Summary: A distributed photovoltaic power station data communication management unit has been designed, which includes both hardware and software systems. The hardware system is based on an ARM embedded processor, collecting and parsing real-time data from photovoltaic power station terminal equipment via RS485/232 serial ports. It connects to the server of the photovoltaic power station control system via Ethernet/GPRS to enable remote wireless communication and data transmission. The software system is based on a multitasking real-time Linux operating system, utilizing a modular design and including function modules such as task management, memory management, time management, and synchronous communication. This communication management unit is capable of real-time collection, storage, analysis, upload, and forwarding of monitoring data for distributed photovoltaic power stations.
Keywords: Distributed Photovoltaic Power Station; Data Management Unit; Embedded System; Remote Wireless; Communication and Transmission
The PV monitoring system can perform real-time monitoring and control of various equipment in solar photovoltaic power stations, such as battery arrays, combiner boxes, inverters, DC/AC distribution cabinets, and meters. It collects, stores, analyzes, manages, and transmits data related to environmental parameters, equipment information, and power parameters associated with the power station's operation efficiency to ensure the safe, reliable, and stable operation of the photovoltaic power generation system and grid metering. It is a component of modern photovoltaic power station systems. The PV power station monitoring system has evolved from early wired data collection to present-day wireless real-time monitoring, with data storage transitioning from storage cards to network data transmission and cloud storage. In recent years, with the extensive construction and grid connection of industrial rooftop and residential rooftop power stations, the development of data monitoring systems for distributed photovoltaic power stations has received attention. In distributed PV monitoring systems, the PV data communication management unit is responsible for collecting all relevant data from the PV power station operations and remotely transmitting it to the PV power station monitoring center via Ethernet/GPRS, allowing users and managers of PV power stations to monitor the operation status in real-time. It serves as the bridge between the PV power station monitoring center and the PV power station equipment. This article describes the design and development of a distributed PV power station data communication management unit based on an ARM embedded processor, which performs real-time collection, analysis and processing, storage, and remote transmission of operation data for distributed PV power stations, enabling remote management and maintenance of the distributed PV power stations.
System Hardware Design
The system hardware of the distributed photovoltaic power plant data communication management unit is based on the SAM9X25 embedded microprocessor. It collects and parses real-time data from terminal devices such as inverters, meteorological environment monitors, concentrators, and meters through RS485/232 serial ports. It connects to the server via GPRS/Ethernet to transmit data to the remote data center. Administrators can configure and manage the photovoltaic data management unit by logging into the WEB interface. Users can view the real-time operation status and historical data of the power station by logging in to the WEB terminal or mobile terminal. The hardware structure is shown in Figure 1.
Figure 1: Hardware Structure Diagram of the Communication Management Unit
1. Serial Port Circuit
The communication management unit employs UART (Universal Asynchronous Receiver Transmitter) and USART (Universal Synchronous Asynchronous Receiver Transmitter) interfaces to convert RS-485/RS-422 serial ports for remote data collection. Figure 2 illustrates the circuit structure of this serial port.
Figure 2 Serial Port Circuit Diagram
The serial data transmission employs balanced transmission and differential reception methods. The UART and USART interfaces of the ARM chip are connected to the four-channel isolator ADUM1400, separating the data transmission signals and the transmitter/receiver control signals. This supports multiple channel configurations and data transmission rates, ensuring system safety and stability. After passing through the isolator, the data transmission signals RX and TX are directly connected to the RS484/Rs422/Rs232 transceivers. The control signal EN for the transceivers must pass through a Schmidt trigger before connecting to the RS484/Rs422/Rs232 transceivers to control the operating mode of the transceivers. The Schmidt trigger converts slowly changing input signals into clear, stable square waves, enhancing the system's anti-interference capability. The RS484/Rs422/Rs232 transceivers communicate with the equipment under the control of end signals.
