Design of outdoor performance test platform for photovoltaic modules

　　An outdoor photovoltaic module test platform proposed on the basis of the traditional working mode electronic load, with a programmable electronic load that automatically switches the working mode as the core, realizes a more accurate and complete measurement of the IV characteristic curve of the photovoltaic module. It can keep the photovoltaic module in the set working state for a long time in the outdoor environment according to the user's settings, and monitor its output characteristics in real time. A large amount of stored IV characteristic curves and environmental parameter data is helpful to analyze the actual outdoor working performance of photovoltaic modules. Photovoltaic system designers can select suitable modules for specific working environments by comparing the outdoor characteristics of different types of modules. The platform also provides a reliable basis for photovoltaic module manufacturers to evaluate their products.

　　1 Introduction

　　With the expansion of the domestic photovoltaic market and the development of distributed photovoltaic power generation systems in recent years, power station designers have also put forward higher requirements for the performance of various photovoltaic module products. At present, the electrical performance test of photovoltaic modules mainly relies on the solar simulator in the laboratory to detect its output characteristic curve. This method is convenient for controlling environmental parameters such as irradiance and temperature. However, photovoltaic modules actually work in complex outdoor environments, and their output power is easily affected by factors such as dust, gravel, rain and snow. The output characteristics may also change due to periodic shadows such as buildings and tree shades. Therefore, the actual output power of photovoltaic modules is generally much lower than the output power under the ideal environment in the laboratory. At present, some methods have been proposed for measuring the IV characteristics of photovoltaic arrays at home and abroad, mainly using dynamic capacitor charging methods to synchronously measure the IV characteristics of photovoltaic arrays on site. This method has a fast measurement speed and requires a high sampling rate for the controller. In addition, there is also a field measurement method based on a variable electronic load. It has more measurement points near the maximum power point on the IV characteristic curve of the photovoltaic array, but fewer measurement points near the short circuit and open circuit points. When the photovoltaic array is in a slightly mismatched or shaded working condition, this method is difficult to achieve accurate measurement of this phenomenon. In order to more accurately reflect the outdoor output performance of photovoltaic modules, an outdoor photovoltaic module test platform is proposed. It allows photovoltaic modules to work in outdoor environments for a long time, monitor their output characteristics in real time, and accumulate measurement data to evaluate the output performance of modules in outdoor environments for a long time, so that power station designers can choose more reasonable photovoltaic modules to build photovoltaic systems according to specific environments. It also provides better protection and technical support for module manufacturers and scientific research experiments.

　　2 Outdoor test platform design

　　2.1 Requirements for outdoor performance testing of photovoltaic modules

　　The output characteristics of photovoltaic modules are mainly affected by solar irradiance and ambient temperature. When photovoltaic modules are working in a specific outdoor environment, it is necessary to measure the ambient irradiance and module temperature. The traditional IV characteristic curve measurement method is to make the programmable electronic load work in constant voltage or constant current working mode and scan with a fixed step size. Since the IV characteristic curve of photovoltaic modules has a logarithmic curve shape similar to that of semiconductor diodes, when photovoltaic modules work in constant current or constant voltage sections, only using electronic load measurement in constant current or constant voltage working mode will result in sparse measurement points in the corresponding part of the curve. In addition, in order to test the outdoor performance of photovoltaic modules, it is also necessary to keep the tested photovoltaic modules working in open circuit, short circuit or maximum power working states for a long time according to user settings. Therefore, the traditional on-site capacitor charging or electronic load instantaneous measurement of IV characteristic curve methods cannot be used.

　　Therefore, a programmable electronic load that can automatically switch working modes is proposed. The constant voltage and constant current control methods of the electronic load are used for the constant current section and constant voltage section on the IV characteristic curve, respectively, to comprehensively measure 256 working points on the IV characteristic curve. The photovoltaic module outputs energy and dissipates it through the heat sink. The rapid scanning of the IV characteristic curve reduces the impact of sudden changes in irradiance on its output characteristics in outdoor environments. After measuring the electrical characteristics and environmental parameters of the photovoltaic module, the data is sent to the host computer and stored. In order to avoid the host computer shutdown or network communication failure, the data needs to be temporarily stored in the platform to ensure the security and integrity of the data.

