Photovoltaic Inverters: Important Features Forgotten

For photovoltaic power plants, PV modules and inverters are undoubtedly the core equipment. The  panels convert sunlight into electrical energy. The inverter turns DC into AC and can be connected to the grid.

The industry has clearly understood the energy conversion function of the inverter. It is undoubtedly the efficiency index that determines the inverter conversion quality. The industry generally has this understanding: the DC output power is determined after the component is selected, and the AC output power of the system is also determined after the inverter is selected.The inverter efficiency parameters of each manufacturer is almost the same (the highest efficiency is about 98.7%, the European efficiency is about 98.4%), then the main factor determining the system efficiency is the cells,it doesn’t matter which inverter to choose.

Is this really the case? We look at the test data of an inverter test platform built by an institution in Hainan. The installation angles and orientations of components, brackets, and components are all the same. Six inverters are selected to actually test the power generation. Some data has reached 6% difference. Is the efficiency parameter given by the inverter manufacturer such a large amount of water? Actually not, if we take the power analyzer to measure the DC input and AC output power of the inverter separately, we will find that although the inverter conversion efficiency is not necessarily claimed High, but the efficiency of the two inverters is not much different, all below 1%. Can it be explained by the difference in DC power of the panel? For example, if there is a set of panels, there is a quality problem that causes a significant drop in output power, but when we just exchange the inverter, we will find that the original power generation after switching the inverter is high. It still generates a high amount of power, which means that it is not a problem determined by the difference in the component array itself.

Where is the problem? To understand this problem, we must first review the basic characteristics of the panel and the working principle of the inverter MPPT.

From the above figure, an important working characteristic of the component can be seen: the output power of the component is determined by the working voltage relationship, that is, the maximum power point of the component output power, and the voltage deviating from the maximum power point is too low or high, which will cause the output power of the component to decrease. That is to say, if the actual operating voltage of the components in a power station system deviates from its maximum power voltage, then the output power of the PV module array will decrease, which will result in a reduction in the power generation of the power station.

Then how is the working voltage of the component determined? This is the working principle of the MPPT tracking of the inverter. The inverter can adjust its input DC voltage, and the DC input of the inverter is directly connected to the positive and negative components of the component. At this time, the inverter detects an increase or decrease of the DC input voltage by detecting a change in the output power, and is finally relatively stable near the maximum power voltage value of the component.

In the power station system, whether the PV module works in the best state and how much capacity can be exerted is not determined by itself, but by the inverter, and the working voltage has a great influence on the power output of the component. %the above. It can be seen that in addition to the energy conversion function, the inverter has a more important function: the control of the power station system. Moreover, for the power generation capacity of the power station, the inverter conversion efficiency may affect the power generation by 0.5~1%, and the control of the inverter system will affect the power generation capacity of the power plant by 5~10%, 10 times the impact of its own efficiency. .

Therefore, as the owner of the power station purchases the inverter, in addition to looking at the inverter’s own conversion efficiency, it also needs to pay more attention to the control capability of the power station system. The difference in power generation caused by the system control capability is far greater than the efficiency difference of the inverter itself. Much bigger. Among the current common inverter types, the string inverter has multi-channel MPPT characteristics, which is more precise and accurate in the control function of the power station system, and can often bring high power generation. It is the preferred inverter type for power plant owners. .

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