Primer: How many PV modules should be designed for each PV string in the PV power plant design? One day from Jun Yang Peng received a plan of 24 PV modules from Huawei to Jun Yang. After earnestly studying, I gained some experience and shared it with everyone.
Traditional PV string design
In the "Design Specification for Photovoltaic Power Plants (GB 50797-2012)", the following formula is proposed.
N—Number of PV modules in series (N is an integer number)
Kv—Open Circuit Voltage Temperature Coefficient of Photovoltaic Modules
K'v—operating voltage temperature coefficient of photovoltaic modules
T—the ultimate low temperature under the working conditions of photovoltaic modules
T'—the ultimate high temperature of photovoltaic modules operating conditions
Voc - open circuit voltage of photovoltaic modules
Vmppt - operating voltage of photovoltaic modules
Vdcmax—The maximum tributary input voltage allowed by the inverter
Vmpptmax—Inverter MPPT Voltage Maximum
Vmpptmin—Inverter MPPT voltage minimum
The above formula is also a common design method for engineers. I have always used this method myself. However, some parameters are not available in this formula.
1, the working voltage temperature coefficient K'v
In the specifications of each component manufacturer, there is no such item. Generally, the open circuit voltage temperature coefficient Kv is used for calculation.
2. Extreme low temperature t under working conditions
First of all, the operating temperature of the PV module will be higher than the ambient temperature; secondly, the PV system can only operate during the day and it will not operate at night. Therefore, t should theoretically be the extreme low temperature of the photovoltaic modules in the daytime.
However, how much higher is the temperature of the PV module than the ambient temperature? This is inconclusive; however, among the many meteorological stations in the country, there are only a few weather stations that can collect extremely high temperatures in the daytime and extreme low temperatures in the daytime, and most weather stations cannot obtain it.
Therefore, the extreme low temperature t generally takes the extreme low temperature of the environment.
3. Extreme high temperature t' under working conditions
As mentioned earlier, the operating temperature of the PV module will be higher than the ambient temperature, but the specific high level is still unclear. The figure below is a graph of the changes in the component temperature and ambient temperature.
Figure 1 Relationship between ambient temperature, irradiance, and component temperature
It can be seen that the component temperature can be more than 20 degrees higher than the ambient temperature. According to the document “Temperature Effects in Solar Photovoltaic Power Generation Applications†(Wang Jianjun), Xining can measure backplate temperatures as high as 70°C. Therefore, when I am calculating, the extreme high temperature generally adopts the scheme that the ambient temperature increases by 25°C, or directly adopts 70°C.
Example 1: Taking the Tianhe 260W polysilicon component as an example, assume that the local extreme low temperature is -30°C; the maximum open circuit voltage of the inverter is 1000V, and the MPPT voltage is 420~850V.
Table 1 Technical parameters of Tianhe PV modules
According to the above table, Voc=38.2V, Vmppt=30.6V, Kv=K'v=-0.32%/°C. The calculation results using the formula in GB 50797-2012 are as follows.
Table 2 Calculation of Series Number of Tianhe 260W PV Modules
It can be seen that if the maximum open circuit voltage of the inverter is 1000V and the MPPT voltage is between 420 and 850V, then the open circuit voltage: N1 ≤ 22; the MPPT voltage: 17 ≤ N2 ≤ 23, and the final series number is taken as 22.
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