Application of Mitsubishi PLC in high voltage inverter control system

1. Introduction

In the design of domestic high-voltage inverters, in order to improve the flexibility of internal control of high-voltage inverters and the scalability of field applications, PLCs are usually built into high-voltage inverters. Since the birth of the first PLC in the 1970s, PLCs have been used more and more widely and their functions have become more and more complete. In addition to powerful logic control functions, they also have other extended functions: A/D and D/A conversion, PID closed-loop control, high-speed counting, communication networking, interrupt control and special function calculations, etc., and can be displayed, alarmed, recorded, and human-computer dialogue can be performed through the host computer, greatly improving its control level.

This article takes the domestic high-voltage inverter ZINVERT-H800/B10 designed and produced by Guangzhou Zhiguang Electric 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 inverter control system.

2. Introduction to Guangzhou Zhiguang Motor High Voltage Inverter

The new generation of high-performance ZINVERT series intelligent high-voltage variable frequency speed regulation system launched by Guangzhou Zhiguang Motor Co., Ltd. is a direct high-to-high variable frequency speed regulation system. It adjusts the motor speed by directly adjusting the power frequency and voltage connected to the stator winding of the high-voltage motor to achieve energy saving. It is a high-tech product integrating high-power power electronic control technology, microelectronics technology, high-speed optical fiber communication technology, automation control technology, and high-voltage technology. This product adopts the mainstream high-performance dedicated dual DSP control system and large-scale integrated circuit design, and realizes the flexible adjustment and energy consumption control of high-voltage motors through precise digital phase shifting technology and waveform control technology.

3. Design and application of PLC in domestic high-voltage inverters

3.1 PLC main logic control

(1) Users require that the high-voltage inverter can quickly and automatically switch to the power frequency bypass operation when a fault occurs and the inverter stops. The author specially configures a bypass cabinet that can realize the automatic bypass function for the high-voltage inverter, as shown in Figure 1. K1~K4 are manually operated knife switches, and J1~J3 are high-voltage vacuum contactors. When the inverter fails, the bypass cabinet can complete the conversion from variable frequency to power frequency within a few seconds; and when the inverter is running at power frequency, the inverter can be converted from power frequency to variable frequency with a single button. Such control requirements increase the complexity of the inverter control logic.

Figure 1 Automatic bypass cabinet

The control logic of the automatic bypass cabinet is briefly introduced as follows:

There are two ways for the variable frequency speed regulation system to exit variable frequency and switch to industrial frequency operation, one is automatic and the other is manual. When the automatic mode is selected, the frequency converter automatically switches from variable frequency to industrial frequency when a shutdown fault occurs; when the manual mode is selected, manual operation is required.

There are also two ways for the frequency conversion system to exit the power frequency and switch to the frequency conversion operation, one is automatic mode and the other is manual mode. When the automatic mode is selected, just press a button on the control cabinet and the frequency converter will automatically complete the frequency conversion from the power frequency; when the manual mode is selected, manual operation is required.

(2) PLC control system schematic diagram

The PLC host uses 48 input and output points, model FX2N-48MR. As the control core of the system logic control, PLC plays a vital 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 the bypass cabinet are relatively complex. Using PLC control simplifies wiring and improves reliability. The logic change of the bypass cabinet also becomes very simple. You only need to modify the PLC ladder diagram program, which is very convenient to meet the user's on-site control requirements.

(3) PLC function instructions to achieve high-voltage inverter PID closed-loop control

The user's on-site requirements for the inverter closed-loop control are: the inverter can 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 can also be set on the LCD screen.

In response to user requirements, the PLC is also equipped with analog special modules FX2N-4AD and FX2N-2DA. FX2N-4AD is an analog input module with four input channels, a maximum resolution of 12 bits, and an analog value input range of -10V to 10V or 4 to 20mA; FX2N-2DA is an analog output module with 2 output channels, a maximum resolution of 12 bits, and an analog value output range of -10V to 10V or 4 to 20 mA. In this way, through the read instruction (FROM) and the write instruction (TO), as well as the PID closed-loop control function instruction of the PLC (as shown in Figure 3), PID closed-loop control of the water pressure of the pipe network at the user's site 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 through the instruction (FROM), and stores the feedback value into the D12 data address in the PID instruction through the move instruction (MOV); stores the user's water pressure setting value into the D10 data address in the PID instruction through the move instruction (MOV); D200~D222 save the PID operating parameters; D14 is the operation value output of the PID instruction, and the PID closed-loop operation result D14 is written into the analog output module through the PLC write instruction (TO), and then converted into an analog signal of -10V~10V or 4~20mA through the analog output module and sent to the high-voltage inverter controller for frequency setting.

When setting PID operation parameters, the parameter settings of P, I, and D are particularly important, and the quality of their settings is directly related to the quality of pipe network water pressure control. P represents proportional gain, and the setting range is 0~99(%). If the proportional adjustment is set large, when the system deviates, the adjustment can be accelerated to reduce the error, but too large a proportional gain will cause system instability; I represents integral time, and the setting range is 0~32767(*100ms). The smaller the integral time, the stronger the integral effect, and vice versa, the larger I is, the weaker the integral effect; D represents differential time, and the setting range is 0~32767(*10ms). Differential adjustment has an advanced control effect, and a suitable differential time can improve the dynamic performance of the system.

The water supply network of Panzhihua Iron and Steel Wastewater Treatment Plant is relatively large, and the water pressure in the network responds slowly to changes in the speed of the water pump. Therefore, the PID calculation speed cannot be too fast, that is, the proportional adjustment cannot be too fast. Otherwise, if the water pressure in the network changes suddenly, the adjustment of the inverter is likely to form a long-term oscillation. According to this situation, as shown in Figure 3, the PID interval calculation time (T0) and the PID calculation dead zone (M0) can be added to the PLC control program, so that the PID calculation speed can be adjusted to be consistent with the speed of change of the water pressure in the network, avoiding the oscillation of the water pressure in the network.

(4) PLC function instructions realize communication between PLC and inverter host computer

In order to enable the inverter host computer to display, alarm and record the PLC, the PLC is also equipped with a communication module FX2N-232BD to achieve serial port communication with the inverter host computer. The communication programming instructions are shown in Figure 4.

Figure 4 Communication programming instructions

PLC RS232 serial communication can use no protocol (RS instruction) or special protocol to communicate with the host computer. 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 indicates the sending start address, K60 indicates the number of bytes to be sent, D150 indicates the receiving start address, and K20 indicates the number of bytes to be received.

4. Conclusion

The high-voltage inverter automatic bypass cabinet uses PLC for bypass logic control. Through the simulated fault of the Zhiguang high-voltage inverter running in the Panzhihua Sewage Treatment Plant, it is very convenient for the high-voltage inverter automatic bypass cabinet to switch from variable frequency to industrial frequency and from industrial frequency to variable frequency, which can be completed within 10 seconds, greatly improving the reliability of the water pump operation. The on-site PID closed-loop control effect is very ideal, and the water pressure fluctuation is very small. When the fluctuation exceeds 0.1kg, the inverter can quickly adjust the speed and control the water pressure within the set range. No oscillation will be generated when adjusting the speed. At the same time, through the serial port RS-232 communication between PLC and the high-voltage inverter controller, the system pipe network water pressure and various PLC status quantities can be monitored on the high-voltage inverter LCD screen.