Photovoltaic (PV) system due to the attenuation of component power, dust shading, and the existence of line losses, coupled with the differences in light conditions in different regions, in order to optimize the system revenue, experienced design engineers will match the total capacity of PV modules with a larger capacity than that of the solar inverter, which is known as overmatching. This is called overmatching. Appropriate overmatching can improve the overall revenue of the power plant system. In the overmatching design, the centralized large inverter is easy to overmatch due to its large stand-alone power, the power of each string is very small relative to the stand-alone rated power, and it is very flexible in the configuration of DC input, which has been accepted and widely used by power plant owners.
It is advantageous to use string inverters in small rooftop power plants and small hill power plants. Can these string inverters meet the requirements of overmatching design and what is the capability of overmatching design? I have investigated and analyzed some mainstream string inverter manufacturers at home and abroad, and found that the majority of manufacturers' string inverters can reach 1.1 times, or even higher overmatching capability, but also found that individual string inverter manufacturers' products have serious defects in design, not only do they not have overmatching capability, but they even cannot use the inverter's rated power to the full extent. The actual available power of the inverter is greatly reduced, which directly leads to the increase of the user's initial investment.
String inverter overmatching design requirements
String inverters, due to the small power of a single machine and the characteristics of multi-way MPPT, are suitable for small roofs, small hills and other complex distributed power stations, which can effectively solve the problems of irregular component layout, different orientations, and local shading. With the development of domestic distributed applications, the application of string inverters is also increasing.
In the overmatching design, in addition to considering the system loss, the optimal capacity ratio (module capacity: inverter capacity) is mainly determined by the light conditions of the location where the power station is located. Most of the domestic distributed power plants are located in the southeastern part of China, and according to the resource area classification of National Meteorological Administration Wind and Solar Energy Evaluation Center, most of them are in the II, III, IV light resource area, with relatively poor light conditions. In such areas, the capacity ratio needs to be at least 1.1 times or more in order to achieve the optimal system cost of electricity and maximize the return of investors. What are the specific requirements for string inverters when designing overmatching?
Need to assess the actual available AC side power of the inverter
Overmatching is the module capacity of a PV power plant relative to the AC side capacity. For a PV power plant, the capacity should be calibrated in terms of the AC power side capacity. For example, a 20MW plant means that its AC side output power can reach 20MW, not the DC side of the module power is 20MW. the same applies to the inverter, firstly, we should pay attention to its AC power rating parameter, and then analyze its "actual available AC power". The "actual available AC power" of string inverters is the one that is really meaningful for over-allocation. Such as a string inverter, its AC side rated power parameters is 36kW, but according to its DC side of the real maximum power can be configured to the power of only 34KWp, taking into account the inverter's own loss, its "actual available AC side rated power" must be less than 34KW, from the perspective of the coefficient of overmatching 1.1, the real version of the From the point of view of overmatching factor 1.1, the real version of "actual available AC side rated power" may be only 30KW, therefore, "actual available AC side power" is the premise of the system for overmatching design.
The inverter must have good heat dissipation capability
The inverter must have good heat dissipation capability. Since string inverters are mainly used in complex distributed power stations such as small roofs and small hills, the ambient temperature is high and the heat dissipation conditions are relatively poor. For example, in the summer when the weather is hot, the ambient temperature of the roof is at least 10℃ higher than that of the ground power station because of the thermal radiation of the roof tiles or the cement roof after being irradiated by light. In such a scenario, the running time of the inverter at full load and overload will be longer after the system is over allocated, which poses a challenge to the heat dissipation capability of the inverter. Therefore, efficient heat dissipation capability is the guarantee for stable and non-decreasing operation of inverters. When choosing the inverter, the selection of heat dissipation method also needs to be careful, the actual test shows that, for tens of KW power electronic equipment, working in full load state for a long time, the intelligent fan cooling effect is better.
The number of DC input terminals must be sufficiently large
In order to realize the overmatching design, string inverters need a sufficient number of terminals. At present, the domestic commonly used component power are 255W, 260W, 270W, usually each string consists of 22 pieces of components connected in series. Taking the current common AC rated power of 40KW string inverter as an example, the overmatching coefficients corresponding to the different number of terminals can be calculated as shown in Table 1.
