1. Introduction
Since the birth of the first PLC in the 1970s, the application of PLC has become more and more widespread and its functions have become more and more complete. In addition to powerful logic control functions, it also has other extended functions: A/D and D/A conversion, PID closed-loop control, high-speed counting, communication networking, interrupt control and special function operation, etc. It can also be displayed, alarmed, recorded and human-machine dialogue through a host computer, which greatly improves its control level.
This article takes the ZINVERT-H800/B10 domestic high-voltage frequency converter designed and manufactured by Guangzhou Zhiguang Motor Co., Ltd. for the sewage treatment plant of Panzhihua Iron and Steel Group Chengdu Iron and Steel Co., Ltd. as an example to introduce the application of Mitsubishi PLC in the high-voltage frequency converter control system.
2. Introduction to Guangzhou Zhiguang Motor High Voltage Frequency Converter
Guangzhou Zhiguang Motor Co., Ltd. has launched a new generation of high-performance ZINVERT series intelligent high-voltage variable frequency speed control system, a direct high-to-high voltage variable frequency speed control system. It achieves energy saving by directly adjusting the frequency and voltage of the power supply connected to the stator windings of the high-voltage motor, thereby regulating the motor speed. It is a high-tech product integrating high-power power electronic control technology, microelectronics technology, high-speed fiber optic communication technology, automation control technology, and high-voltage technology. The product adopts a mainstream high-performance dedicated dual-DSP control system and large-scale integrated circuit design, achieving flexible adjustment and energy consumption control of the high-voltage motor through precise digital phase-shifting technology and waveform control technology.
3. Design and application of PLC in domestic high-voltage frequency converters
3.1 Main Logic Control of PLC
(1) The user requires that the high-voltage frequency converter can quickly and automatically switch to power frequency bypass operation when a fault occurs and the unit stops. Therefore, the author has specially configured a bypass cabinet with automatic bypass function for the high-voltage frequency converter, as shown in Figure 1. K1～K4 are manually operated knife switches, and J1～J3 are high-voltage vacuum contactors. When the frequency converter fails, the bypass cabinet can complete the switch from frequency conversion to power frequency within a few seconds; while when the frequency converter is running at power frequency, the switch from power frequency to frequency conversion can be achieved with a single button. This control requirement increases the complexity of the overall control logic of the frequency converter.
Figure 1 Automatic bypass cabinet
The control logic of the automatic bypass cabinet is briefly described below:
There are two ways for a variable frequency speed control system to switch from variable frequency to mains frequency operation: automatic mode and manual mode. When automatic mode is selected, the variable frequency drive will automatically switch from variable frequency to mains frequency when a shutdown fault occurs. When manual mode is selected, manual operation is required.
There are two ways for a variable frequency speed control system to switch from power frequency to frequency conversion: automatic and manual. In automatic mode, simply press a button on the control cabinet and the inverter will automatically switch from power frequency to frequency. In manual mode, manual operation is required.
(2) PLC control system schematic diagram
The PLC host selected has 48 input/output points and is model FX2N-48MR. As the control core of the system's logic quantity control, the PLC plays a crucial role in the logic relationship control of the automatic bypass cabinet. The schematic diagram of the PLC control system is shown in Figure 2.
Figure 2 Schematic diagram of PLC control system
The logic control requirements of bypass cabinets are relatively complex. Using PLC control simplifies wiring and improves reliability. Changing the logic of bypass cabinets is also very simple, requiring only modification of the PLC ladder diagram program, which easily meets the user's on-site control requirements.
(3) PLC function instructions realize PID closed-loop control of high voltage frequency converter
The user's requirements for the closed-loop control of the frequency converter are: the frequency converter should be able to automatically adjust the speed of the variable frequency pump according to the changes in the user's system water consumption to achieve constant pressure water supply in the pipeline network; at the same time, the pressure target value should be set on the LCD screen.