　　2.2 Design of outdoor performance test platform for photovoltaic modules

　　In view of the above requirements for outdoor testing of photovoltaic modules, a block diagram as shown in Figure 1 was established. Using DSP as the main controller, the equivalent resistance of the electronic load is controlled by the DAC module, so that the photovoltaic module works at the corresponding working point, and then the 12-bit A/D converter built into the DSP samples the load voltage and current. A Pt100 platinum thermal resistor with high linearity is selected as a temperature sensor to measure the backplane temperature of the photovoltaic module. At the same time, a silicon cell irradiance sensor is installed coplanar with the tested component to measure the irradiation energy absorbed by the photovoltaic module. In addition, a wireless local area network is established between the host computer, which consists of the Ethernet module of the test platform, the test platform router, the host computer router and the host computer network port, so that the host computer can remotely monitor the test platform and receive data. The outdoor test platform also has an SD card storage module to temporarily store the measurement data of recent weeks to achieve data backup.

　　Figure 1 Outdoor photovoltaic module test platform

　　3 Programmable Electronic Load Hardware Design

　　At present, the maximum output power of common photovoltaic modules on the market under standard test conditions (STC) is about 200~300W, the short-circuit current is about 8~9A, and the open-circuit voltage is about 30~40V. Therefore, a programmable electronic load with a rated load of 300W that can automatically switch working modes is designed and used as a load during the module testing process to continuously dissipate the module output power in the form of heat energy during the test. The measurable current and voltage ranges are 0~10A and 0~90V respectively, which meet the current common commercial module measurement requirements.

　　3.1 Constant current working mode control circuit

　　A typical MOSFET has three working regions, namely the cut-off region, the linear region, and the saturation region. When the MOSFET works in the linear region, the current Id flowing through it can be controlled by controlling the voltage VGS between its gate and source, and finally its equivalent impedance can be controlled, thereby testing the output performance of the power supply. Its sub-control circuit is shown in Figure 2. A low-temperature drift sampling resistor is selected to collect the current signal, and then the current signal is differentially amplified and connected to the reverse input terminal of the operational amplifier U1A. U1A compares the current signal with the control signal of the same-direction input terminal, controls the MOSFET gate voltage, and realizes the control of the MOSFET equivalent impedance.

　　Figure 2 MOSFET sub-control circuit

　　Since the power dissipated by a single MOSFET is limited, an 8-way MOSFET parallel structure is selected to shunt the output current of the photovoltaic module, and the 8 MOSFETs are evenly fixed on the heat sink to prevent a single MOSFET from burning due to excessive power. The above-mentioned sub-control circuit is used for each MOSFET respectively, so that the working state of each MOSFET is roughly the same, reducing the working temperature difference of different MOSFETs. Finally, the 8-way differentially amplified current signals are superimposed into a total current signal through an addition circuit, and a peripheral feedback circuit is used to compare the total current signal with the control signal given by the DAC module. At the same time, the output signal is connected to each MOSFET control circuit to form a peripheral feedback control. As shown in Figure 3.

　　Figure 3 Constant current working mode peripheral feedback control circuit 　　3.2 Constant voltage working mode and mode switching

　　For the constant voltage working mode circuit, the control principle is the same as that of the constant current working mode. After the load voltage is differentially processed, it is compared with the control signal of the DAC module, and the output of the op amp is connected to the control circuit of each MOSFET so that its equivalent impedance is controlled by the voltage signal given by the DAC.

　　The proposed programmable electronic load that can automatically switch working modes needs to use constant voltage and constant current working mode scanning curves for the constant current section and constant voltage section of the photovoltaic module output respectively when scanning the IV characteristic curve of the photovoltaic module. Therefore, an analog electronic switch is selected to switch the above control signal. The analog electronic switch is directly controlled by the main controller DSP to realize automatic switching of working modes during the measurement process.

