Central PV Inverter vs String PV Inverter
Are you thinking to build or develop a PV plant, and you don´t know what type of inverter to choose? this article will help you.
Where is your location?
The power capacity of central inverters leads to the need for active cooling. Inverter cabinets are constructed with fans, filters, and vents to allowing cooling.
The smaller capacity of string inverters eliminates the need for active cooling.
The layout of the inverter components inside the enclosure allows for the heatsink to share one wall of the enclosure so that outside air will passively cool the device.
Therefore, fans, air filters, and vents are not required.
In both the central and string inverter plant designs, the output power of the various inverters can be adjusted dynamically to maintain optimal performance of the attached arrays.
In the string inverter case, more granular control is achieved as each inverter has a much small number per MPPT. Therefore, if the performance across arrays varies due to non-uniform shading, arrays with different tilt angles or orientations, or damaged modules, the performance of each array can be optimized at the array level such that output of the system is maximized.
One of the more performance impacts in the plants happens when an inverter fails. If a central inverter goes down, a significant portion of the plant’s output is lost until functionality is restored. The string inverter approach, meanwhile, results in only a small number of arrays going offline if an inverter fails. Depending on the availability requirements of the project, a distributed approach may provide an advantage.
The string inverters provide the ability to capture the performance of the system in more detail than central inverters, allowing for the possibility to optimize and monitor the performance at the array level.
Central inverters employ the use of an isolation transformer in their component architecture where string inverters are transformer-less. The impact on plant design is such that central inverter-based systems will allow for the negative side of the module arrays to be grounded where in the string inverter-based design, the negative side will be floating. In the string inverter case, this means the modules could be exposed to potential induced degradation (PID). PID leads to severe degradation if the modules are not designed to be PID resistant.
The cost of string inverters will be only slightly higher than central inverters.
Central inverters typically have a more involved commissioning process than string inverters.
Our recommendation: bear in mind the work, equipment, quantity of components, maintenance requirements, commissioning, and replacement costs and their impact in the project costs.
The final comparison:
| Central Inverter | String Inverter | |
| Capacity | Up to 5MWp (Master and Slave) | up to 150kWp |
| Installation | Concrete pad/steel skid crane lift | Rack mountable, 2 people is enough to lift |
| Design | Isolation Transformer, typically grounded, air or liquid cooled | Transformer-less, typically floating ground, air or convection cooled |
| Design Life | 20 – 25 years | 20 years |
| warranty | 5 years | 10 years |
Written by Alberto Ruiz