Design of Photovoltaic Power Generation Data Monitoring System Based on Labview

　　The energy output of photovoltaic power generation systems varies greatly due to changes in the surrounding environment. Real-time monitoring of photovoltaic power generation systems can obtain original measurement data, providing useful data for system improvement and optimization as well as future scientific research. Monitoring and analyzing system environmental parameters and the electrical performance of the system itself is a prerequisite for ensuring the normal and efficient operation of the system. The operation of photovoltaic power generation systems is generally carried out without supervision. Monitoring and maintaining photovoltaic systems that are scattered on the ground is very difficult and cumbersome, requiring a lot of time and manpower and material resources. Therefore, the use of remote data monitoring systems in photovoltaic power generation systems is of great significance. Labview can use high-performance modular hardware and combine efficient and flexible software to complete various testing, measurement and automation applications. Flexible and efficient software can create a customized user interface for photovoltaic monitoring systems and provide powerful subsequent data processing capabilities. It can easily set the data processing, conversion and storage methods [4]. Modular hardware can easily provide a full range of system integration. In addition, Labview also has functions such as web publishing, report generation, data management and software connection. This paper uses the powerful functions of Labview and FieldPoint modular distributed I/O to design a photovoltaic power generation data monitoring system, and achieves the purpose of remote monitoring through the web publishing function.

　　2 Principle of Photovoltaic Monitoring System

　　Figure 1 is a block diagram of the photovoltaic data monitoring system. Sensors such as current, voltage, temperature, and wind speed are used to sense information about the photovoltaic power generation system and the surrounding environment to generate measurable electrical signals. Since the signals obtained by the sensors may be very weak or contain a lot of noise, they need to be amplified, attenuated, isolated, multiplexed, filtered, and other operations through signal conditioning devices. The conditioned signal can be connected to the data acquisition device. The monitoring system uses the industrial RS485 bus to realize the communication between the lower computer and the monitoring host PC. The maximum communication distance of the RS485 bus is about 1219m, and the maximum transmission rate is 10Mb/S. The transmission rate is inversely proportional to the transmission distance. The maximum communication distance can be achieved at a transmission rate of 100Kb/S. After adding a repeater, a larger transmission distance can be achieved. Labview software and its supporting DAQ (Data Acquisition) driver and data acquisition hardware form a complete set of data acquisition, analysis, and display systems. At the same time, Labview software can also complete data storage tasks to provide reliable data for future scientific research. Through the Web publishing tool in the software, you can log in to the monitoring system at any time through the Internet for remote data monitoring.

Figure 1 Schematic diagram of photovoltaic data monitoring system

　　3 Photovoltaic Monitoring System Hardware Design

　　3.1 Sensors and Transducers

　　The information that the photovoltaic power generation monitoring system needs to obtain from the site mainly includes: ① The DC current value, voltage value, power value when the photovoltaic array is running, and the battery charging parameters after passing through the power regulator. ② Collect wind speed values, the temperature of the photovoltaic module surface and the surrounding environment, and the solar irradiance. ③ Calculate the cumulative power generation, average temperature, average irradiance and other values ​​through the data collected within a certain period.

　　The data is measured using sensors and converters corresponding to the above information. The temperature sensor uses a precision platinum resistance temperature sensor PT100. The sensor is designed and manufactured in accordance with the IEC751 international standard. The temperature is measured using the characteristic that the resistance value of the platinum resistance changes when the temperature changes. The sensor element is made of platinum wire, with high stability and a wide measurement range. The two temperature sensors can measure the surface temperature and ambient temperature of the photovoltaic module respectively, and the measured temperature is converted into a (4~20)mA DC two-wire standard signal and sent remotely. The voltage is measured using the STCV-800 voltage sensor of the ST-A series of the ST-Tong brand. This series of sensors is widely used in the monitoring of power systems, and the voltage test range is 0~1200V. The HD series high-precision DC large current sensor produced by Wuhan Instrument Company is used for the measurement of DC current. Its working principle is shown in Figure 2.

