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

Abstract: This paper introduces the current status of the coal feeder control system in a thermal power plant, and elaborates on the working principle and method of the modified system, the composition of the control system, the design ideas of the PLC control program, the design of the touch screen and its parameter settings, and the test It proves that the system operation after the transformation has met the predetermined requirements of economy, efficiency and stability.

Keywords: frequency conversion speed regulation; PLC; touch screen; DCS

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 rinciple 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. The coal falling from the coal bunker onto the coal feeder conveyor belt is transported to the coal mill through the conveyor belt. The system principle is shown in Figure 1. The system converts the weight signals measured by the two weighing sensors into the actual coal weight, and compares the weights of the two channels to determine whether the load sensor has failed. If there is no failure, the actual total weight is the sum of the two weights; if it fails, the actual weight is equal to The product of density and volume, the product of actual weight and conveyor belt speed is the actual coal feeding rate. Then according to the deviation between the requested coal feeding rate and the actual coal feeding rate, conventional PID control is used to adjust the coal feeder motor speed, and finally realize the system coal feeding rate. quantity regulation, and also achieves power generation regulation.

1.2 How the system works

(1) 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, which means that the coal feeder accepts control signals from the DCS main control room in this working mode. To achieve coal quantity control, the control signals mainly include the start and stop signals of the coal feeder and the coal feed rate request signal. When the operator selects "remote control mode", the coal feeder realizes constant flow control according to the coal feeding rate requested by the user.

(2) Stop method

There are two methods to stop the coal feeder: ① The main control room sends a coal feeder stop signal; ② Select "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" in this working mode. When the operator selects "stop mode", the coal feeder stops running first, and then the user can select functions such as "check belt scale" to perform related operations.

(3) Local method

The local mode is a working mode that adjusts the coal feeder speed locally. In this working mode, the system does not accept control signals from the DCS main control room. When the operator selects "local mode", the coal feeder runs at the speed set on site. This method is used as a way to verify whether the system speed variable frequency control function is normal.

2. Control system design

2.1 Basic components 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. The control system structure is shown in Figure 2.

The programmable controller PLC adopts CQM1H CPU of Japan OMRON Company