1. Inverter classification
The photovoltaic power generation system is mainly composed of photovoltaic components, inverters, combiner boxes, combiner cables, box transformers and supporting equipment, among which photovoltaic inverters are one of the important equipment.
According to the inverter form, photovoltaic inverters can be divided into centralized inverters, string inverters, distributed inverters and micro inverters. Among them, centralized inverters are mostly container-type monomers with low price and high power quality, but MPPT (maximum power point tracking, maximum power point tracking) tracking accuracy is insufficient; string inverters are small in size and MPPT tracking accuracy High, which helps to increase the power generation. When used in mountainous conditions, the power generation of photovoltaic power plants has increased significantly, but the price is expensive and the power quality is slightly worse; the distributed inverter combines the centralized and string inverters. The advantages, the price is medium, but the reliability needs to be tested; the power of the micro inverter is small, the volume is small, and the conditions of use are flexible. 1 to 2 photovoltaic modules can be used to form a micro power generation system through the micro inverter, but the inverter price High, more suitable for small projects.
2. efficiency index
As an important income indicator of photovoltaic power stations, the power generation indicator is getting more and more attention. As the core component of photovoltaic power generation system, grid-connected inverter is particularly important to evaluate its efficiency objectively and reasonably.
The key technical indicators of photovoltaic inverters have a variety of efficiency indicators that measure the performance of photovoltaic inverters, such as maximum efficiency, European efficiency, California efficiency, and Chinese efficiency. To understand photovoltaic inverters, you must first understand these efficiency indicators Meaning.
3. Basic efficiency indicators
(1) Maximum power point tracking efficiency
According to the “Technical Specification for Grid-connected Inverters for Photovoltaic Power Generation”, the maximum power point tracking efficiency refers to the DC power obtained by the inverter under test within the specified test period Tm and the theoretical photovoltaic square array simulator (or photovoltaic cell array) The ratio of the electric energy provided by working at the maximum power point during this period of time, the calculation formula is:
Where: PDC(t)—the instantaneous value of the input power of the inverter at the DC port;
PMPP(t)——Theoretically, the instantaneous maximum electric power provided by the photovoltaic square array simulator (or photovoltaic cell array).
(2) Conversion efficiency
Conversion efficiency refers to the ratio of the power output from the inverter at the AC port to the power input at the DC port within the specified test period TM. The calculation formula is:
In the formula: PAC(t)——The instantaneous value of the inverter output power at the AC port;
PDC(t)——The instantaneous value of the input power of the inverter at the DC port.
(3) Total efficiency
The total efficiency refers to the ratio of the electrical energy output by the inverter at the AC port to the electrical energy provided by the theoretical photovoltaic square array simulator (or photovoltaic cell array) during the specified test period TM. The calculation formula is:
Before the formulation of “China Efficiency”, my country has always used the “maximum conversion efficiency” to evaluate the power generation efficiency of inverters, and there are certain limitations in the evaluation method. In actual work, the inverter will not always work at the “maximum conversion efficiency” load point. Its input voltage and load point will change with changes in irradiance and temperature, and the on-site power generation of the inverter with the maximum conversion efficiency may not necessarily Is the largest. Figure 1 shows the conversion efficiency curve of an inverter. The maximum conversion efficiency in Figure 1(a) is 99.37%; the maximum conversion efficiency in Figure 1(b) is 98.67%, which is lower than that in Figure 1(a), but the curve is relatively stable in the entire working range. There are differences in solar resources in different places of use, so their duration under a certain irradiation intensity will also be different. This is reflected in the difference in the residence time of different load points on the inverter, and the energy accounted for under corresponding load conditions The ratio is also different.
Therefore, before the “Chinese efficiency” was formulated, some inverter manufacturers marked the maximum European efficiency or the maximum California efficiency in addition to the maximum efficiency.
In 1990, the European Joint Research Center (EUJRC), based on the characteristics of solar energy resources in Munich, Germany, collected annual sunshine intensity data without catastrophic disasters, respectively corresponding to the European efficiency bins, and counted the annual cumulative power generation in different intervals. Based on the calculation of the weighting proportion of the total annual power generation at each power level.
In a given irradiance interval, the inverter will operate under a certain power condition. By calculating the annual cumulative power generation of the inverter in different irradiance intervals, it is calculated that the irradiance interval is Finally, 6 effective weighting coefficients are given as the energy proportion weight of, and this method is called European efficiency afterwards, as shown in the formula:
Where: Effx%——The conversion efficiency of the inverter under the condition of x% load.
However, this evaluation method does not introduce the selection principle of load points and the reason for dividing the interval represented by each load point in detail.
② California efficiency
California efficiency (CEC efficiency) is an efficiency standard proposed in 2004 by the California Energy Commission based on the California solar irradiance data based on the California efficiency (CEC efficiency) calculation method based on European efficiency, combined with domestic solar resource conditions and adjusted weighting coefficients. The standard selects the radiation intensity of the Los Angeles area and Dallas area in the United States for one year, and the classification interval corresponding to the CEC efficiency according to the principle of the above range selection, and the annual cumulative power generation of different intervals is calculated, and on this basis, the power classification of each segment is calculated The weight ratio of the total annual power generation on the level. The expression of California efficiency is as follows:
European efficiency and California efficiency consider the irradiance conditions of the inverter operation site and the actual operating conditions, and give a concept of calculating the average efficiency of the inverter. The evaluation method is widely adopted and recognized by the corresponding regions. However, it only counts the annual irradiance data of the two regions, and does not fully consider the difference in irradiance between multiple regions, and the sampling data interval is relatively large when the irradiance data is collected.
China’s solar energy resources are quite different from those in Europe and the United States. Under different solar irradiance conditions and given power, the corresponding weighting coefficients will be different, so the European efficiency and CEC efficiency cannot be blindly directly compared with the operating time. The calculation formula is used to evaluate the power generation performance of inverters in China. Based on the analysis of European efficiency and CEC efficiency models, combined with the conditions of solar resource areas in China, China began to set up a drafting team in 2012 and in 2013, based on the characteristics of China’s photovoltaic power generation construction and operating environment, it was the first in China to propose “Chinese efficiency” The concept, that is, the average value of the weighted total efficiency of the inverter reflecting the characteristics of China’s sunshine resources under different input voltages is called the average weighted total efficiency.
“China Efficiency” adapts to China’s climatic conditions and also to the characteristics of China’s photovoltaic power generation construction and operating environment. It can accurately reflect the power generation of inverters in actual operation and better evaluate the characteristics of inverters used in China.
China’s solar energy resources are divided into four categories. In each category, representative regions are selected to analyze the annual cumulative power generation in different power ranges. According to the principle of European efficiency and CEC efficiency, the selection is relatively stable and can cover all The statistical interval of the power range, and calculate the weight proportion of the annual power generation in each power bin.
According to this, the efficiency of photovoltaic inverters in China is the average weighted total efficiency under different input voltages. Given the weight coefficient, the Chinese efficiency of the grid-connected inverter can be calculated according to the formula: