Wind power generation communication system based on rockwell automation PLC

1 Introduction

Wind power generation technology is developing rapidly, and the installed capacity is increasing. It has received widespread attention all over the world. Among the current variable speed constant frequency wind power systems, the doubly fed wind power system using a doubly fed induction generator (DFIG) has the largest market share, and the direct drive system using a permanent magnet synchronous generator (PMSG) has developed rapidly [1-2]. Whether it is a doubly fed or direct drive wind power system, its overall control is relatively complex, and a master control system is required to coordinate multiple links such as pitch, yaw, converter, measurement, protection and monitoring. In addition, the operating environment of the wind power system is usually relatively harsh, and there may be a certain distance between the various actuators, so the communication problem is crucial [3-4].

A programmable logic controller (PLC) is an electronic system designed for industrial environments. It uses programmable memory to store instructions for performing logic operations, sequential control, timing, counting, and arithmetic operations, and controls various types of production processes through digital and analog inputs and outputs. PLC has the advantages of simple programming, easy use, strong anti-interference ability, reliable operation in special environments, short fault repair time, easy maintenance, and strong interface functions [5]. Therefore, it is very suitable for use in wind power systems.

This paper first explains the structure of the wind power communication system, selects Rockwell Automation's Controllogix as the main control PLC, and implements a PLC-based wind power communication system; it implements upper-level monitoring of the communication system based on VC++, discusses the implementation principle of VC++, and gives the monitoring effect of the direct-drive wind power communication system based on Controllogix.

2. Wind power communication system structure description

The structure diagram of the direct-drive wind power generation system is shown in Figure 1, which includes a wind turbine, a permanent magnet synchronous generator, a back-to-back converter, a converter controller with DSP as the core, a wind power generation main control system with PLC as the core, and a host computer. The communication system is mainly composed of PLC and the host computer. PLC also communicates with the converter control DSP. The communication system realizes the monitoring of the direct-drive wind power system. The host computer and PLC use serial communication. PLC is used as a lower computer to complete control, data acquisition, and state judgment. The host computer is used to complete data analysis, calculation, information storage, state display, print output and other functions, so as to realize real-time monitoring of the wind power system.

As can be seen from Figure 1, the PLC must be connected to both the host computer and the converter control DSP. The converter in Figure 1 uses dual DSP control, and there are other variable pitch controllers, etc., which may involve multiple processors and need to be coordinated and controlled by the PLC. At the same time, it needs to be centrally monitored by the host computer in the central control room. Therefore, the role of the PLC-based wind power communication system is very important.

Figure 1 Structure diagram of direct drive wind power generation system

This paper uses Rockwell Automation's Controllogix as the main control PLC to conduct a preliminary exploration of the direct-drive wind power communication system. Controllogix is ​​a modular PLC of the AB series launched by Rockwell in 1998. It is one of the most competitive control systems in the world. Controllogix integrates sequential control, process control, transmission control and motion control, communication, and IO technology on a single platform, which can provide strong support for various industrial applications and is suitable for various occasions. The biggest feature is that it can be connected to each other using the network, and each control station can exchange information according to customer requirements. For Controllogix, Ethernet/ip and controlnet are more commonly used communication protocols when building a communication network. In addition, Controllogix also supports devicenet, DH+, RS232, DH485, etc., and the RS-232/DF1 port distributor expands the communication capabilities of the controller. Therefore, Controllogix is ​​more suitable for building a wind power communication system.

3 Communication system upper monitoring based on VC++

In order to more flexibly monitor the operation of the lower computer system and facilitate the debugging and expansion of the lower computer functions, this paper develops the upper computer software CMonitor based on VC++6.0 to match the direct-drive wind power communication system. It can provide a good user interface and multiple operation methods such as toolbars and menus, and cooperate with the vivid bitmap animation function to display the actual status and topology of the system in real time. It can control, modify and monitor the operation mode and operation parameters of the wind power system, complete the collection and analysis of historical data, and facilitate users to remotely monitor and debug the wind power system.

