AXIS Explores
Inverters
Overview
Inverters convert the Direct Current (DC) generated by the PV modules into Alternating Current (AC) for export to the grid.
For utility scale solar PV, two system designs dominate: the central inverter and string inverter approaches.
Larger projects tend to use central inverters since they represent a lower cost per watt, although string inverters are growing in size (1 MW +) and efficiency, becoming more viable.
Inverters have an average lifespan of approximately 10 years and inverter replacements account for approximately 10% of a site’s Operations & Maintenance (O&M) costs.
Central inverters
Central inverters are up to 4 MW in size and centralise the site’s power into fewer places than string inverters.
Central inverters are more complex and not all O&M teams are able to repair them on site. Newer models incorporate medium voltage transformers, adding complexity and cost.
It is uncommon for spare central inverters to be held on site.
The loss of a central inverter has a far larger impact on business interruption than the loss of a single string inverter and may stop the site from exporting any power.
The majority of large solar PV farms built 5-10 years ago used the centralized approach. These may now be coming to the end of their operational life.
String inverters
String inverters are smaller (typically up to 300 KW) and located at the end of each solar array. They are better able to optimize power production from each array, but there can be hundreds of them per site which may increase maintenance costs.
Failure of one string inverter only takes that specific array offline and they can be quickly swapped out with a spare unit.
On site O&M teams are usually trained in repairing string inverters.
All inverters should be housed in weather proof enclosures on concrete plinths high enough to prevent water ingress in the event of a flood, although quality can vary.
Types of failure
Inverters are the most heavily worked part of the PV system and failures are typically caused by:
- Capacitor failures caused by electro-mechanical wear. This can result in a fire
- Failure of the inverter bridge due to overuse, or overcurrent/overvoltage. This can be avoided by using fuses and circuit breakers, overvoltage requires surge protection devices
- Manufacturing defects e.g. inadequate insulation on the Insulated-Gate Bipolar Transistor (IGBT) causing an arc fire
- Overheating. Inverters require robust cooling, and dehumidifying systems in some regions
- Low quality equipment used on DC connectors or inadequate insulation
- Ultrasonic vibrations cause friction and increased heat. Often caused by dirty terminals on internal connections, maintenance is key here
