Challenge: Build a custom real-time monitoring system in a short time at a low cost, using a variety of measurement equipment to evaluate the performance and characteristics of grid-connected PV systems.
Solution: Design the monitoring system and dedicated interface software using the open NI LabVIEW software platform, input the output of multiple measurement devices to the PXI system through the serial interface, and transmit the data using the off-the-shelf User Datagram Protocol (UDP) function. Go to the PC and provide real-time display of different measurement results.
"The heart of the entire device is the NI PXI-8184 real-time controller, which provides data storage, high system reliability, compactness, ruggedness, and ease of configurability."
In 2007, the Singapore government invested 350 million Singapore dollars to transform the city into a global clean energy hub, with a focus on developing solar energy. To achieve this goal, the Singapore Government and the Clean Energy Research and Experiments Program (CERT) under the Economic Development Board have developed a number of measures. To this end, we have opened a solar technology center with five different photovoltaic panels, with a total power of 14.2 kW. The center is used to display different grid-connected photovoltaic systems and serves as a test laboratory for studying the performance and characteristics of long-term operation of different photovoltaic modules under tropical climate conditions.
In order to study the performance and characteristics of these photovoltaic modules, we asked a team from the electronic engineering department to build a custom real-time monitoring system capable of measuring different parameters and actual climatic conditions of the PV array to be tested at low cost in a short time.
The team uses LabVIEW software for development because it provides an open development platform, versatility, and an original graphical user interface. The software allows them to quickly develop non-standard Modbus serial communication protocols, which are key interfaces for delivering multiple measurement device outputs to a PXI real-time controller system. The UDP communication function is then used to transfer the data to the PC for real-time display, analysis, and storage.
Photovoltaic system measuring instrument
At the heart of the entire plant is the NI PXI-8184 real-time controller, which provides data storage, high system reliability, compactness, ruggedness and ease of configurability. Software development is based on LabVIEW and LabVIEW Real-Time Modules. The PV measurement and monitoring system is shown in Figure 1.
Develop Modbus serial communication function
The weather monitoring system consists of seven measurement parameters, including global solar irradiance, scattered solar irradiance, temperature, humidity, wind speed, wind direction and rainfall. The measurement parameters of the PV monitoring system include DC voltage, DC current and panel temperature. AC power and power generation takes place using an electronic power meter. In addition, a total of 22 measurement data points were collected from the measuring instrument and five electronic power meters for a total of 23 measurements. In order to be able to process multiple measurement data and minimize the connection between the device and the PXI controller, we use a serial interface.
The team uses the Modbus Remote Terminal Unit (RTU), an open serial (RS232 or RS485) protocol that provides master/slave communication between devices connected via a network. It is simple, reliable, low-cost, and capable of exchanging data in binary format for increased throughput. However, each Modubs device manufacturer implements this protocol with different function codes, data formats, and cyclic redundancy verification (CRC) code. Therefore, it is necessary to develop a Modbus driver to allow the user to modify the implementation of the protocol according to the requirements of different manufacturers.
Because LabVIEW provides a Virtual Instrument Software Architecture (VISA) serial function, designing programs to communicate between these devices is straightforward. In addition, because of the data manipulation functions, such as splitting digits, carry-shift right shift, swap byte, type conversion, etc., we can conveniently implement the Modbus message structure using the corresponding function code, data format and CRC error verification algorithm. In this way we have developed and tested the Modbus RTU interface program and are able to work with devices from different manufacturers.
Data transfer from PXI real-time controller to PC
The team uses the UDP function in LabVIEW to transfer the acquired data to the PC. UDP is simple, convenient, and capable of broadcasting messages to multiple addresses simultaneously. Although it does not guarantee reliable data transfer, it does not affect this system because the system only needs to transfer data to the PC at 10 s 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 a comprehensive display of all measurement data related to the monitoring system and the solar technology center settings. Tab controls in LabVIEW provide users with useful features for placing all information in a single application without making the user interface too complex.
Another challenge is to display different measurement data in the chart. The chart allows the user to visually see the relationship between one parameter and another, as well as the trend of any parameter. There are 22 measurement data points at any one time point, including weather parameters, photovoltaic DC voltage and current, and panel temperature for each type of solar panel. It is impossible to list all the data on one chart. This can be solved by controlling the chart drawing with a programmatic approach. The design of the program is implemented using project selection and chart property nodes. Users can view up to four data curves and measurement parameters 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 designed an excellent user interface with rich information. System reliability is ensured with NI PXI hardware, which has been running 24 hours a day, seven days a week since September 2008. Our team's next task is to evaluate PV system performance, including assessing PV array efficiency, energy generation, and climatic conditions and module conditions.
Figure 2: User Interface Screen Capture
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