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challenge
Build a custom real-time monitoring system in a short period of time at a low cost, and use a variety of measurement devices to evaluate the performance and characteristics of grid-connected PV systems.
solution
Using the open NI LabVIEW software platform to design the monitoring system and dedicated interface software, the output of multiple measurement devices is input to the PXI system through the serial interface, and the data is transmitted to the PC using the ready-made User Datagram Protocol (UDP) function. Provides 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 convenient 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 the development of solar energy. In order to achieve this goal, the Government of Singapore and the Economic Development Bureau's Clean Energy Research and Experimental Program (CERT) have formulated a number of measures. To this end, we have opened a solar technology center with five different photovoltaic panels, which totals 14.2 kW. The center is used to demonstrate different grid-connected photovoltaic systems and serves as a test center for the study of long-term performance and characteristics 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 electrical engineering department to build a custom real-time monitoring system that can measure different parameters of the PV array under test and actual weather conditions at a low cost.
The team developed using LabVIEW software 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 transferring the output of multiple measurement devices to the PXI real-time controller system. Later, using UDP communication functions, data is transferred to the PC for real-time display, analysis, and storage.
Photovoltaic system measuring instruments
At the heart of the package 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 the LabVIEW Real-Time Module. 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. Photovoltaic monitoring system measurement parameters include DC voltage, DC current, and panel temperature. AC power and power generation are collected using an electronic power meter. In addition, a total of 22 measurement data points were collected from measuring instruments and five electronic power meters, a total of 23 measurements. To be able to handle a wide variety of measurement data and to minimize the wiring between the device and the PXI controller, we use a serial interface.
The team uses a Modbus Remote Terminal Unit (RTU), which is an open serial (RS232 or RS485) protocol that provides master/slave communication between devices connected over a network. It is simple, reliable, low cost, and capable of exchanging data in binary format, which improves throughput. However, each Modubs device manufacturer implements this protocol with different function codes, data formats, and cyclic redundancy verification (CRC) codes. 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 virtual instrument software architecture (VISA) serial functions, it is simple to design programs to communicate between these devices. In addition, due to the data manipulation functions provided, such as splitting numbers, right shift with carry, swap bytes, 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 can work for different manufacturers' equipment.
Data transfer from PXI real-time controller to PC
The team used UDP functions in LabVIEW to transfer the collected data to the PC. UDP is simple, convenient, and capable of broadcasting messages to multiple addresses at the same time. Although it does not guarantee reliable data transfer, in this system, this will not cause any impact, because the system only needs to transfer data to the PC in 10 s intervals, so even if some data points are lost It will not cause any problems.
Designing a graphical user interface
One of the system requirements is to provide comprehensive information that shows all the measurement data associated with the monitoring system and the settings of the solar technology center. The tab control in LabVIEW provides users with practical features to place all information in a single application without making the user interface too complicated.
Another challenge is to display different measurement data in the chart. The chart allows the user to intuitively 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, 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 a chart. This can be solved by using a procedural method to control chart rendering. The design of the program was implemented using the project selection and chart properties nodes. Users can view up to four data curves and measurement parameters at one time in one 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 informative and excellent user interface. Using NI PXI hardware to ensure system reliability, it has been running 24 hours a day, seven days a week since September 2008. The next task of our team is to evaluate the performance of photovoltaic systems, including evaluating the efficiency of photovoltaic arrays, energy generation, and the relationship between climate conditions and module conditions.