1. 2 SD card storage circuits
The communication management unit includes an SD card slot, as shown in Figure 3 for its circuit structure. The SD card is physically connected to the HSMCI (High-Speed Multimedia Card Interface) module of the ARM chip via the PIO interface. HSMC1 supports the reading and writing of streams, blocks, and multi-block data, while also being controlled by the DMAC (Direct Memory Access Controller), greatly enhancing the processor's data transmission efficiency. The SD card supports three transmission modes: SPI mode, 1-bit mode, and 4-bit mode, with this system utilizing the 4-bit mode.
Figure 3: SD Card Storage Circuit Structure Diagram
1. 3 Ethernet port circuit
The communication management unit features two Ethernet ports, as illustrated in Figure 4, which depicts its circuit structure. The Ethernet connects to the shared transmission medium via RJ-45 interfaces for differential data transmission. Filters convert the voltage at the RJ-45 network interfaces to the voltage required by the network card chip DM9161, while filtering out other interfering signals to reduce the error rate in information transmission. The physical layer interface of the network card chip receives the output signal from the filter, converts it to MII (Media Independent Interface) signals, and then sends the MII signals to the ARM controller's EMAC (Ethernet Media Access Controller 10/100) module via the data interface. This system utilizes standard network cables, where both ends have the same twisted pair wiring sequence with a RJ-45 connector, for connecting terminal devices to HUBs or LAN switches.
Figure 4: Ethernet Port Circuit Structure Diagram
2 System Software Design
The communication management machine software system employs a modular design based on a multi-tasking real-time system, encompassing functional modules such as task management, task scheduling, memory management, time management, and inter-task communication and synchronization. The system extends the operating system by designing driver modules, utilizing the API functions of the operating system, system tasks, and task scheduling modules, all based on the task kernel provided by Linux. The program is divided into multiple task modules according to the correlation of various functions, including the main program, data collection, data transmission, and data storage.
The system's main program must complete the initialization settings for the ARM chip and peripherals, the Linux system, and the semaphore, then start the system. Additionally, the main program needs to initiate task threads for driver scheduling, network communication, and the database. The flowchart of the main program is shown in Figure 5.
2.1 Data Collection
The data collection task involves gathering data from various terminal devices. As shown in Figure 6, the program reads variable information, then initiates a serial communication thread and calls the device driver program to initialize the interface. At this point, the collection task sends a command to read the "collection data" interface data to the data bus. All sensors connected to the data bus receive the read command and determine if it's intended for them based on the data packets. If so, they return the device parameter data to the system until the data transmission is complete.
Figure 5: Main Program Flowchart
Figure 6: Data Collection Task Process Flowchart
2.2 Data Transmission
Data transmission encompasses two main parts: data sending and data receiving. The data sending section primarily transfers data from the backend database to the frontend WEB page, while the data receiving section mainly receives and parses data uploaded from the network. The data transmission flowchart is shown in Figure 7.
2.3 Data Storage
Data storage primarily handles the storage of uploaded data. As shown in Figure 8, upon receiving a communication interruption message from the network communication layer, the program reads information on all variables from the memory database and selects different storage solutions based on the data types. First, it determines the data type in the memory database. If the data is a state variable (O or 1) that changes, the current data is written to the historical data storage cache queue. Subsequently, if the data is an analog quantity, it is stored according to the required storage method. If periodic storage is chosen, it cyclically checks if the set period time has been reached; upon reaching the period, the data is written to the historical data storage cache queue. If precision storage is selected, the current data is judged using the rotating gate algorithm, and data that meets the criteria is written to the historical data storage cache queue.
Figure 7: Data Transmission Task Process Flow Diagram
Figure 8: Data Storage Process Diagram
3 Applications
The Chitic Distributed Photovoltaic Power Station Data Communication Management Unit has been designed and developed based on the aforementioned design scheme. The local WEB configuration interface of this management unit includes seven major functions: data view, serial port configuration, channel configuration, network settings, transmission configuration, services, and system maintenance. Each functional module also contains several sub-functions, as shown in Figure 9.
Under the Data View, the power station can view the current power parameters, environmental parameters, and energy-saving parameters; under the Equipment Information section, the number of equipment at the power station and the operating status of each device can be viewed.
Figure 9: Chitic Distributed Photovoltaic Power Plant Data Communication Management Machine WEB Configuration Interface
Serial port configuration allows for setting various parameters of the serial port, enabling data collection and local viewing functions.