To meet user requirements, the PLC is additionally configured with analog special modules FX2N-4AD and FX2N-2DA. The FX2N-4AD is an analog input module with four input channels, a maximum resolution of 12 bits, and an analog input range of -10V to 10V or 4 to 20mA. The FX2N-2DA is an analog output module with two output channels, a maximum resolution of 12 bits, and an analog output range of -10V to 10V or 4 to 20 mA. Thus, by using read instructions (FROM) and write instructions (TO), as well as the PLC's built-in PID closed-loop control function instructions (as shown in Figure 3), PID closed-loop control of the water pressure in the user's on-site pipeline can be achieved.
Figure 3. PLC program with PID closed-loop control function instructions.
The specific programming process is as follows: The PLC reads the user's water pressure feedback value using the FROM instruction and stores the feedback value in the D12 data address of the PID instruction using the MOV instruction; the user's water pressure setpoint is stored in the D10 data address of the PID instruction using the MOV instruction; D200 to D222 store the PID's operating parameters; D14 is the output of the PID instruction's calculation value. The PLC writes the PID closed-loop calculation result D14 to the analog output module using the TO instruction, and then converts it into an analog signal of -10V to 10V or 4 to 20mA through the analog output module and sends it to the high-voltage frequency converter controller for frequency setting.
When setting PID operating parameters, the settings of P, I, and D parameters are particularly important, as their quality directly affects the effectiveness of pipeline water pressure control. P represents proportional gain, with a setting range of 0–99%. A larger proportional gain setting can accelerate adjustment and reduce errors when system deviations occur; however, an excessively large proportional gain can cause system instability. I represents integral time, with a setting range of 0–32767 (*100ms). A smaller integral time results in a stronger integral action, and vice versa. D represents derivative time, with a setting range of 0–32767 (*10ms). Derivative adjustment has a leading control effect, and an appropriate derivative time can improve the dynamic performance of the system.
The water supply network of the Panzhihua Iron and Steel Group's wastewater treatment plant is quite extensive, and the water pressure in the network responds slowly to changes in pump speed. Therefore, the PID calculation speed cannot be too fast, meaning the proportional regulation cannot be too rapid. Otherwise, if the network water pressure changes suddenly and significantly, the frequency converter's adjustment may experience prolonged oscillations. Based on this situation, as shown in Figure 3, a PID interval calculation time (T0) and a PID dead zone (M0) can be added to the PLC control program. This allows the PID calculation speed to be adjusted to match the rate of change in network water pressure, preventing network water pressure oscillations.
(4) PLC function instructions enable communication between the PLC and the inverter's host computer.
In order to enable the host computer of the frequency converter to display, alarm and record the PLC, the PLC is also equipped with a communication module FX2N-232BD to realize serial communication with the host computer of the frequency converter. The communication programming instructions are shown in Figure 4.
Figure 4 Communication Programming Instructions
PLC RS232 serial communication can communicate with the host computer using either no protocol (RS instruction) or a dedicated protocol. In this example, no protocol is used to communicate with the host computer, as shown in Figure 4: D8120 is used to set the PLC communication format, D50 represents the send start address, K60 represents the number of bytes to send, D150 represents the receive start address, and K20 represents the number of bytes to receive.
4. Conclusion
The high-voltage frequency converter automatic bypass cabinet uses a PLC for bypass logic control. Through simulated fault demonstrations using the Zhiguang high-voltage frequency converter operating at the Panzhihua Iron and Steel Group wastewater treatment plant, it was shown that the automatic bypass cabinet's switching between frequency conversion and power frequency is very convenient, completing within 10 seconds, significantly improving the reliability of the water pump operation. The on-site PID closed-loop control effect is excellent, with very small water pressure fluctuations. When fluctuations exceed 0.1 kg, the frequency converter can quickly adjust the speed to control the water pressure within the set range, without any oscillation during speed adjustment. Simultaneously, through RS-232 serial communication between the PLC and the high-voltage frequency converter controller, the system network water pressure and various PLC status variables can be monitored on the high-voltage frequency converter's LCD screen.