Table 1 The power corresponding to different terminal numbers and the corresponding overmatching coefficients
As seen from Table 1, for a 40KW AC rated power string inverter, for the common 270W and below components, the 40KW string inverter needs to be configured with at least 8 strings in order to satisfy the overmatching design requirement of 1.1 or more. Different from the centralized inverter scheme, string inverter is directly connected to the components, there is no DC convergence link in the middle, the number of connected component strings is limited by the number of its own input terminals, therefore, sufficient number of input terminals to achieve the necessary guarantee of overmatching design.
Inverter overload capacity needs to be as large as possible
The inverter needs to have a strong overload capacity. On the one hand, when the output energy of the module is still larger than the rated power of the inverter after deducting the DC side line loss, the inverter with overload capacity can minimize the time of limiting the generation and reduce the loss of power generation. On the other hand, as more and more users use inverters to replace the SVG function of power stations, inverters with overload capability can output active power exceeding the rated capacity while responding to reactive power dispatch.
Research on overload capability of string inverters of mainstream manufacturers
What is the real situation of string inverters' over-allocation capability of mainstream manufacturers? Does the actual available AC power match the parameter table? Is the heat dissipation capacity sufficient? Is the number of terminals on the DC side sufficient? How about the overload capacity of the inverter? With these questions in mind, Shielden conducted a research on domestic and international mainstream manufacturers such as SMA, Delta, Sunny Power, Guriwatts, and ShanYi. The main parameters of each inverter are summarized in Table 2. According to the data in the table and combined with the research, it can be concluded:
(1) Most string inverters are calibrated with the rated power on the AC side, and the actual available power on the AC side is sufficient, but there is one inverter whose actual available power is on the small side, which is not only unable to be configured to the full load, let alone over-allocated. Combined with the analysis of Table 1 and Table 2, it can be seen that the maximum access capacity of the DC side of the 6-channel input is far less than the claimed 40KW power value of the product model, and cannot even reach the claimed rated output power of 36KW, so the real usable capacity of the inverter is greatly reduced, and the design of the over-allocation is even more impossible to talk about. This greatly increases the cost of the system's kWh, which seriously affects the investor's income.
(2) Most string inverters use fans to dissipate heat, which can ensure stable operation under overload for a long time when overmatching, but there is one inverter without external fan, even under normal power, the life of the inverter itself is challenged, and overmatching is even more impossible to talk about.
From the results of the survey in Table 2, mainstream manufacturers use fan cooling as the mainstream cooling solution. In order to compare the difference in performance between fan cooling and natural cooling, I investigated the cooling effect of 40KW string inverters from different manufacturers, and the results showed that with the same 45℃ ambient temperature under full load operation, the temperature rise of key components such as IGBTs and capacitors inside the 40KW inverter from manufacturer A with natural cooling is at least 10℃ higher than that from manufacturer B with intelligent fan cooling, which means that the temperature rise of key components is at least 10℃ higher than that of manufacturer B with intelligent fan cooling, which means that the life span of the same components is at least 10℃ higher than that of manufacturer B with smart fan cooling. According to the law of ten degrees, it means that under the same conditions, the product life of the natural cooling method will be reduced by half, and this impact on the life is often ignored by users because it cannot be shown in the short term.
(3) The number of DC terminals of most string inverters is more than enough to cope with the overmatching design, and there is only one inverter that does not have enough DC terminals. The DC side of this inverter manufacturer's 40KW string inverter is designed with three MPPTs, but each MPPT can only be connected to a maximum of 2 strings, i.e., the number of terminals on the DC side of the inverter is only 6. According to Table 1, the maximum DC power accessed by 6-way terminals is only 34320W, and the maximum over-allocation coefficient is only 0.95, which does not have over-allocation capability.
Table 2.Summary of acceptable overmatching ratio of string inverters of international mainstream inverter manufacturers
Summary
The overmatching design of PV system has been widely concerned by users, and through appropriate overmatching, the overall return of investors can be improved. For the distributed projects that are being vigorously promoted in China, an overmatching factor of at least 1.1 times is required. By analyzing and comparing, not all string inverters can be designed for overmatching. Among the current common 40KW inverters, only string inverters with more than 8 DC input terminals can realize the overmatching scheme of more than 1.1 times. And the inverter needs to have enough power margin, good heat dissipation performance and certain overload capacity. International mainstream manufacturers in the product design are oriented to maximize customer revenue, fully considering the importance of DC side overmatching to the system revenue, the actual power calibration of the inverter products, heat dissipation, the number of DC input terminals and overload capacity, are fully considered to meet the needs of the system overmatching design.
