The development of photovoltaic power plants is in full swing, and more and more people are paying attention to this clean and green renewable energy source. When investing in photovoltaic power plants, power generation is one of the important considerations. Apart from the quality of the components themselves, they are also affected by other external factors. Today Xiao Bian took everyone to analyze an actual case and quickly learn together.
I. Project Background
The 10MW Phase I project of a certain site has been connected to the grid for a period of low power generation. The first phase of the project has a designed installed capacity of 1 MW. The components are installed vertically in two rows with a support angle of 34°. One string per 22 modules is collected by the combine box. After the access to two 500kW centralized inverter.
Second, the scene
The project site is located on the top of the mountain and it is difficult to level the land. The components are arranged along the mountain, uneven and ups and downs. According to different orientations of the slopes, although the tilt angles of the arrays of each row are the same, there are many azimuths: south east, south south, and south west. In the field layout of the combiner box, the nearest principle is adopted, and the adjacent two to three arrays of modules are connected to one combiner box, resulting in multiple clusters of different azimuth angles in the same combiner box.
Third, the cause analysis
The working voltage and current of photovoltaic modules are not constant. The IV curve of conventional components is as follows. At a certain voltage value, the output power of the module is the largest. This voltage value is called the peak voltage; the power output of the module is affected by the incident angle of light, irradiance, and Temperature and other factors have a great influence, as shown in the figure below. The nominal power on the component nameplate is the maximum output power measured under standard conditions (AM1.5, 25°C, 1000W/m2).
IV curves at different irradiances
IV curves at different temperatures
Due to the output uncertainty of photovoltaic modules, the PV inverter must include an MPPT (Maximum Power Point Tracking) module to detect the operating voltage of the PV strings in real time so that the PV modules always operate near the peak voltage, ensuring that the PV system is at its maximum. Power output to the outside world. Based on the above characteristics of the photovoltaic module, the light intensity received at the same moment in different strings with different azimuth angles is different, and the operating temperature is different, and different IV curve characteristics are exhibited, and the corresponding peak voltages are also different. As the inverters used in this project are centralized inverters, each inverter has only one MPPT module. All strings connected to the inverter can only work at the same voltage, resulting in that some strings cannot be maximized. The power output reduces the overall output power of the PV array, resulting in a low overall power generation. IV. Findings
There is no land leveling at the site of the power station, resulting in a large difference in the azimuth angles of the different strings. When connecting strings, the different azimuth strings are connected to the same combiner box and then they are connected to the same centralized inverter, resulting in a large difference between the strings in each group. The single-channel MPPT centralized inverter cannot guarantee that all strings are Maximum power output, large mismatch between strings, affecting power generation.
It is recommended that the owner adopt a multi-channel MPPT distributed or string inverter in the follow-up project, which can effectively reduce the mismatch loss between the strings and increase the power generation for a similar mountainous project. Or flatten the land as much as possible to ensure that all groups have the same azimuth. It is not possible to improve the string convergence method. Connect groups with the same or less difference in azimuth angle to the same inverter. The east and west groups can be connected to different inverters, and the group string can also be improved. Match conditions to increase power generation.
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