1. Overview The Gongzui Hydropower Plant is located on the Dadu River in Sichuan Province. Currently, it consists of two power stations, Gongzui and Tongjiezi, forming a cascade hydropower system. Gongzui has an installed capacity of 7×100 MW, and Tongjiezi has an installed capacity of 4×150 MW, for a total installed capacity of 1300 MW. The Gongzui Hydropower Station has upper and lower powerhouses. The upper powerhouse is a dam-mounted open-air powerhouse with an installed capacity of 4×100 MW, while the lower powerhouse is a tunnel-mounted powerhouse with an installed capacity of 3×100 MW. The Tongjiezi power station is located approximately 33 km downstream of the Gongzui power station and is also a dam-mounted open-air powerhouse. The Gongzui Hydropower Plant site is located in Shawan District, Leshan City, approximately 37 km from the Gongzui power station and approximately 32 km from the Tongjiezi power station. In 1995, the Gongzui Hydropower Plant was designated by the State Power Corporation as one of the second batch of pilot units for unmanned operation. To achieve "unmanned operation (minimal staffing)," the Gongzui Hydropower Plant began a comprehensive automation upgrade of the Gong and Tong power stations in 1996. After the upgrade, both power stations are controlled from the Shawan Cascade Dispatch Center, with full computer monitoring. [b]2 Introduction to Programmable Logic Controllers[/b] Programmable logic controllers (PLCs) have a long history of application in industrial control and have now developed into a very mature industrial control technology. They possess multiple functions such as logic sequencing, timing, counting, arithmetic, control, signal and alarm, protection, remote control, and remote transmission. Their flexibility, reliability, ease of programming, and ease of adjustment are widely recognized, and PLC technology has been extensively used in thermal power, hydropower, substations, and various industries such as chemical, metallurgical, and machinery. Currently, there are many types of PLC products on the Chinese market. Our factory primarily uses Mitsubishi Electric programmable logic controllers (FX series). The following is a brief introduction to the basic components and performance of a Mitsubishi Electric Programmable Controller (FX series) as an example: 2.1 Basic Logic Instructions Basic logic instructions include start operation, series connection, parallel connection, circuit block series connection, circuit block parallel connection, coil drive instruction, no-operation, coil on-and-hold instruction, coil on-and-clear instruction, operation memory, memory read and reset, and program end instructions. 2.2 Step Sequential Control Instructions STL (Relay Ladder Diagram or Step Ladder Diagram) is a diagram that is easy to understand and eliminates the need for specific mechanical actions. Users can program the programmable controller very simply, even without complex programming. STL diagrams are entirely represented in the form of relay ladder diagrams, and have the characteristic of comprehensively expressing load drive circuits and transfer condition circuits. The flow forms include single flow, selective branching and merging flow, jump flow, parallel branching and merging flow, intermediate state STL, initial state STL, and operating mode selection control. 2.3 Soft Components: These include input/output relays (XY), auxiliary relays (M), status elements (S), pointers (P/I), constant handling (K/H), timers (T), counters (C), data registers (D), and index registers (Z/V). 2.4 Application Instructions: Application instructions include program flow, transfer and comparison instructions, arithmetic and logical operation instructions, loops and shifts, data processing, high-speed processing, convenience instructions, external I/O devices, and external modules. 2.5 Special Soft Components: These include PC status, clock, flags, step sequence control, interrupt disable, error detection, high-speed meters, function extensions, pulse capture, alternative functions, internal up/down counters, and high-speed counters. 2.6 Programming Languages: One of the main aspects of PLC application technology is writing application programs, and ladder diagram language is one of the most widely used programming languages. PLC ladder diagrams evolved from traditional electrical control circuit schematics, making them easily accepted and mastered by technical personnel. The main similarities between ladder diagrams and electrical control circuit schematics are: (1) They have the same graphic structure, both being unfolded circuit schematics. Traditional electrical control circuit schematics can be seen as relay ladder diagrams; while when used for PLC programming, they are called PLC ladder diagrams. (2) Relay ladder diagrams use graphic symbols of various control electrical appliances, while LC ladder diagrams use equivalent circuit symbols of PLC internal resources - programming elements. (3) The signal input, output forms and control functions are the same, both outputting the input signal after the circuit's logical operation to complete a certain control function. The main differences between the two are: (1) They work in different ways. Relay ladder diagrams are parallel working modes where power is applied to each branch simultaneously, while LC ladder diagrams are serial working modes with sequential scanning. (2) They have different numbers of available contacts. The number of action contacts in a hardware relay is only a limited number of pairs; while the number of