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)
Inverter classification according to Interconnection types is discussed in EME 812 (11.4. Grid connection and role of inverters).
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.
- Central inverters, which are usually around several kW to 100 MW range.
- String inverters, typically rated around a few hundred Watts to a few kW.
- Multi-string inverters, typically rated around 1 kW to 10 kW range.
- 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 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.
Advantages of a Central Inverter
- The most traditional inverter topology
- Easy system design and implementation
- Low cost per Watt
- Easy accessibility for maintenance and troubleshooting
Disadvantages of a Central Inverter
- High DC wiring costs and power loss due to Voltage Drop.
- Single MPPT for the entire PV system
- System output can be drastically reduced in case of partial shading and string mismatch
- Difficult to add strings or arrays for future expansion
- Single failure point for the entire system
- Monitoring at array level
- Huge size! (It is a disadvantage because the bigger size requires more land and creates a shading issue for the PV array.)
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.
Advantages of a String Inverter
- Smaller in size when compared to central inverters
- Better MPPT capability per string
- Scalability for future expansion by adding parallel strings
- Short DC wires
- Monitoring at string level
Disadvantages of a String Inverter
- The installation requires special racking for the inverter for each string
- Poor flexibility at partial shading
- Higher per Watt cost than central inverter
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.
Advantages of Micro Inverters
- Resilience to partial shading effects as compared to the central and string inverters.
- MPPT at module level
- Highest system flexibility for future expansion
- Minimum DC wiring costs
- Monitoring at module level
Disadvantages of Micro Inverters
- High per Watt cost
- High maintenance costs
- Difficult access for maintenance since the installation is under the PV modules
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.