As one of the important technical indicators to measure the working state of the inverter, the inverter load rate has not received due attention in the operation practice for a long time, and the application of this indicator is also lacking in the existing background monitoring and analysis software. In the string inverter data analysis system jointly developed by Shanghai Power Energy Technology Co., Ltd. and Shanghai Linshi Technology Development Co., Ltd., a lot of tracking and in-depth analysis of the inverter load rate have been carried out, and some experience and experience have been obtained.
In photovoltaic power generation sites, affected by light intensity and the environment, photovoltaic power generation is often in variable load conditions, which makes data analysis more difficult than stable load conditions. In order to simplify the analysis, the load rate can be divided into several intervals according to the analysis object, and the operating parameters in this interval can be regarded as a stable working condition, so as to control the analysis error within an acceptable range.
For example, there is a significant difference in the inverter conversion efficiency during the low load rate period and the high load rate period. In a climate with sufficient light, the inverter conversion efficiency is improved by about 2-3% compared with cloudy days. Therefore, the evaluation of the inverter conversion efficiency A more efficient solution is to compare and analyze within the same load rate range.
For another example, according to the distribution of the load rate throughout the year, the power generation after the change of the inverter capacity ratio can be accurately calculated, using the law that the power generation space is high during the low load rate period, and the power generation space is small during the high load rate period. , simulate the incremental power generation and conduct economic analysis. This method can cover a series of uncertain factors such as insufficient irradiation, component attenuation, series-parallel loss, DC line loss, and component damage, replacement, occlusion and cleaning, reducing computational complexity and improving simulation accuracy. Since the solar altitude and azimuth change periodically in units of years, this scheme is substituted into the annual data set for calculation, and the obtained results are more credible.
According to the corresponding power generation in the load rate range, some operating parameters can be simulated more accurately, such as the “Photovoltaic Power Generation System Performance Specification” (NB/T10394-2020) to calculate the operating temperature of the components in the cycle that needs to be confirmed for PR. The temperature represents the data in the whole cycle. If the arithmetic average of all the values in the interval is simply performed, the influencing factors of the power generation time are increased, and the influence factors of the power generation amount are reduced, and the error is obviously larger. Therefore, a more reasonable method is to divide all the temperatures in the cycle according to the load rate interval, average the values in the interval, and then perform the weighted average calculation with the power generation in this interval as the weight. The proportion of the temperature value is more reasonable. To calculate the operating temperature of a photovoltaic power station module in late summer and early autumn, the arithmetic average calculation method shows that the daily average temperature of the module is 42.26°C, and the corresponding temperature is 47.61°C calculated by the weighted load rate, with a difference of 11.2%.
For real-time PR analysis, due to load fluctuations, various technical indicators also show different fluctuation characteristics. In addition to the aforementioned inverter conversion efficiency, series loss, parallel loss, line loss, shading and dust accumulation have an impact on power generation loss. The impact rate will change and should be treated with caution.
The inverter load rate index also plays an important role in the refined management of photovoltaic power plants. Typical inverter load rate power generation and power generation time ratio are shown in Figure 1(a) and Figure 1(b). Although the power generation time is long, the contribution of power generation is very small. Therefore, from the point of view of station operation and maintenance, if daily operation and maintenance work is carried out in different regions during the low load rate period, it will have little impact on the overall power generation of the power station. The inverter is not disconnected from the grid and the low load rate is carried out, which can not only reduce the electricity consumption for lighting at night, but also help to ensure the safety of equipment and personal safety. In addition, from the perspective of operation and maintenance KPI assessment, it is unreasonable to simply assess the duration of the fault, and it is more accurate to calculate the power loss based on the load rate during the fault period.
(a) Power generation (b) Power generation time
Figure. 1 The proportion of power generation on sunny days and the load ratio of power generation time