At present, there are many inverter models on the market, and many EPC manufacturers tend to use a limited number of inverter models to do as many case designs as possible in order to simplify management and explore the advantages of centralized procurement, which will inevitably involve the use of inverter capacity reduction. A small reduction in capacity is not a big problem, but a large reduction in capacity will not only cause a waste of inverter costs, but also fail to obtain the desired power generation.
I. Waste of resources
For example, if a 10kW inverter is used to carry 6kW PV modules, it is equivalent to using only 60% of the inverter's workload, which will cause waste of resources over time and is not conducive to the optimization of the overall cost of electricity (LCOE). Of course, the small case is fine, if it is a MW-class large case, the waste is not negligible.
Second, shorten the length of the string, the overall power generation is reduced
1. Not conducive to string optimization design
For a certain power level inverter, the number of DC input terminals is closely related to the rated power of its MPPT module. If we want to derate the power of 10kW MPPT module to 6kW, the length of the module string will be shortened to 60% of the original, and the string open circuit and working voltage will be reduced to 60% of the original, which is obviously not the optimal design!
2. Not conducive to efficient inverter work
As mentioned above, with the string open circuit and working voltage reduced to 60% of the original, the reduced capacity use will have a significant impact on the working behavior of the inverter.
Impact on inverter start-up and shutdown time: After the string voltage is reduced, the inverter start-up time in the morning is pushed back and shutdown time in the evening is advanced, the daily power generation time is shortened and the power generation capacity is reduced!
Impact on the working mode of inverter: String inverter is composed of two power conversion units, front DC/DC to realize MPPT function and back DC/AC to realize inverter function. Among them, the front stage DC/DC is realized by boost topology (boost architecture), and its working time is determined by the string input voltage, which works when the input voltage is lower than the set value and stops working when the input voltage is higher than the set value. When the string voltage is reduced, the DC/DC module operating time increases and the average conversion efficiency of the inverter decreases, resulting in lower overall power generation!
As you can see, the inverter capacity reduction is achieved at the cost of shortening the string length and reducing the string input voltage. The advantage is to reduce the inverter model, and the life of the inverter will be prolonged if the inverter works in a light load state for a long time; the disadvantage is that the use of reduced capacity not only wastes the inverter resources, but also does not help the inverter work more efficiently, which leads to the reduction of overall power generation!
Feasibility Analysis of Reducing the Number of String Inputs to Achieve Inverter Capacity Reduction
Generally speaking, we think that capacity reduction is not desirable because it often comes at the cost of shortening the overall string length, but if reducing the number of string inputs (without considering the waste of inverter resources) is the path to achieve it, is capacity reduction still undesirable? The answer is clearly no! Because such a design not only does not change the working behavior of the inverter, but is more beneficial for the inverter's lifetime.