contacts in a soft relay is unlimited and can be used any number of times. (3) They have different programming methods. In relay ladder diagrams, a limited number of relay contacts are used, and only the operational sequence between components is considered. Appropriate mutual constraints are set. LC ladder diagrams operate in a sequential scanning mode, without the problem of simultaneous operation of parallel branches, but the correct sequence of actions must be considered to minimize the number of program steps. PLC ladder diagrams are primarily written using a horizontal drawing method, mainly representing equivalent circuits and energy flow direction. During writing, attention must be paid to the arrangement order of contacts, the arrangement order of coils, the prohibition of series connections to the right of coils, and the requirement that the contact arrangement conforms to energy flow rules. 2.7 Basic Performance of Mitsubishi Programmable Controllers (FX Series) Operation Control Method: Stored Program, Iterative Operation; Input/Output Control Method: Batch Processing; Operation Processing Speed: Basic Instruction 0.48 μs/instruction; Application Instruction Count + to Hundreds μs; Number of Instructions: 20 Basic Instructions, 2 Step Instructions, 228 Application Instructions; Input Relays: 154 Points X0 to X267; Output Relays: 84 Points Y0 to Y267; Auxiliary Relays: M0 to M8255; Status Relays: S0 to S999; Timers: T0 to T255; Counters: C0 to C199; Data Registers: 0 to D7999. [b]3 Application of Programmable Controllers[/b] The most commonly used programmable controllers in our factory are the Mitsubishi Electric Programmable Controller (FX Series) and the AB SLC-5 PLC. During the comprehensive automation transformation of Gongzui Hydropower Plant, on the one hand, an advanced computer monitoring system was adopted, and the original unreliable and inaccurate automatic components were replaced with automatic components that were sensitive, reliable and stable. On the other hand, PLCs were used to replace the original automatic waterwheel control panel, unit hydraulic device control panel and air compressor and water collection well control panel. Using PLCs, the corresponding equipment can be monitored and controlled independently as a system, and data can be easily exchanged with the computer monitoring system. It has the advantages of accurate measurement, low failure rate and stable operation, so that it can operate independently under normal circumstances. Only when the PLC fails will the computer monitoring system intervene and control the corresponding equipment. 3.1 Configuration and Function 3.1.1 Air Compressor Control System (1) Configuration: Mitsubishi programmable controller FX2-80T, 4A/D module, power supply, relays, etc. Motor control and protection: air switch, contactor, thermal protection element (with phase loss protection). Measurement and control elements: pressure transmitter, solenoid valve, electronic flow switch, temperature switch. (2) Function: The core of the system's measurement and control is the PLC, which is responsible for processing all information. All monitored switch quantities are sent to the PLC input port, and the transmitter sends the measured air pressure electrical signal to the PLC's dedicated A/D conversion module for data processing. The PLC completes the automatic measurement and control of the air compressor system according to the preset program, realizing the automatic start and stop of the air compressor, and the automatic control of the water supply valve and the pressure tank drain valve. The system can record the cumulative number of runs and the cumulative running time of each air compressor. 3.1.2 Oil Pressure Device Control System (1) Configuration: Mitsubishi programmable controller FX2-80T, SGK AC solid motor controller, pressure transmitter, differential pressure transmitter, liquid level transmitter, combined air replenishment valve B302, pressure switch. (2) Function: The whole set of equipment completes the automatic control of the oil pressurization device, realizes the automatic start and stop of the oil pump and the automatic air replenishment, and maintains the pressure and oil level within the normal working range; during automatic operation, the "working" and "standby" pumps automatically cycle and take turns. When the "working" pump fails, the "standby" pump automatically takes over. The system controls the electromagnetic air replenishment valve group to automatically replenish air based on the continuously and accurately measured oil level and air replenishment oil level set value. After the oil level reaches the normal level, the air replenishment stops. When the oil level in the pressurization tank and the oil level in the return tank are abnormal, the corresponding fault signal is issued. 3.1.3 Water collection well control system (1) Configuration: Mitsubishi programmable controller FX2-80T, 4A/D module, power supply, relays, etc. Motor control: soft starter, air switch; control elements: level transmitter, float switch, solenoid valve (normally open), thermal conductivity flow switch; (2) Function: PLC is the core of the system's measurement and control. First, it performs self-check and mutual check on the water level signals measured by the level transmitter and float switch. PLC executes the start and stop control of the water pump motor according to the preset program, maintains the normal water level in the collection well, and automatically completes the "working-standby" state switching of the water pump motor. When the water level reaches the standby water level and the high water level, the corresponding signal is issued. The system adopts two water level measurement control signals with different measurement principles. The level transmitter and float switch take turns to serve as the "main" working state. When one of them fails in the "main" state during operation, the other automatically takes over its operation. The control measurement mode composed of the level transmitter and float switch has mutual check function. 