipment, and switchyard. The LCU uses a Controllogix PLC from AB (Allenstein) as its core. The Controllogix PLC is AB's latest generation PLC, reflecting the latest developments in control systems, and features: a passive multi-master bus design; multiple processors can simultaneously perform multi-task control on a single rack; hot-swapping of any module without affecting other modules; a large I/O capacity (12800 discrete I/Os and 4000 analog I/Os); powerful network sharing capabilities; and high cost-effectiveness. Considering the high price of the PLC's temperature acquisition module, a microcomputer-based temperature monitoring and protection device was selected for temperature measurement and protection. A color touchscreen display was configured to view the unit's operating parameters on the LCU. System I/O configuration table: LCU Power Supply: Before the upgrade, each LCU was powered by only one 220V AC power supply and equipped with one UPS. Operating experience shows that the internal batteries of small UPSs have a short lifespan, and their capacity tends to decrease after a few years of use, rendering them ineffective as UPS systems. Because the hydropower plant has a highly reliable 220V DC power supply system, the LCU power supply in this upgrade adopts a dual AC/DC power supply, greatly improving power reliability. 5. The control network host, engineer workstations, communication workstations, and each LCU are connected to a Fast Ethernet network via a 10/100M network switch. The TCP/IP protocol is used. The host, engineer workstations, and communication workstations are connected to the network via twisted-pair cables, while the LCUs are connected via fiber optic cables, with a network speed of 100Mbps. 6. The GPS satellite clock system provides a standard clock for the host and each LCU to obtain the power grid's standard clock and facilitate event sequence analysis in case of accidents. II. System Functions This system consists of a plant-level computer and LCUs. The LCU is responsible for data acquisition and control, sending the acquired data and event information to the host computer, receiving downlink commands from the host computer to control the equipment, and can work independently of the host computer. Therefore, in terms of functional division, data acquisition and control are performed by the LCU, while the host computer performs database functions and plant-wide control functions (such as optimal load distribution control between units). 1. Data Acquisition and Processing The LCU collects and quantifies the operating status, operating parameters and measured values ​​of the main and auxiliary equipment of the power plant in real time and stores them in the database as the basis for real-time monitoring, alarm, control, calculation and processing. The objects collected are divided into analog quantities, switch quantities, pulse quantities and digital quantities. (1) Analog quantities Analog quantity processing includes disconnection detection, digital filtering, data rationality judgment, scaling conversion, and limit over-limit alarm. (2) Switch quantities Switch quantities include two types: sequence of events (SOE) switch quantities and ordinary switch quantities. SOE switch quantities refer to the action signals of accidents, circuit breaker opening and closing and important relay protection. The monitoring system uses the interrupt method to quickly respond to the action signals and records them with a time resolution of milliseconds. Ordinary switch quantities refer to switch quantities other than SOE switch quantities. (3) Pulse quantities are mainly used to accumulate the power pulses output by the meter to realize power accumulation and management. However, based on the operating experience of the past few years, once the LCU loses power or is out of maintenance, it cannot collect pulses, which causes the power accumulation to be inaccurate. Therefore, we have switched to digital smart meters, which have the power accumulation function. The monitoring system reads the meter data through the serial port and cancels the pulse quantity acquisition. (4) Comprehensive quantity Comprehensive quantity includes comprehensive switch quantity and comprehensive analog quantity, which is the quantity obtained after performing logical and arithmetic operations on the acquired quantity, such as the unit status, etc. 2. Control operation The control mode is divided into three levels: grid dispatch, hydropower station central control and equipment local. The monitoring system mainly uses the first two levels in normal times, and the local level is only used for debugging and emergency handling. The monitoring system controls and regulates the power plant equipment according to the current operation status of the power plant and the remote and local control commands, automatic generation control (AGC) and automatic voltage control (AVC) calculations, including unit status conversion; closed-loop regulation of unit active and reactive power; operation of switches, disconnectors and grounding switches; start and stop operation of auxiliary equipment and public equipment; gate opening and closing operation, etc. The active and reactive power of the unit can be input by the on-duty personnel in the central control room or by the upper-level dispatch center remotely, and is also given according to the predetermined load curve. The monitoring system will check the legality of the given value and, during the adjustment process, will use PID calculation to send adjustment commands to the speed