Application of PLC and VFD (Variable-frequency Drive) in power plant control system

　Reconstruction Design for Coal Feeder Control System

for Thermal Power Plant Based on PLC

QIAO Dong - ka i

（Maoming College Electromechanical Project College, Maoming 525000, China）

Abstract: The status quo of coal feeder control system for a thermal power p lant is introduced. Elaborates on the working p rincip le and way of the reconstructed control system, system components, design ideas of PLC control program, design of touch - screen and the setting of parameters. Tests show that the operation of reconstructed control system can meet a expected requirement as it is economic, efficient, and steady.

Key words: frequency conversion; PLC; touch - screen; DCS?

1 System description

1. 1 System working principle

The main function of the coal feeder system is to adjust the amount of coal required for combustion according to the power generation requirements. The coal feeder is mainly composed of a conveyor belt and a motor, and the coal falling from the coal bin to the coal feeder conveyor belt is transported to the coal mill through the conveyor belt. The principle of the system is shown in Figure 1. The system converts the weight signals measured by the two load sensors to obtain the actual coal weight, and compares the weights of the two channels to determine whether the load sensor is invalid. If there is no failure, the actual total weight is the sum of the two weights;

1. 2 System working mode

(1) The remote control mode is the main working mode of the coal feeder control system. The so-called "remote control" is relative to the DCS main control room, that is to say, the coal feeder receives the control signal from the DCS main control room in this working mode to realize coal quantity control. The control signals mainly include the coal feeder start-stop signal and the coal feed rate signal. When the operator selects the "remote control mode", the coal feeder realizes constant flow control according to the coal feed rate requested by the user.

(2) Stop method

There are two ways to stop the coal feeder: ① The main control room sends a signal to stop the coal feeder; ② Select the "stop mode" on the touch screen of the local control cabinet. The stop mode of the coal feeder is mainly to realize the work of "checking the belt scale" under this working mode. When the operator selects the "stop mode", the coal feeder will stop running first, and then the user can select functions such as "check the belt scale" to perform related operations.

(3) In-place method

The local mode is a working mode of adjusting the speed of the coal feeder locally. In this working mode, the system does not accept the control signal from the DCS main control room. When the operator selects "on-site mode", the coal feeder runs at the speed set on site. This mode is used as a way to verify whether the system speed conversion control function is normal.

2 Control system design

2. 1 Basic composition of the system

The control device is mainly composed of PLC, NT (industrial touch screen), VFD (Variable-frequency Drive), weight detection mechanism, coal feeder motor speed detection mechanism and various actuators, etc. The structure of the control system is shown in Figure 2.

Programmable controller PLC adopts CQM1H and CPU of Japan OMRON Company