Challenge: To build a custom real-time monitoring system at low cost and in a short time, capable of evaluating the performance and characteristics of grid-connected photovoltaic systems using a variety of measurement devices.
Solution: The monitoring system and dedicated interface software are designed using the open NI LabVIEW software platform. The outputs of multiple measurement devices are input to the PXI system through a serial interface, while the readily available User Datagram Protocol (UDP) functions are used to transmit the data to the PC, providing real-time display of different measurement results.
"The core of the entire system is the NI PXI-8184 real-time controller, which provides data storage capabilities, extremely high system reliability, compactness, robustness, and easy configurability."
In 2007, the Singapore government invested S$350 million to transform the city into a global clean energy hub, with a focus on solar energy development. To achieve this goal, the Singapore government and the Clean Energy Research and Experimentation Programme (CERT) under the Economic Development Board implemented several measures. To this end, we opened a solar technology centre featuring five different types of photovoltaic panels with a total power output of 14.2 kW. The centre showcases different grid-connected photovoltaic systems and serves as a testing ground for studying the performance and characteristics of different photovoltaic modules operating over long periods in tropical climates.
To study the performance and characteristics of these photovoltaic modules, we commissioned a team from the Department of Electrical Engineering to build a custom real-time monitoring system that could measure different parameters of the photovoltaic array under test and actual climatic conditions in a short time and at low cost.
The team used LabVIEW software for development because it offers an open development platform, versatility, and a unique graphical user interface. The software allowed them to quickly develop a non-standard Modbus serial communication protocol, a key interface for transmitting outputs from multiple measurement devices to the PXI real-time controller system. Subsequently, UDP communication functions were used to transmit the data to a PC for real-time display, analysis, and storage.
Photovoltaic system measuring instruments
The core of the entire system is the NI PXI-8184 real-time controller, which provides data storage capabilities and boasts extremely high system reliability, compactness, robustness, and easy configurability. Software development was based on LabVIEW and the LabVIEW Real-Time module. The PV measurement and monitoring system is shown in Figure 1.
Develop Modbus serial communication functionality
The weather monitoring system consists of seven measurement parameters: global solar irradiance, diffuse solar irradiance, temperature, humidity, wind speed, wind direction, and rainfall. The photovoltaic monitoring system measures DC voltage, DC current, and panel temperature. AC power supply and power generation are collected using electronic power meters. In addition, a total of 22 measurement data points are collected from the measuring instrument and five electronic power meters, for a total of 23 measurements. To handle a variety of measurement data and minimize cabling between the equipment and the PXI controller, a serial interface is used.
The team used Modbus Remote Terminal Units (RTUs), an open serial (RS232 or RS485) protocol that provides master/slave communication between network-connected devices. It is simple, reliable, low-cost, and capable of exchanging binary data, thus improving throughput. However, each Modbus device manufacturer implements this protocol with different function codes, data formats, and cyclic redundancy check (CRC) codes. Therefore, it was necessary to develop Modbus drivers that would allow users to modify the protocol implementation according to the requirements of different manufacturers.
Because LabVIEW provides Virtual Instrument Software Architecture (VISA) serial functions, designing programs for communication between these devices is straightforward. Furthermore, the availability of data manipulation functions, such as splitting numbers, right shifting with carry, swapping bytes, and type conversions, allows us to easily implement Modbus message structures using the corresponding function code, data formats, and CRC error verification algorithms. This enabled us to develop and test a Modbus RTU interface program that works with devices from different manufacturers.
Data transfer from PXI real-time controller to PC
The team used UDP functions in LabVIEW to transmit the acquired data to the PC. UDP is simple, convenient, and can broadcast messages to multiple addresses simultaneously. Although it does not guarantee reliable data transmission, this does not have any impact in this system, as the system only needs to transmit data to the PC at 10-second intervals, so even if some data points are lost, it will not cause any problems.
Design a graphical user interface
One of the system requirements is to provide comprehensive information displaying all measurement data from the monitoring system and the settings of the solar technology center. LabVIEW's tabbed controls offer users a practical way to place all this information in a single application without making the user interface overly complex.
Another challenge is displaying the different measurement data in charts. Charts allow users to visually see the relationship between one parameter and another, as well as the trend of any parameter's change. There are 22 measurement data points at any given time, including weather parameters, photovoltaic DC voltage and current, and panel temperature for each type of solar panel. Listing all the data on a single chart is impossible. This can be solved by controlling the chart drawing programmatically. The program is designed using item selection and chart attribute nodes. Users can view up to four data curves and measurement parameters at once in a single chart. Figure 2 shows a screenshot of the user interface.
in conclusion
LabVIEW helped our team quickly develop the Modbus interface program within three months and design an informative and user-friendly interface. The use of NI PXI hardware ensured system reliability; it has been running 24/7 since September 2008. Our team's next task is to evaluate the photovoltaic system's performance, including assessing the photovoltaic array efficiency and the relationship between energy generation and climatic and module conditions.
Figure 1: Schematic diagram of PV measurement and monitoring system
Figure 2: Screenshot of the user interface
Author information:
Chin Siet Choo
Ngee Ann Polytechnic
Singapore