3.1.4 Transformer Cooler Control System (1) Configuration: FX2-80 MT programmable controller, 4A/D module, temperature transmitter, temperature switch, flow switch, differential pressure transmitter, electric valve. (2) Functions: The PLC controls the start and stop of the cooler and provides early warning and alarm for temperature rise based on the main transformer load and the four set temperature values ​​sent by the three temperature switches and one temperature sensor; it enables automatic start and stop of the oil pump and cooler for air-cooled transformer coolers to maintain the main transformer oil temperature within the normal operating range; it enables automatic start and stop of the cooler for water-cooled transformer coolers and can alarm for cooler blockage, and can automatically switch the cooling water of the cooler in both directions. In addition to automatic adjustment according to the main transformer oil temperature, its operation mode also automatically performs timed cyclic switching of each unit. When a cooler malfunctions during operation, the monitoring and control device can automatically switch between "working" and "standby" states; at the same time, it sends the alarm signal to the power station computer monitoring system. In order to ensure the reliability of the emergency shutdown operation of the main transformer cooler, the system requires the measurement and control device to perform self-test and mutual test, as well as a three-choice-two correlation action between the two (i.e., when the two temperature switches and one temperature sensor are at the emergency temperature, at least two of their outputs must reach the emergency temperature set value) before the device will send a trip signal; otherwise, it will not operate. 3.1.5 Dam Spillway Control System (1) System Structure and Configuration: The system is composed of a PLC controlling each spillway, and the PLCs of the five spillway gates are connected to form a PLC network and connected to the dam LCU-PLC; the operation and control can be performed on the panel of the local control unit PLC or on the gate LCU-PLC. The local PLC consists of: A-BSLC-5 programmable controller, power supply, panel, communication converter 1770-KF3, relay, solenoid valve, pressure switch, displacement sensor, air switch, soft starter controller, etc. (2) Functions: (a) Data Acquisition and Operation Monitoring: Both the dam LCU and the local PLC can acquire and monitor various pressures, oil levels, gate openings, and the operation of automated components. They can also monitor the operating status of the oil pump and the gate position, and trigger an alarm when a fault or abnormality occurs. The pressure sensor output value is directly connected to the PLC, which outputs upper and lower limit control contacts. (b) Gate Control: The dam LCU and the local PLC can independently execute defined tasks. Even when disconnected from the dam LCU, the PLC can still control and display the gate through the operation panel. Operators can set the gate opening, switch between working and standby pumps, and control the gate's lifting, lowering, and stopping through the touch buttons on the local PLC control panel. Automatic control of the water lubrication device of the arc gate water seal system can also be completed. The gate's sinking and rising are achieved through the PLC. When the gate is in any set position, if it slides down by more than 200 mm due to oil leakage or other reasons, the working pump is started, and the gate rises back to its original position. If the working pump fails, the gate will continue to slide down to 250 mm, at which point the standby pump will start and the gate will rise to its original position. If the standby pump fails, the gate will continue to slide down to 300 mm and an alarm signal will be issued. During automatic operation, the working pump and the standby pump will automatically cycle and take turns operating. When the working pump fails, the standby pump will automatically take over. (c) The analog and digital signals of the local PLC are shared with the dam LCU to ensure automatic and smooth switching between the two systems. When the local unit PLC fails, it can automatically and smoothly switch to the dam LCU and report a fault signal, but it will not affect the normal operation of other PLCs. When the dam LCU fails, the local PLC can automatically switch to independent operation. (d) Protection function: The spillway gate PLC monitoring system can determine whether the gate is operating normally based on the collected data and operating conditions. If electrical or mechanical abnormalities occur, protection actions can be taken according to different situations to avoid equipment damage. The spillway gate PLC control power supply is powered by two power supplies and can switch automatically. (e) Communication function: The spillway gate PLC can be used as part of a computer monitoring system. It can operate independently or be managed by the computer monitoring system. 