Abstract : The belt conveyor coal conveying system in a thermal power plant is a crucial support system and an essential condition for ensuring the normal operation of the unit. This paper analyzes the process control of the belt conveyor coal conveying system in a thermal power plant and develops a belt conveyor coal conveying monitoring and control system using Siemens PLC as the control core. Practice shows that the system is reliable in operation, requires minimal maintenance, has a high degree of automation, and also has remote monitoring capabilities.
Keywords : Thermal power plant, Belt conveyor system, Monitoring system, MCGS PLC
0 Introduction
The coal conveyor system in a thermal power plant is a crucial support system, essential for ensuring stable and full-capacity operation of the units. The coal conveyor system at Qingdao Jiaonan Yitong Thermal Power Plant includes vibrating feeders, belt conveyors, rotary screens, crushers, and electric tee switches, among other equipment. The system comprises numerous, geographically dispersed devices, making manual control by on-site personnel essential. This not only requires a significant workload but also offers limited resilience against short circuits, overloads, belt misalignment, and blockages, potentially leading to coal accumulation and other accidents. Furthermore, the start-up and shutdown sequence of the coal conveyor system requires strict adherence to regulations, necessitating robust interlocking mechanisms between equipment. This paper utilizes a Siemens PLC as the control core and MCGS configuration as the host computer software. Based on the actual requirements of the coal conveyor system, it implements local control, remote manual control, and remote monitoring. Real-time monitoring of parameters such as equipment current and belt misalignment switches is conducted, and fault handling procedures for overload, short circuits, and belt misalignment are designed to ensure the safety and reliability of the coal conveyor system.
1. Analysis of the Coal Conveying System in Power Plants
The coal conveying system of Jiaonan Yitong Thermal Power Plant is shown in Figure 1. The system consists of two conveying lines, each including a vibrating feeder, belt conveyor, rotary screen, coal crusher, iron separator, and electric tee. The coal flow direction changes after entering the electric tee. Based on the direction of the coal flow, the power plant's coal conveying system has four conveying lines.
To extend equipment lifespan and prevent damage from overload during startup and shutdown, the equipment must be in an unloaded state during startup and shutdown. During coal supply, the startup and shutdown of each piece of equipment must follow a specific sequence, i.e., interlocking control of the equipment. System startup proceeds against the coal flow direction, meaning downstream equipment is started first, followed by upstream equipment. System shutdown proceeds with the coal flow direction, stopping upstream equipment first, followed by downstream equipment. During system operation, if any equipment malfunctions, it must be stopped immediately. To prevent coal accumulation at the faulty equipment, upstream equipment must stop immediately, while downstream equipment stops with a delay or continues normal operation until the fault is resolved, at which point the equipment restarts after a delay. To ensure no coal residue remains on the equipment, appropriate delay times are set during the interlocking shutdown process.
Figure 1. Power plant coal conveying system diagram
2. Control System Composition
The control system is shown in the figure. It mainly includes a computer, a PLC control master station, an electrical control box, a current transmitter, and a belt misalignment limit switch.
Figure 2 System Composition
The control system adopts a three-level control structure consisting of an industrial control computer, a programmable logic controller (PLC), and a local control box.
The industrial control computer serves as the monitoring center of the coal conveying control system. It communicates with the programmable logic controller (PLC) in a host-slave relationship to realize real-time monitoring of the operating status of field equipment and belt misalignment, and intuitively describes the operating status of the coal conveying system.
In this paper, a Siemens S7-226 programmable logic controller (PLC) is selected, equipped with a digital signal expansion module (EM222), an analog signal acquisition module (EM231), and an Ethernet module (CP243). The system utilizes the PLC to acquire and control data from the entire coal conveying system, and issues control commands to the system equipment through the human-machine interface of an industrial control computer. The host computer uses MCGS configuration software to work with the slave computer to complete the system's automatic control and remote manual control functions, monitor the system's operating status, and achieve unmanned operation in the work area.
The local control box is placed near the equipment to enable manual/automatic selection and manual start/stop control of the equipment, and acts as the actuator of the PLC to realize manual/automatic control of the equipment.
Belt misalignment switches, electric three-way position switches, current transformers, etc., are used to collect data from field equipment, which is then processed and analyzed by the PLC and displayed on the host computer.
3 System Functions
(1) Automatic control: After the coal flow enters the tee, it can be redirected to either belt conveyor A or belt conveyor B. Based on the direction of the coal flow, four working modes are set to interlock the equipment. After the operator selects the working mode through the upper computer configuration interface, the system will start automatically. The equipment will start automatically in the predetermined sequence.
① When starting up, start from the last conveyor belt (and related equipment) in the opposite direction of coal flow, and set a reasonable delay time. Coal supply will only begin after all equipment has started. The bell will ring for 30 seconds before each conveyor belt starts.
