Now that we understand why we need an inverter for PV systems, it is time to introduce the different types of inverters that exist in the market and discover the advantages and disadvantages of each type. Inverters are classified based on their size, mode of operation, or configuration topology.
Considering the classification based on the mode of operation, inverters can be classified into three broad categories:
Inverter classification according to Interconnection types is discussed in EME 812 (11.4. Grid connection and role of inverters) [1].
Aside from the modes of operation, grid-connected inverters are also classified according to configuration topology. There are four different categories under this classification.
Let's start with the central inverter, as shown in Figure 4.1. This is a PV array that consists of three strings, where each string has three series connected modules. Before these strings are connected to the utility grid, a power conditioning unit is required as an interface between the array and the grid. Designers can use one central inverter as illustrated in Figure 4.1, where all strings are connected to the DC side of the inverter and the single AC output is connected to the utility grid.
What consequences can the size of a central inverter have on a PV array?
ANSWER: The huge size of a central inverter needs to be taken into account when designing the PV array to avoid a shading effect.
Now, we are moving to the String inverters as shown in Figure 4.2. Assuming the same PV array that consists of three strings, another way to connect it to the grid is using three string inverter as illustrated in Figure 4.2. In this case, each PV string is connected to a single string inverter at the DC side, and all AC outputs of inverters are combined and connected to the utility grid.
As the name indicates, each string of PV modules has its own inverter. In this case, we are moving closer to the PV modules level.
There is another topology of string inverters called the multi-string inverter. It utilizes string DC-DC converter for MPPT and then central inverter. This type is not very common and is beyond our discussion for this class.
Finally, let's look at the micro inverters. These are also referred to as module inverters. In this case, each module has one dedicated inverter connected on the back of the module. The module DC terminals are connected to the DC side of the inverter and then all AC wires of all terminals are combined and then connected to the utility interconnection point as illustrated in Figure 4.3.
As the name suggests, each module has a dedicated inverter with an MPP tracker.
What consequences can the micro-inverter installation location have on the PV module?
ANSWER: Micro-inverters can increase the heat mass under the PV module.