3.2 Application Status To achieve "unmanned operation and minimal staffing," the Gongzui Hydropower Plant carried out comprehensive automation upgrades, including extensive modifications to auxiliary equipment and public facilities at the Gongzui and Tong stations. PLC technology was widely used in the upgrades to the auxiliary equipment control systems. Specifically, four sets of automatic air compressor monitoring and control systems were used in the high-pressure and low-pressure air compressor systems at the Gongzui and Tong stations; four sets of automatic oil pressurization device monitoring and control systems were used at the Tong station and seven sets at the Gong station; nine sets of automatic leakage and drainage monitoring and control systems were used; one set of automatic monitoring and control system for the dam spillway gate was used; and five sets of automatic monitoring and control systems were used for the main transformer coolers. Based on the actual operation of the PLC devices put into operation, the PLC devices operate safely, reliably, and stably. The automatic measurement and control system, composed of a programmable controller as its core and highly reliable peripheral automation components, has completely changed the traditional measurement and control methods. It replaces the traditional contact pressure gauges and mechanical pressure relays, which are difficult to adjust and prone to fatigue damage. This new system measurement and control mode has a high degree of automation, reliable operation, and complete functions, possessing comprehensive automatic control, protection, and information transmission functions. It can reliably realize the self-checking of the system's own working status, the judgment of peripheral equipment fault conditions, and has strong fault tolerance capabilities. Comprehensive data display functions provide a basis for checking and analyzing the equipment's operating status, thereby improving the overall management level. [b]4 Conclusion[/b] Gongzui Hydropower Plant began comprehensive automation transformation work in 1996, aiming to achieve "unmanned operation and minimal staffing" at the hydropower station. However, the original auxiliary machine control could hardly meet the requirements of "unmanned operation and minimal staffing." In the renovation of the auxiliary equipment and public equipment control system, our plant chose PLC control instead of using the computer monitoring system for local LCU control for the following reasons: (1) Even when the unit is shut down, the auxiliary equipment of the hydropower station (such as oil pressurization device, top cover drainage pump, etc.) must be automatically controlled to start and stop according to the monitored working conditions. If the control of these auxiliary equipment is all completed by the computer monitoring system, it will inevitably require the unit LCU to be unable to exit maintenance when the unit is shut down. At this time, it puts unrealistic requirements on the reliability and availability of the LCU, which is not advisable in terms of technology and economy. At present, with the development of new technologies, PLC has been used to realize control to improve reliability. When the computer monitoring system cannot be put into operation synchronously with the unit, or when the monitoring system has a temporary failure and is temporarily shut down, in order to ensure the safe operation of the unit, except for a few auxiliary equipment (such as technical water supply system, main shaft sealing water, etc. that work at the same time as the unit operation) which can be automatically controlled by the unit LCU, other auxiliary equipment should be equipped with special automatic devices. (2) The common equipment of hydropower plants mainly includes high and low pressure gas systems, drainage systems, etc. These equipment are installed in scattered locations, are independent of each other and form their own systems. They only need to rely on their own automation components to independently control according to their monitoring status and given operating mode. The logic of automatic control is relatively simple. In order to meet the needs of commissioning and operation management, local control panels are generally set up separately. If the automatic control of these scattered equipment is concentrated on the common LCU of the computer monitoring system, the number of connecting cables will increase a lot, interference will be difficult to prevent, and the reliability requirements of the common LCU will be too high, thus increasing the difficulty of programming and debugging of the computer monitoring system, making automatic control difficult to achieve. (3) PLC is easy to operate and maintain. PLC programming language is standardized and step ladder diagram is easy to understand. (4) From an economic point of view, the price of PLC is much lower than that of LCU modules with the same number of measuring points. In terms of safety, because PLC has the above-mentioned characteristics of being able to independently monitor and control equipment reliably, even if the computer monitoring system or other systems fail to operate, it will not affect the normal operation of the system. Therefore, adopting a computer monitoring system for the entire power plant, consisting of a main computer monitoring system supplemented by several PLC subsystems (primarily used for monitoring and controlling auxiliary and public equipment), is a very good configuration method from both an economic and safety perspective.