After initializing the serial communication module of the lower computer PLC, data can be sent and received through the SCIRX and SCITX of the PLC. Since the data received by the PLC may contain other data information in addition to the command word, different processing methods must be used for different types of command words. Define a variable cmd to save the current command word information.

struct {int ID; int counter;} cmd; ID is used to identify the current command word, and counter assists in recording the total number of data bytes processed under the current command word. The use of the variable cmd can effectively simplify the implementation process of the lower computer communication function and improve the stability of the communication function. As shown in Figure 2, in the SCI communication service function, the program enters different branches according to the value of cmd.ID. After the task of each command word is executed, cmd.ID is assigned to 0, so that it enters the 0x0 branch when idle, and continuously detects new instructions. The modification or expansion of the function only needs to modify the corresponding branch, which is easy to maintain.

Figure 2 Flowchart of the serial communication function of the lower computer

Figure 3 lists the processing flow of several typical command words, which are all part of the flowchart shown in Figure 2. In Figure 3 (a), cmd.ID is 0, indicating that there is no command word at present. The program will try to read data from the serial port, so that once there is a new command word, the program can immediately know it. Figure 3 (b) shows the processing flow when cmd.ID is 0x01, which indicates that the host computer is testing whether the communication is normal. If data can be sent to the serial port, the program will reassign cmd.ID to 0 after sending the data 0x01 indicating success; otherwise, the program returns, so that cmd.ID is not modified, and the interrupt function will still process the 0x01 command word when it runs next time. Figure 3 (c) shows the process of processing the 0x02 command word. According to the pre-defined instruction corresponding to 0x02, PWM output is disabled. When cmd.ID is 0x02, the corresponding register is modified to disable PWM output. After that, the value of cmd.ID is modified to 0x01 (the command word 0x01 will assign cmd.ID to 0 after execution) because a successful execution signal is to be sent to the host computer. In this way, the 0x01 command word branch will be directly entered when the communication service function is executed next time. The processing flow of command words 0x03, 0x04, and 0x05 is similar to Figure 3 (c).

Figure 3 Processing flow of several typical command words

The instruction corresponding to command word 0x06 is to modify the operating parameters of the system, including active current reference (2 bytes), reactive current reference (2 bytes), whether to use carrier phase shift and whether to use SVM (1 byte). Therefore, there are 5 additional data. The processing flow is shown in Figure 3 (d). The program first determines whether there is data to be read from the serial port. If so, the corresponding data is read and stored, and then the count value is increased by 1. After that, it is determined whether the count value has reached 5. If so, it means that the 5 additional data have been read. At this time, the corresponding variables in the lower computer program are updated according to the read data. Finally, cmd.ID is changed to 0x01, and a response signal of successful execution is sent to the upper computer. The instruction corresponding to command word 0x07 is to collect DC voltage. The processing flow is shown in Figure 3 (e). The program determines whether data can be sent to the serial port. If it can be sent, the low or high bit is determined according to the count value, and the count value is increased by 1. Then it is determined whether the count value is 2. If so, it means that the DC voltage has been sent. Then cmd.ID is assigned to 0, and the program returns.

Figure 3 (f) shows the processing flow of command word 0x12, and its corresponding instruction is to prohibit PLC from storing new data and receive stored data from PLC, with a total of 1600 bytes of data. The program first determines whether data can be sent to the serial port. If it can be sent, it sends the corresponding data according to the count value and adds 1 to the count value. Then it determines whether the count value reaches 1600. If it does, it assigns cmd.ID to 0 and enters the waiting stage for new instructions.

The processing flow of each command word in FIG3 is typical, and the processing flow of other command words in FIG2 can find a corresponding type in FIG3, so the implementation process becomes simple and intuitive, and the degree of modularization is high.

4. Achieve the effect

This section gives the interface graphics of the upper computer software CMonitor. The software has relatively complete functions and can be applied to the development, optimization and monitoring of lower computer programs, and its effectiveness has been confirmed through actual operation.

