1 Overview
Currently, the Anqing Petrochemical Thermal Power Plant's fuel—coal—is mainly supplied by both rail and waterway. The rail section of its coal conveying system is PLC-controlled, with both automatic and manual operation modes. However, the waterway section still uses conventional relay logic control, operable only via console buttons. This equipment is outdated, with numerous relays that operate frequently, leading to easily damaged contacts and a high failure rate. Furthermore, it only allows for manual control on-site, requiring telephone communication between the control room and the field. The entire system is started and stopped in stages, resulting in low automation, hindering accident handling, and severely impacting normal production operations.
To ensure the safe and stable operation of the coal conveying system, the water circuit of the power plant's coal conveying control system was modified. An industrial control computer was used as the human-machine interface, and a PLC program control system was adopted. The original control methods of control panel button operation, relay interlocking and simple analog panel display were eliminated. The entire operation process was drawn into a dynamic flowchart screen, which was monitored and operated through the industrial control computer.
2 System Structure
2.1 Control Scope
The control scope covers the entire process of transporting coal from the riverside coal wharf to the coal yard or coal bunker via belt conveyors. The control system upgrade includes two parts: a coal wharf loading and unloading monitoring interlocking system (see Figure 1) and an in-plant coal conveying interlocking system (see diagram). The interlocking relationship is actually generated in the opposite direction of coal flow; that is, the former can only start when the latter is activated and in normal operating condition. The belt conveyors in the in-plant coal conveying system are dual-belt systems, forming two independent single-system operation modes (A and B) and two independent cross-operation modes.
2.2 Controlled Objects
The control objects of the entire interlocking system mainly include the control, detection, and alarm of 23 belt conveyors and their auxiliary equipment. Among them, the control parameters mainly complete the start and stop of the 23 belt conveyors and 14 dust collectors; the detection parameters mainly include the display of the status of each belt conveyor (slight deviation, heavy deviation, start/stop status, pull wire switch, speed switch, changeover switch, etc.) and the display of motor current values, as well as the status and parameters of auxiliary equipment such as fork pipe position, coal chute baffle, coal plow, bucket wheel excavator, iron separator, belt scale, nuclear scale, and coal bunker level. The number of input and output points of the entire interlocking system is listed in Table 1.
Table 1 Number of input and output points of the interlocking system
Device Name DI.dot.AI.dot
Belt conveyors (23 units) 7×232323
Dust collectors (14 units) 1414
Fork pipe positions (6) 6
Coal chute baffles (18 units) 18
Except for large woodenware 1
Coal plow 1
Bucket wheel excavator 1
Iron separator (3 units)
Belt scale, nuclear scale 3
Coal bunker level 3
Other 41
Total 2,093,829
2.3 Interlocking Control System Structure
The entire waterway coal transportation system spans a large area, with equipment distributed across a relatively dispersed location and harsh on-site conditions. Adopting a centralized management and distributed control system not only reduces the overall cable length and construction workload but also significantly improves system reliability and automation. Given that the system's inputs and outputs are primarily switching quantities, and the entire control is sequential logic control, the AB SLC500 small programmable logic controller was selected. This series of products allows for the expansion of multiple remote control stations from a single main controller, making it suitable for situations where the controlled objects are dispersed and the area spans a large distance. However, expanding by one remote station also incurs additional costs. The number and location of remote stations must be determined by considering their performance-price ratio. Taking all factors into account, this system upgrade adopts a structure with one main control station and three remote control stations.
