Types of Grid-connected Inverters

String Inverter

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. String inverters are commonly used in residential and commercial sites. Also, as technology improves allowing string inverters to have greater power density in smaller sizes, string inverters are becoming a popular alternative over central inverters in small utility installations smaller than 1 MW.

Advantages of a String Inverter

1. Smaller in size when compared to central inverters
2. Better MPPT capability per string
3. Scalability for future expansion by adding parallel strings
4. Short DC wires
5. Monitoring at string level

Disadvantages of a String Inverter

1. The installation requires special racking for the inverter for each string
2. Poor flexibility at partial shading
3. Higher per Watt cost than central inverter

Central Inverter

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 (combiner box) is required as an interface between the array and the grid. Designers can use one central inverter where all strings are connected to the DC side of the inverter and the single AC output is connected to the utility grid. They are best suited for large installations with consistent production across the array.

Advantages of a Central Inverter

1. The most traditional inverter topology
2. Easy system design and implementation
3. Low cost per Watt
4. Easy accessibility for maintenance and troubleshooting

Disadvantages of a Central Inverter

1. High DC wiring costs and power loss due to Voltage Drop.
2. Single MPPT for the entire PV system
3. System output can be drastically reduced in case of partial shading and string mismatch
4. Difficult to add strings or arrays for future expansion
5. Single failure point for the entire system
6. Monitoring at array level
7. Huge size! (It is a disadvantage because the bigger size requires more land and creates a shading issue for the PV array.)

Micro Inverter

Micro inverters are also referred 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. Each module has a dedicated inverter with an MPP tracker. Systems with microinverters can be more efficient, but these often cost more than string inverters.

Advantages of Micro Inverters

1. Resilience to partial shading effects as compared to the central and string inverters.
2. MPPT at module level
3. Highest system flexibility for future expansion
4. Minimum DC wiring costs
5. Monitoring at module level

Disadvantages of Micro Inverters

1. High per Watt cost
2. High maintenance costs
3. Difficult access for maintenance since the installation is under the PV modules

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