② When shutting down, in the direction of coal flow, first stop the power supply equipment, and then stop the equipment in sequence from the first to the last. For each piece of equipment, issue a shutdown command according to the predetermined delay time, that is, set an appropriate delay time, and stop the operation of the equipment after the remaining coal is cleared.
③ In the event of a fault, the equipment at the fault point and upstream equipment will stop instantly, while the equipment downstream of the fault point will remain in its original operating state. After the fault is cleared, the equipment can be restarted upstream from the fault point, or, if the fault is not cleared, the equipment can be delayed and stopped downstream from the fault point. The interlocking mechanism can prevent any equipment from starting out of sequence.
(2) Remote control: All coal conveying equipment can be operated one-to-one through PLC and industrial control computer.
(3) Local control: Manual control of equipment can be achieved on site without going through PLC.
(4) When there is a possibility of endangering personal safety or damaging the equipment during the operation of the equipment, and the operator does not have time to judge how to deal with it, the operation of all field equipment can be stopped by using the emergency stop button.
(5) Monitoring function: Monitor the operation of the system, including the current of the equipment, the working mode of the equipment, the operation of the belt, etc., and process the fault information by alarming, storing reports, etc., for the staff to handle.
(6) Protection functions: Set up protection actions such as short circuit, overload, deviation, blockage, etc., and interlock to trip related equipment.
4. Visual Software Design
This paper selects MCGS configuration software as the host computer software for the system. MCGS is a software system developed by Beijing Kunlun Tongtai Automation Software Technology Co., Ltd. for quickly building computer monitoring systems. Due to its flexibility in system construction and stability, and the fact that the system has numerous drivers for connecting to underlying devices such as general-purpose devices, PLC devices, data acquisition boards, network devices, and user-customized devices, as well as its various exchange methods such as OPC, ODBC, OLE, DDE, and ActiveX for interaction with other applications, MCGS configuration software has strong versatility and has been widely used in the field of automation.
MCGS serves as the main human-machine interface of the system, responsible for displaying the status of system devices, issuing fault alarms, and issuing control commands. It is the monitoring center of the system.
(1) Main control screen
The MCGS main control screen, as shown in Figure 3, mainly consists of system simulation operation status monitoring, equipment status monitoring, manual/automatic selection, interlock control, and electric three-way monitoring.
Figure 3 System main control screen
The system's simulated operation status monitoring section displays the system's operating status in real time based on the actual collected equipment parameters. When equipment is turned on, coal flows through the corresponding equipment, allowing staff to observe the system's operation in real time. The equipment status monitoring section mainly includes equipment operating mode (remote/local) display, operating status display, equipment current, etc. The manual/automatic selection section provides a selector switch. When the switch is in automatic mode, all equipment operates in an interlocked manner; upstream equipment can only start after downstream equipment starts. If a piece of equipment stops, its upstream equipment stops immediately. When the switch is in manual mode, the system will display a manual control screen, allowing staff to start any equipment as needed. The interlocking control consists of two parts: line selection and start/stop control. There are four coal conveying lines based on the direction of the electric tee. After pressing the start button, the system starts the equipment against the coal flow direction according to the line selection status, and presses the stop button to stop the equipment with the coal flow direction. The electric tee control section is the monitoring part of electric tee A and electric tee B, mainly completing the remote control function of the system's electric tee.
(2) Auxiliary monitoring screen
The auxiliary monitoring screen mainly consists of historical current curves and historical data reports. By observing the historical curves and historical data reports, staff can have a general understanding of the recent operating status of the equipment. The occurrence of overcurrent and overload situations allows staff to make an advance prediction of the coal feed rate.
(3) Fault alarm handling
The fault alarm consists of two parts: visual alarm and voice alarm. When the equipment experiences faults such as short circuit, overload, or belt misalignment, the system will provide visual prompts and voice alarms, and offer alarm handling strategies. There are two fault alarm strategies to choose from: one is to delay shutdown from the fault point; the other is to start from the fault point. Starting from the fault point requires confirmation that the equipment fault has been resolved.
(4) Stop processing with one click
The system provides a one-click stop function, which can instantly stop all devices in case of an unexpected situation.
5 Conclusion
This paper presents a coal conveying control system for a thermal power plant, based on Siemens PLC and MCGS as the control core. The system is comprehensive in function, simple to operate, and provides vivid and realistic monitoring visuals that closely resemble the actual site conditions. This greatly facilitates monitoring of the coal conveying system for staff and significantly reduces their workload. The system has been implemented at Jiaonan Yitong Thermal Power Plant. Since its commissioning, the system has operated stably, fully meeting practical needs and receiving high praise from the user.
