The general classification of grid interactive inverters is divided into four types

This is a general classification of grid-interactive inverters. In each category, there are many inverter brands and models with different sizes and different characteristics.

Branch inverter
Branch inverters are used in small systems ranging from 1kWp to 11kWp. The branch inverters all contain a maximum power point tracker (MPPT), and the DC input voltage varies from extra low voltage (ELV) to 1000V (low voltage, LV). The branch inverters are connected in different ways, as shown in Figure 1, Figure 2 and Figure 3.

Figure 1 The branch inverter is connected to a string of photovoltaic modules. The photovoltaic array is composed of two strings of modules connected in parallel, and the inverter has only one input terminal, which means that the inverter can only receive the output power of one string of photovoltaic modules. This is a relatively stable configuration because there is no interference from other component strings to the grid
Figure 2 The same branch inverter may be connected to multi-shen photovoltaic modules, but if the output power of a string is low (due to shading or damage), it will affect the output of the entire array
Figure 3 can use multiple inverters to improve system reliability. If one inverter fails, the other inverters can still work, and the system can continue to output power

Multi-branch inverter
The multi-branch inverter is still one inverter, but has multiple MPPT input terminals. Therefore, the photovoltaic array can be divided into multiple branches, and each branch is connected to an MPPT input of the inverter.

The advantage of this type of inverter is that if the component orientations are different, the array can be divided into different branches, and all components in the same branch have the same orientation. Then each component string is connected to a dedicated MIPPT terminal, so compared with an inverter with only one MPPT terminal, the system outputs more energy. Multi-branch inverters are generally cheaper than using multiple inverters, and have the advantage of higher output energy when the various parts of the photovoltaic array have different orientations or different degrees of shielding.

Figure 4 Multi-branch inverter installed on Cockatoo Island by the Sydney Harbour Association Trust Fund
Figure 5 Two component strings are connected to different MPPT ends. If one of them is blocked, the output of the other string will not be reduced.

Centralized inverter
Centralized inverters are very similar to branch inverters with multiple component strings. The difference is that centralized inverters are generally used in large systems (>10kWp). In these systems, the array can be divided into many sub-arrays, and each sub-array is composed of multiple component strings.

In some systems, a large inverter may be suitable for the entire photovoltaic array, or a centralized inverter may be a cabinet containing several small multi-branch inverters and exhibit an electrical output. Otherwise, many inverters will be needed, for example, a 100kW system requires 5 x20kW inverters.

Some large-scale centralized inverter manufacturers have optimized the design. For example, a centralized inverter is composed of multiple smaller inverters, and operates selectively according to the power generated by effective light. This configuration scheme improves the working efficiency of the centralized inverter, especially when operating at lower than peak load.

Figure 6 Multiple component strings connected to a centralized inverter

Component inverter
Component inverters are also called micro inverters, which are designed as small transformerless inverters installed on the back of photovoltaic modules (some inverters have an isolation transformer to reduce DC injection current). Many 100~300W component inverters appeared many years ago. In 2009, this product appeared again in the grid-connected photovoltaic system market. The component inverter has two main advantages: Since each component can output AC power, it is no longer necessary to draw a DC cable from the array; AC cables can be connected in parallel at each component, and then connected to the grid at a suitable location. Inverters are also small, easy to manufacture, and have the benefits of modularity (similar to photovoltaic modules), which means that more components and inverters can be integrated into the system at a very low cost in the future.

Figure 7 For very large systems, a whole house of centralized inverters may be required. This is the centralized inverter

The past module inverters are usually more expensive, that is, the cost per unit power ($/W) is higher than other inverters. However, the latest module inverters on the market are related to the installation of an inverter in photovoltaic system engineering. The cost is comparable.
The disadvantage of the module inverter is related to the installation on the back of the photovoltaic module. If the inverter fails, maintenance and replacement of the inverter requires removing these components from the array to find the inverter behind the components. As discussed in Chapter 4, the temperature of photovoltaic modules rises a lot during the day, so compared with the case where the inverter is placed on a sheltered wall or indoors, the working temperature of the module inverter is higher, which increases the failure of the inverter. risks of. The manufacturer shall provide the high temperature operating characteristics of the component inverter.

Figure 8 Micro-inverter attached to the back of the module
Figure 9 Micro-inverters on the US market

The latest micro-inverters provide data recording and communication features similar to other inverters on the market, either through the local AC grid or through the website.

Inverter typeComponent inverterBranch inverterMulti-branch inverterCentralized road changer
Power range100 ~ 300W700~11000W2000 ~ 17000W10000~30000W
Typical efficiency95% 93% ~97%97%97%
AdvantageNo need for DC cables; easy to integrate more componentsMature technologyMultiple MPPT, mature technologyLowest cost ($/W), installed in one location
DisadvantageDifficult to replace the faulty inverterOnly one MPPTIf the inverter fails, there is no redundancy
Table 1 Types and characteristics of inverters