　　4. Test process development

　　As mentioned above, to measure the IV characteristic curve of a photovoltaic module once, it is necessary to measure the solar irradiance, module temperature and ambient temperature under its working conditions at the same time. Referring to the relevant content in IEC 60904-1, the outdoor test process of photovoltaic modules has been formulated, and the steps are as follows:

　　1) Synchronously measure the solar irradiance, component temperature and ambient temperature in the environment and record the data;

　　2) Measure the open circuit voltage VOC and short circuit current ISC of the photovoltaic module, calculate the voltage Vapp = 0.8VOC at the approximate maximum power point, and calculate the number of measurement points NCV in the constant voltage mode;

　　3) Calculate the voltage change step size ΔV = Vapp/NCV, set the programmable electronic load to constant voltage operation mode, and measure each point on the IV characteristic curve in turn with a step size of ΔV;

　　4) When the NCV point measurement is completed, the working voltage of the photovoltaic module is Vapp, and the corresponding working current Iapp is measured. The number of measurement points NCC in the constant current mode is calculated from Iapp;

　　5) Calculate the current change step ΔI = Iapp/NCC, set the programmable electronic load to constant current working mode, and continue to scan the IV characteristic curve from the current working point with a step size of ΔI until the remaining points are measured;

　　6) Synchronously measure the solar irradiance, component temperature and ambient temperature in the environment again to ensure that the irradiance and temperature do not change suddenly during the IV characteristic curve measurement;

　　7) Based on the measured data, calculate the maximum power point, fill factor and other characteristic parameters on the IV characteristic curve, package all the data and store them in the SD card, and the IV characteristic curve scan is completed.

　　When a set of data measurements is completed, the platform can control the PV modules to operate in open circuit, short circuit or maximum power states according to user settings until the next measurement begins. This can detect the long-term working performance of PV modules in a specific state.

　　In order to avoid the influence of environmental irradiance or temperature changes on the measured IV characteristic curve and make the measured curve smoother, it is very important to be able to quickly scan the IV characteristic curve. In the above measurement process, the AD converter takes about 80μs to synchronously measure each point on the IV characteristic curve of the photovoltaic module, and it takes about 22ms to measure a set of IV characteristic curve data. Generally speaking, there will be almost no sudden changes in environmental irradiance or temperature during this measurement time.

　　After the measured data is stored in the SD card, the DSP also encapsulates the measured data into UDP packets, and sends them to the host computer through the Ethernet module and the test platform router. After receiving each UDP packet, the host computer gives a reception response. Based on VB.NET programming technology, a host computer monitoring program is designed, which communicates with the DSP and stores the data in the SQLServer database, which is convenient for users to analyze and evaluate the long-term outdoor working performance of the components.

　　5 Test Results and Analysis

　　In order to verify the performance of the outdoor test platform for photovoltaic modules, four EG50W modules produced by Yijing Company were used to form a 2×2 array to replace the 200W modules commonly found on the market. The experiment was conducted on March 19, 2013. The weather was cloudy, the solar irradiance fluctuated around 200W/m2, the module temperature was about 19℃, and the outdoor test platform scanned the IV characteristic curve of the photovoltaic module once every 5s. In order to facilitate the comparison with the traditional IV curve scanning method, the programmable electronic load of this outdoor test platform was controlled to work in the traditional constant current mode, constant voltage mode and the automatic switching working mode proposed in this paper. The IV characteristic curve of the photovoltaic module was scanned by three methods respectively, and the measured curves are shown in Figures 4 to 6.

　　Figure 6 Curve measured by the electronic load that can automatically switch working modes

　　As can be seen from Figure 4, in the part of the IV characteristic curve close to the short-circuit current, due to the small change in the component operating current, it is difficult to completely scan the curve using the electronic load in constant current working mode with a fixed step size, resulting in the problem of scarce measurement points mentioned above. Correspondingly, Figure 5 shows that when the IV characteristic curve is scanned by the electronic load in constant voltage working mode, the phenomenon of a significant reduction in measurement points also occurs near the open circuit voltage. If the test process proposed above is adopted, the entire curve can be measured more completely by scanning the IV characteristic curve through a programmable electronic load that automatically switches the working mode. As shown in Figure 6, the 256 points on the measured curve are closely arranged, and the data does not need to be smoothed. At the same time, near its maximum power point, the measured points are more densely distributed, ensuring a more accurate measurement of the maximum power value of the photovoltaic module.

　　6 Conclusion

　　The developed outdoor photovoltaic module test platform has flexible programming, which effectively realizes the accurate and complete measurement of the outdoor IV characteristic curve of photovoltaic modules. By analyzing the measurement data, the performance of photovoltaic modules in a specific environment can be evaluated. For photovoltaic system designers, by analyzing the output capacity of different modules in a specific outdoor environment, they can better select photovoltaic modules suitable for working in the environment, so as to optimize the output efficiency of the photovoltaic system. It also provides product testing basis for photovoltaic module manufacturers.