Figure 2 Schematic diagram of current sensor

　　Using the magnetic comparison method, M is a high permeability material core, and is a proportional winding, and is provided to and DC current respectively. The DC magnetic potentials obtained are, and because the two magnetic potentials and are in opposite directions, when, that is, when the synthetic magnetic flux in the core is zero, the magnetic potential balance equation is, and when,. The above shows that even if a single current with a large value is large, as long as there are enough turns, it can be balanced with a smaller one, and can be used to represent the corresponding value. The value is small, which is convenient for direct precision measurement, and it is a constant and is not affected by other quantities. Therefore, the magnetic comparison method can achieve higher accuracy in measuring large DC currents. Similarly, the measurement of wind speed, solar radiation and other signals uses sensors and converters that match the photovoltaic power generation system to convert the signal into a standard electrical signal before it can be sent to the data conditioning unit.

　3.2 Signal conditioning and data acquisition device

　　The directly collected signal may not meet the requirements of the acquisition system due to noise and other reasons. In order to fit the input range of the data acquisition device, the electrical signal generated by the sensor must be processed. The signal conditioning device can amplify or reduce the voltage and current range as required, and perform isolation and filtering on the signal. The signal conditioning diagram of the photovoltaic monitoring system is shown in Figure 3.

Figure 3 Signal conditioning of photovoltaic monitoring system

　　The signal conditioning device SCXI (Signal Conditioning Extension for Instrumentation) consists of a signal conditioning chassis, a signal conditioning module and a signal connection port. The distributed signal acquisition system uses a modular approach to complete the functions of signal conditioning, data acquisition and network communication. The distributed signal acquisition system is very suitable for industrial field testing, as it allows signal conditioning to be performed close to the sensor. The monitoring system uses NI's FieldPoint modular distributed I/O products, which can be easily connected to the local PC using the RS485 serial interface. FieldPoint has built-in signal conditioning components that can be directly connected to the sensor. It has accurate and reliable 16-bit analog inputs and can be used in harsh environments with independent I/O modules that can be mixed and matched. In addition, FieldPoint has an innovative structure that modularizes I/O functions, signal terminals and communication methods. The system has a short design cycle and stable performance. The FieldPoint system includes a large number of isolated analog and digital I/O modules, terminal blocks, and network interfaces to make it easier to connect to standard open networks [9]. Users can individually select the most suitable network interface module, I/O module or signal terminal type for a specific application. The photovoltaic power generation monitoring system uses NI FP-AI-110 single-ended input module, NI FP-TC-120 thermocouple module and NI FP-1001 network interface module.

　　Since the solar radiation sensor uses the photoelectric detector on its top to measure solar radiation and can convert light signals into voltage signals, the NI FP-AI-110 module is selected for acquisition. It is an 8-channel single-ended input module used to directly measure voltage and current signals from various sensors. The NI FP-TC-120, an 8-channel thermocouple module with an operating temperature range of -40 to 70°C, is used to measure the temperature of standard J, K, T, N, R, S, E and B thermocouples. It has signal conditioning, double-layer insulation isolation, input noise filtering and a high-precision delta-sigma 16-bit analog-to-digital converter to ensure the accuracy of the measured data. Both modules provide HotPnP (hot-swap) operation and simple configuration. They can self-diagnose and automatically adjust to engineering units. They are designed for efficient and reliable measurements. They provide low-noise 16-bit resolution analog input with filtering and over-range protection, and on-board diagnostic functions to ensure trouble-free installation and maintenance. They are also accompanied by NIST calibration certificates to ensure accurate and reliable analog measurements, which are very suitable for use in photovoltaic power generation monitoring systems. In order to achieve communication between FieldPoint and RS485 buses, NI FP-1001 network interface modules are also used. Each FP-1001 network module can connect up to 9 FieldPoint I/O modules as nodes to the RS485 network. FP-1001 manages the communication between PC and I/O modules through the local high-speed bus connected to the FieldPoint terminal base. FP-1001 also provides several diagnostic and automation functions to simplify installation, use and maintenance.