4.1 Startup and Login Interface

The startup and login interface of CMonitor is shown in Figure 4. The startup interface displays the software name (Converter Monitor, CMonitor), version (V1.0), and unit information (Institute of Electrical Engineering, Chinese Academy of Sciences), etc. After logging in, CMonitor automatically tests whether the communication is correct and detects the working status of MSI. Only when everything is normal can the various functions of the software be used to prevent possible misoperation of the lower computer, etc., thereby improving the security and stability of the system.

Figure 4 Cmonitor startup and login interface

4.2 Control Panel Interface

The control panel is the main panel for controlling the direct-drive wind power system, and mainly includes the following three parts.

(1) Topology control part. It shows the electrical connection of the system, including permanent magnet synchronous generator, motor side PWM converter, DC bus, grid side PWM converter, pulse switch, grid inductor, grid relay (3-Phase Breaker), three-phase grid, etc. Click the pulse switch position in the topology diagram to turn the pulse switch on or off.

Thus, the control pulse can be controlled; by clicking the grid-connected relay position in the topology diagram, the three-phase relay can be opened or closed to realize the grid connection and disconnection of the wind power converter. The graphics of the pulse switch and the grid-connected relay will change with the actual circuit, so the actual state of the system can be intuitively controlled and reflected.

(2) Parameter control part: The active current (Iq) and reactive current (Id) of the wind power system during operation can be modified to control whether the wind power system converter uses the SVM or SPWM modulation method.

(3) Log recording section. It displays all the user's operations on the current panel and gives the operation results. It can review the user's various operation steps, monitor the communication status of MSI, and provide reference for accident analysis.

4.3 Data Panel Interface

The function of the data panel is to collect data during system operation. It provides two collection modes: real-time data collection and historical data collection. Both can collect ten types of data, including DC voltage, grid A phase voltage, grid B phase voltage, grid C phase voltage, modulation wave A phase voltage, modulation wave B phase voltage, modulation wave C phase voltage, and inverter output A phase current, B phase current and C phase current.

In the data acquisition panel interface shown in Figure 5, the left side is the real-time data acquisition part. Clicking the corresponding acquisition button will complete the acquisition and display it; the right side is the historical data acquisition part. Clicking the human-shaped button under the upper right indicator light can collect historical data and draw the corresponding waveform. When the data acquisition process shown in Figure 5 (a) is completed, the virtual oscilloscope will display the collected waveform. The virtual oscilloscope interface of the data panel shown in Figure 5 (b) allows users to display, hide, move, and zoom in and out the waveforms of up to ten variables, which can be used to monitor program operation, verify program functions, and understand the working status of the program.

Figure 5 Data acquisition panel interface

4.4 Tool Panel Interface

The tool panel interface of CMonitor is shown in Figure 6. It can convert the waveform captured by the Tek oscilloscope waveform and set the color of each channel waveform of the virtual oscilloscope. The cover of the program displays information such as the author and copyright of the software. Its functional diagram is shown in Figures 6 (a) and (b). Figure 6 (a) shows an experimental waveform file opened by the tool panel. It can be seen that the background color of the figure is black, and each waveform is colored (with different degrees of brightness). The waveform processed by the tool panel is shown in Figure 6 (b). Comparing Figures 6 (a) and 6 (b), it can be seen that Figure 6 (a) keeps the waveforms of each channel the same as Figure 6 (b), but the background color has changed to white. The tool panel completes the function of converting the oscilloscope waveform into a standard experimental waveform, which facilitates the analysis of the waveform.

Figure 6 Tool panel interface

5 Conclusion

This paper implements a direct-drive wind power communication system based on Rockwell Automation PLC-Controllogix. It mainly studies the serial port communication between PLC and host computer, and builds host monitoring software based on VC++, which can effectively monitor the operation of the wind power system, display the operating status, record historical data and operations, draw test waveforms, and process experimental waveforms; it can provide a good human-computer interface, and through further optimization and improvement of functions, it can facilitate the debugging and monitoring of direct-drive wind power systems.