Air compressor systems are essential equipment in hydropower stations. While their operation is not complex, their start-up and shutdown processes have strict requirements. With the rapid development of electronic, software, and control technologies, PLCs (Programmable Logic Controllers) have also advanced rapidly, offering superior performance and significant advantages over traditional relay-based control circuits. PLCs feature high reliability, abundant I/O interface modules, modular structure, easy-to-learn programming, and convenient installation and maintenance. As the automation level of hydropower stations continues to improve, it is necessary to adopt fully automatic PLC control for the air compressor operation process and install monitoring and alarm devices in a remote control room to achieve unattended operation on-site and remote monitoring and alarm functions.
1. General requirements for the control system
The PLC automatic control system for the air compressor in a hydropower station should meet the following requirements:
(1) The power supply of the control system is an AC/DC online switching type to ensure that the PLC data processing and control can work reliably in abnormal situations (power switching).
(2) High and low pressure air compressor PLC control panel, using pressure feedback as the criterion to realize the local PLC automatic start and stop of air compressor.
(3) The control system should be equipped with I/O modules, central processing modules, communication modules, power supply modules, analog quantity modules and other equipment required for operation. All modules are solid-state plug-in standardized structural components and should meet or exceed the industrial control level standards.
(4) It must meet the on-site operating conditions of the power plant and have high stability and anti-interference performance.
2. Control System Hardware Design
2.1 System Solution
Based on the technical requirements of the power plant's air compression equipment, the designed control system structure is shown in Figure 1.
2.2 Hardware Configuration of the Control System
(1) TSX3721 CPU module. It features a real-time clock, 20K words of RAM, and 16K words of backup FlashROM, allowing for increased application memory capacity. It can also connect to a communication module, with a maximum of 248 I/O points. It comes with a display module that can categorize, summarize, and display all the data required for controlling, diagnosing, and maintaining the PLC and its modules, providing a simple human-machine interface.
(2) TSXAEZ-802 Analog Module. It features 8 high-precision, multi-range current channels, each selectable between 0-20mA or 4-20mA. The module uses steady-state multiplexing technology to scan the input channels (normal or fast) to obtain 12-bit A/D conversion values. In addition to the above functions, the PLC processor can also perform input overflow monitoring and measurement value filtering.
(3) TSXDMZ-28DT digital input module. 16 digital inputs, 12 digital outputs, 24VDC power supply.
(4) Sensors. The temperature sensor provides a 4-20mA current signal with a range of 0-120%. The pressure sensor is an MPM/MDM580 series electronic pressure sensor with a 24VDC power supply, providing a 4-20mA current signal with a range of 0-10MPa and 0-1.0MPa .
Based on the above module requirements, the PLC hardware configuration is shown in Figure 2.
Flowchart of the control software 2.3
The PLC program control diagram is shown in Figure 3.
3. PLC Control System Software Design
3.1 Control Strategy
(1) As shown in Figure 1, the pressures of the air tank are P1, P2, P3, and P4. The PLC control system automatically starts the main and standby air compressors according to the pressure range of the pressure acquisition signal.
(2) The two air compressors serve as hot standby for each other.
(3) After each air compressor has worked for a total of 30 minutes, start the drain valve for 15 seconds.
(4) All start/stop air compressor and abnormal signals are sent to the central control room.
(5) Three control modes can be selected on the control panel: manual, automatic, and remote.
3.2 PLC and Central Control Room Communication Data Sheet
The PLC and the central control room communicate using the MB+ network interface. Table 1 lists some of the communication data.
4. Application
This article describes a PLC-based air compressor control system that has been applied to the Mollsadra Hydropower Station. The Mollsadra Hydropower Station is located on the Tang-e-Boragh River, a major tributary of the Kor River, within Fars Province of the Islamic Republic of Iran. The air compressor control system comprises two high-pressure compressed air systems and two low-pressure compressed air systems, all operating in a one-main-one-standby configuration. The hydropower station's air compressor system primarily provides air for the hydropower station's governor hydraulic system, industrial equipment, and braking systems.
In this system, the rated discharge capacity of the high-pressure air compressor is 0.82 m³/min, and the discharge pressure is 7.0 MPa; the rated discharge capacity of the low-pressure air compressor is 2.8 m³/min, and the discharge pressure is 0.8 MPa ; the rated volume of both is 2 m³ . In the design, P = 6.3 MPa, P2 = 6.4 MPa, P3 = 6.5 MPa, P4 = 6.8 MPa, P5 = 6.9 MPa, and T = 100℃.
After on-site commissioning and acceptance, the equipment has been running on-site for more than a year and has been operating reliably, playing an important role in the safe and stable operation of the hydropower station.
5. Conclusion
a . The PLC air compressor control system has been successfully applied to the Moll-sadra hydropower station. It is safe and reliable in operation, highly intelligent, and its excellent real-time adjustment can overcome adverse factors such as adjustment lag caused by human factors. It is simple to operate and can achieve unattended operation .
b. During system implementation, fault detection and diagnosis procedures can be introduced to improve the system's intelligence and further enhance the effectiveness and reliability of the automatic control system. By optimizing scheduling strategies and applying automatic control function modes such as software interlock protection, it is expected that the level of automation can be raised to a higher level, providing a basis for determining the condition-based maintenance points of air compressor equipment and thereby obtaining greater benefits.
