What is the “throughput” capability of string inverters?

Are the component capacity and inverter capacity still designed at a 1:1 ratio? If so, you are out. The ratio of component capacity to inverter capacity is called capacity ratio. When the capacity ratio is greater than 1, it is called over-capacity. The benefits of over-capacity have been repeatedly verified in practice and widely accepted by the industry. At present, the common capacity ratio in domestic photovoltaic power stations has reached 1.05-1.1, and some power stations have reached more than 1.2. This is because the capacity ratio is strongly related to light resources. my country has a vast territory and uneven light distribution. The optimal capacity ratio in different regions varies greatly.

The current development trend of China's photovoltaic market is "from west to east" and "subsidy reduction", that is, photovoltaic power stations are gradually transferred from Class I resource areas with better light resources to Class II, III, and IV resource areas with poorer light resources; and the annual reduction in photovoltaic electricity prices has a serious impact on investment returns. In the face of such a trend, how to maximize the reduction of system costs and increase power generation is the core demand of the photovoltaic industry. Through the refined design of the system, especially the correct capacity ratio design, the initial investment cost of the system can be greatly reduced. For example, according to the configuration ratio of 1.2:1, 100MW components only need 83MW inverters and box transformers. The saved 17MW inverter and box transformer and other electrical equipment costs are the pure profit obtained by customers. Why not do it?

In the face of the demand for a higher capacity-to-capacity ratio, when selecting a string inverter, its DC input and AC output capabilities, that is, the inverter's "throughput" capacity, become an important basis for inverter selection. Why is over-capacity related to the "throughput" capacity of the string inverter? Let's look at two cases.

Examples show that the “throughput” capacity of string inverters affects power station revenue

Positive case: Class I resource area, good light resources

A 50MW PV power station in Xiaojin County, Sichuan, uses a 60kW string inverter. The DC side of a single inverter is equipped with 62.92kWp components, that is, the component to inverter capacity ratio is 1.05:1. Due to the high irradiance at the project construction site, the actual maximum output active power of the inverter is as high as 66.1kW, that is, the inverter is operated at 1.1 times overload for a long time, as shown in Figure 1(a).

Negative example: Class III and IV resource areas, poor light resources

A power station in Hubei uses a 30kW string inverter. Due to the insufficient DC input capacity of the selected inverter, the DC side of a single inverter can only be configured with a maximum of 30kWp components, that is, the capacity-to-capacity ratio is 1:1. When the light resources are relatively poor, even when the radiation is the highest throughout the year, the actual output power of the inverter is only 25kW, as shown in Figure 1(b). The electrical systems such as inverters and transformers operate under light load for a long time, and the system utilization and efficiency are greatly reduced.

Figure 1 Analysis of actual power station inverter operation

From the above two actual cases, we can see that (1) over-allocation can significantly reduce system costs and increase profits; when not over-allocated, system utilization is reduced and inverter capacity is wasted; (2) when over-allocation is designed, the string inverter not only requires a sufficient number of input terminals on the DC side, but also a certain overload capacity on the AC output, that is, the "throughput" capacity of the inverter is very critical.

Correctly selecting the “throughput” capacity of string inverters can improve the power generation revenue of power plants

For some Class I resource areas with good lighting conditions, the inverter needs to have good overload capacity, that is, the "spitting" capability is required to be high; while in other resource areas with relatively poor lighting resources, the DC side of the inverter needs to be able to connect more components and have a sufficient number of input terminals, that is, the "swallowing" capability is required to be high.

With stronger AC side overload capacity

As described in the positive case, when environmental conditions such as irradiance or temperature change, the output power of the module will exceed the maximum power specified in the specification, that is, "module over-generation". This situation usually occurs in some Class I resource areas with good light resources. This requires the inverter to have the ability to convert all the energy of the module, that is, the inverter's "spitting" ability is required to be high. Otherwise, there will be a phenomenon of abandoned light, reducing power generation and affecting user benefits, as shown in Figure 2(a).

Figure 2 The value of a reasonable design of a 50kW string inverter

With stronger DC side access capability

In Class II, III, and IV regions with poor sunlight resources, if the capacity of the inverter connected to the components is equal to or less than the inverter AC power rating, the inverter, transformer, and back-end electrical system will be under light load for a long time, greatly reducing system utilization and indirectly increasing system investment costs. As shown in Figure 2(b), for a 50kW inverter, if a 50kW component is connected, the actual output will be less than 50kW.

The above analysis only considers the light resources, but in the actual photovoltaic system, due to dust blocking, the output of the components is reduced by at least 2-3%. Considering the factors such as component attenuation and cable loss, the DC power actually transmitted to the input of the inverter will be reduced by about 5-10%. In addition, according to the cost calculation of the 1.6MW typical system design scheme, the 9-string scheme of the 50kW string inverter DC side is connected to the 8-string scheme, and the number of inverters is reduced by more than 12%, which can save the initial investment of 0.07 yuan/W of the system, and 100MW can save the initial investment of 7 million yuan. This is the benefit brought by the higher "swallowing" capacity of the inverter, as shown in Figure 3.

Figure 3 The impact of different numbers of DC side accesses on system cost

Therefore, for Class II and III resource areas, it is generally recommended that the capacity of components connected to the inverter be more than 1.2 times the rated capacity of the inverter, that is, the DC side of the inverter needs to be configured with sufficient input terminals to ensure that more than 1.2 times the number of components can be connected.

How to reduce costs from a system perspective is the core demand of power station investors. Reasonable selection of the capacity ratio of components and inverters is an effective way to improve the utilization of system components and reduce the initial investment of the system. The most critical factor in reasonable capacity ratio design is the "throughput" capacity of the inverter, that is, whether the rated capacity of the inverter and the AC/DC input and output matching design are reasonable. On the one hand, the DC input end of the inverter needs to have enough input terminals to connect more components to ensure that the system utilization is maximized in areas with poor lighting resources. At the same time, the AC side has a certain overload capacity to ensure that the system is not derated, minimize the initial investment of the system, and improve the overall benefits of the power station.