governor and excitation regulator. To ensure safe and reliable adjustment and stable operation of the unit, a series of restrictions and protection measures are taken, such as power limitation, voltage protection, current protection, and avoiding the vibration zone of the unit. 3. Safe operation monitoring and accident alarm (1) Real-time monitoring: The on-duty personnel can monitor the operating status and operating parameters of all main and auxiliary equipment and public equipment in the plant in real time through the large screen monitor of the monitoring system. (2) Parameter over-limit alarm: The monitoring system can alarm for certain parameters and calculated data over-limit and record them automatically. (3) Accident sequence recording: When an accident occurs in the power plant, the monitoring system will respond immediately to the event and record it at a resolution of milliseconds for accident analysis. (4) Fault and status display recording: The monitoring system will scan each fault and status signal at regular intervals. Once a status change occurs, it will be recorded and the fault status name and the time of occurrence will be displayed. (5) Voice alarm: Once an accident or fault occurs in the power plant, the system can automatically realize Chinese voice alarm and send fault information to relevant personnel's mobile phones and pagers, which is conducive to fault handling. 4. Automatic Generation Control (AGC) AGC refers to the technology of automatically controlling the active power of the hydropower plant in an economical and rapid manner to meet the needs of the system according to specified conditions and requirements. Based on the water inflow and power system requirements, considering the operating limitations of the power plant and units, the number of operating units, unit combination and load distribution among units are determined according to the principle of economic operation. Main functions: control the active power of the whole plant according to the load curve; control the total active power of the whole plant according to the given load; frequency regulation function; unit start-up and shutdown guidance. 5. Automatic Voltage Control (AVC) AVC refers to automatically controlling the bus voltage or reactive power of the hydropower plant according to predetermined conditions and requirements. Under the condition of ensuring the safety of the units, it provides the system with fully utilized reactive power and reduces the power loss of the power plant. Main functions: control the reactive load distribution of the whole plant according to the given reactive power mode. 6. The establishment of a historical database and statistical records is primarily based on the following reasons: A. To compensate for the shortcomings of the computer monitoring system: Previously, the hydropower plant's computer monitoring system, being a real-time control system, lacked a historical database, making historical data and event retrieval difficult. Furthermore, many events requiring human judgment, such as the dates of unit commissioning/decommissioning, could not be statistically analyzed by the monitoring system, resulting in inaccurate data accumulation and statistics, rendering statistical functions unusable. B. To improve operational management: This system allows for the computerization of operation logs, inspection management, and work/operation ticket management, modernizing operational management. The historical database system uses a dedicated PC server with a Windows NT operating system and an SQL database. It collects real-time data and events from the monitoring system, as well as manually entered data and events. After classification and processing, a centralized historical database is formed. Serving as the hydropower plant's production data center, including both real-time and historical data, it also isolates the power plant's computer LAN from the monitoring system. Data can be displayed graphically and published to the entire plant via a browser, greatly facilitating hydropower plant production management. 7. Data Communication: Since Qingxi Hydropower Plant lacks microwave communication with the provincial power dispatch center, we use VSAT satellite communication. Satellite communication equipment and leased channel operation costs are very low, and the transmission rate reaches 40-50 Kbps, basically meeting the requirements for dispatch data transmission. It is an effective communication method for remote areas. 8. System Self-Diagnosis and Redundancy Switching: The monitoring system has online self-diagnosis capabilities. When a fault is detected, the primary and backup computers can automatically switch, ensuring that the backup computer can correctly take over when the primary computer fails. This improves the reliability of the monitoring system. III. Conclusion The Qingxi Hydropower Plant's computer monitoring system represents the development level and advancement of computer control in hydropower plants both domestically and internationally in terms of its mode, functions, and LCU configuration. Its features, particularly PLC selection, satellite communication, GPS time synchronization, and the establishment of a historical database for open real-time data, are particularly noteworthy. It provides valuable reference for the upgrading of control systems in large and medium-sized hydropower plants in China. References: 1. NC-2000 Hydropower Plant Computer Monitoring System User Manual, Nanjing NARI Group, 2001. 2. Controllogix PLC User Manual, Rockwell Automation, USA, 2000.