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Mechanical Engineering

Inverter types and classification

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.

Inverters based on PV system type

Considering the classification based on the mode of operation, inverters can be classified into three broad categories:

  • Stand-alone inverters (supplies stable voltage and frequency to load)
  • Grid-connected inverters (the most commonly used option)
  • Bimodal inverters (usually more expensive and are used less often)

Types of Grid-connected Inverters

Aside from the modes of operation, grid-connected inverters are also classified according to configuration topology. There are four different categories under this classification.

  • 1. Central inverters, which are usually around several kW to 100 MW range.
  • 2. String inverters, typically rated around a few hundred Watts to a few kW.
  • 3. Multi-string inverters, typically rated around 1 kW to 10 kW range.
  • 4. And finally, Module Inverters or Micro Inverters, typically rated around 50 to 500 W.

Let’s start with the central inverter, as shown in Figure 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 Figure1, where all strings are connected to the DC side of the inverter and the single AC output is connected to the utility grid.

Let’s start with the central inverter, as shown in Figure 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 Figure1, where all strings are connected to the DC side of the inverter and the single AC output is connected to the utility grid.

Now, we are moving to the String inverters as shown in Figure 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 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.

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