High-voltage electrostatic dust removal control system solution based on PLC and touch screen (Part 2)

2.1.6 Thyristor trigger circuit

　　The dust removal control system controls the power supply by changing the trigger angle of the thyristor. The increase or decrease of the trigger angle is changed by the trigger pulse, as shown in Figure 8. The trigger pulse circuit is mainly composed of a pulse amplifier and a transformer. When the PLC generates a zero-crossing interrupt, it starts the timer T0 (T0 is the initial value, corresponding to the conduction angle). T0 overflow will cause an interrupt. At this time, the PLC outputs a trigger pulse to control the conduction of the transistor. The primary side of the pulse transformer will generate a trigger pulse that is added to the G and K poles of the thyristor respectively. When the transistor is cut off, the trigger pulse will be blocked.

　　
Figure 8 SCR trigger circuit

　　2.1.7 Data Acquisition Circuit

　　The analog quantities that need to be detected mainly include the average value of the primary voltage and current, the average value of the secondary voltage and current, and the peak value of the secondary current. The values ​​of these analog quantities are generally large.

　　On-site sampling is required before entering the control system.

　　Sampling of primary voltage: The primary voltage is stepped down through a small transformer.

　　Sampling of primary current: conversion by current transformer connected in series in the primary circuit.

　　Sampling of secondary voltage: voltage division of anode high voltage circuit in transformer rectifier.

　　Sampling of secondary current: A sampling resistor is connected in parallel to the ground end of the anode-secondary circuit in the transformer-rectifier and then converted.

　　
Figure 9 Secondary voltage acquisition circuit

　　The following is an explanation using the secondary voltage as an example. The sampling circuit is shown in FIG9 . The secondary voltage collected on site is divided, passed through a proportional adjustment circuit composed of a sliding rheostat VR4, and finally detected by a Hall voltage sensor.

　　2.2 Software Design

　　2.2.1 Main flow chart of the program

　　As shown in Figure 10, after the control system is powered on, the system is initialized first. If the reset button is pressed, it will restart. If the start button is pressed, the parameter setting stage will be carried out. The CPU will process the collected parameters. If sparks appear, it will enter the spark processing subroutine. If a parameter exceeds the limit, it will enter the alarm and fault processing subroutine. Finally, the system will rescan the data and start a new cycle.

　　
Figure 10 Main flow chart of the program

2.2.2 Touch screen configuration

　　The configuration of the touch screen is mainly the configuration of the screen. Designing a friendly and convenient touch screen interface can not only increase the convenience of operation, but also reduce the error rate of operation, so that the machine and equipment can play the maximum function. The screen includes the main screen, parameter setting, display screen, trend curve and help. The detailed pictures of the display screen and parameter setting screen are omitted.

　　In the parameter setting screen, click the input field on the screen, and a numeric keyboard will appear for input. The KTP touch screen can ensure that when a fault occurs, an alarm will be displayed on the current page for immediate processing. There are two types of alarms: errors and warnings. Warnings do not require confirmation and do not cause direct harm to the equipment; errors require operational confirmation and are harmful to the equipment. The equipment will trip and shut down. Warnings include primary voltage overlimit, primary current overlimit, secondary voltage overlimit, etc. Errors include secondary overcurrent fault, secondary overvoltage fault, oil temperature overlimit fault, biased excitation, etc.

　　3. Implementation of remote control

　　With the help of modern network interconnection technology, the controllers of each electric field of the dust collector can exchange data with the host computer monitoring software through Ethernet. The host computer can realize remote start, stop, boost, buck control and parameter adjustment of the dust removal equipment. The operating parameters and operating status of each device of the electrostatic precipitator can also be intuitively displayed on the host computer screen, so that managers can realize remote control as if they were on site.

　　The upper computer monitoring software uses Siemens' WinCC configuration software, which has operation screen, monitoring screen, control screen, alarm screen, real-time trend curve, historical trend curve, and can print reports.

　　The communication between WinCC and S7-1200 CPU is realized through Ethernet and can only be realized through OPC. S7-1200 CPU only needs to set the IP address. The host computer establishes PC Starion through SIMATIC NET software to communicate with S7-1200 CPU. S7-1200 CPU has an integrated PROFINET port that supports Ethernet and TCP/IP-based communication standards. The remote control of the three-field dust collector is shown in Figure 11.

　　
Figure 11 Schematic diagram of three-field remote control

　　4 Conclusion

　　The system not only has good automatic voltage regulation and remote control capabilities, but also simplifies on-site operations through PLC and touch screen, improves the flexibility of control programs and human-machine interfaces, and is easy to maintain. Although the initial investment in the equipment is high, the operating cost is acceptable in the long run.