Channel configuration allows for setting up device channel information, connecting each device in a tree structure. Each serial port is a channel, with a total of 4 channels available. Up to 16 devices can be connected under one channel, and the system collects device data centrally. Network configuration supports IP settings, with data upload options including NET, GPRS, and WIFI. Transmission configuration allows setting the remote data center IP, and the system uploads power station data to the data center IP based on the configured upload method. Service item view displays supported protocol types and partial functions of ModBus register retention. System maintenance is used to check system version and serial number, and to perform system upgrades, time synchronization, and device restarts.
AnkoRui Gateway Introduction
4.1 Communication Management Unit
4.1.1 Overview
This series of intelligent communication management units employs an embedded hardware computer platform, featuring multiple downstream communication interfaces and one or more upstream network interfaces. It is designed to collate and aggregate the communication data from all intelligent monitoring/protection devices within a target area, then upload it to the main station system in real-time, completing functions such as remote signaling and remote measurement for energy data collection.
Additionally, this series of intelligent communication management units supports receiving commands from the superior master station system and forwarding them to intelligent series units within the target area. It completes remote control of the opening and closing of switchgear equipment or parameter setting of devices within the power station, realizing remote control and remote adjustment functions to achieve the goal of remote dispatching commands.
4.1.2 Product Introduction
4.2 Data Transformation Module
4.2.1 Overview
AF-GSM is a new 4G remote wireless data collection device launched by Anke Electrical, featuring an embedded design. It incorporates a TCP/IP protocol stack and utilizes a powerful microprocessor chip, coupled with an integrated watchdog for reliable and stable performance.
This product offers a standard RS485 data interface, enabling easy connection to devices such as RTUs, PLCs, and industrial computers. With a one-time initial configuration, it can complete data collection from MODBUS devices and communicate with AnkeRui servers.
5.2.2 Product Introduction
4.3 Wireless Communication Terminal
4.3.1 Overview
The AWT100 Data Conversion Module is a new data conversion DTU launched by Ankerui Electric. It supports communication data conversion via 2G, 4G, NB, LoRa, LoRaWAN, GPS, WiFi, CE, DP, and other communication methods. The downlink interface provides a standard RS485 data interface, allowing for easy connection to power meters, RTUs, PLCs, industrial computers, and other equipment. With a one-time initialization and configuration, it can complete data collection for MODBUS devices. Additionally, the AWT100 series wireless communication terminals utilize powerful microprocessor chips, combined with built-in watchdog technology, ensuring reliable and stable performance.
The AWT200 Data Communication Gateway is applied to data collection and analysis for various terminal devices. It facilitates equipment monitoring, control, and computation, establishing a communication link between systems and devices for bidirectional data communication. It monitors in real-time, detects anomalies promptly, and makes logical judgments based on user-defined rules, significantly reducing labor and communication costs.
4.4.2 Product Introduction
5 Conclusion
A distributed photovoltaic power station data communication and management unit has been designed, with its hardware system based on ARM embedded processors. It collects and processes real-time data from photovoltaic power station terminal equipment via RS485/232 serial ports, and connects to the server of the power station control system using Ethernet/GPRS, achieving remote wireless communication and data transmission. The software system is based on a multi-tasking real-time Linux operating system and employs a modular design, including modules for task management, memory management, time management, and synchronous communication. This communication management unit features real-time collection, storage, analysis, upload, and forwarding of monitoring data for distributed photovoltaic power stations. The system utilizes WEB pages for viewing and configuring photovoltaic power station data, supports multiple communication protocols, offers breakpoint resuming, and includes functions such as data collection, parsing, historical data storage, synchronous upload, and local forwarding. Additionally, the system boasts low cost, low power consumption, large data storage capacity, fast data processing speed, and the ability to perform real-time multi-tasking operations, making it an essential component of the distributed photovoltaic power station monitoring system.
Reference
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Yang Qikai. Design and Implementation of a Real-time Remote Monitoring System for Photovoltaic Power Generation [D]. Chongqing: Chongqing University, 2013
