In the past year, solar industry growth has become more sustainable. After facing countless challenges over the past decade, the industry has created solutions to improve a photovoltaic (PV) system operator’s overall solar energy experience, which includes the technology and usage of string inverters. Improvements in string inverter serviceability, higher National Electrical Manufacturers Association (NEMA) classification, and advanced power control features within these products are all contributing to increased solar industry confidence, which makes the future of solar power look promising.
Recent Developments
In basic terms, string inverters connect an array of multiple solar panels, and can be mounted either indoors or outdoors. This differs fundamentally from micro inverters, which are always roof-mounted directly under the solar panel it serves. For microinverter servicing, a panel must first be located and then disassembled by two technicians for safety reasons, according to U.S. Occupational Health & Safety Administration (OSHA) rules. Conversely, string inverters can be easily serviced where they are mounted, minimizing PV system downtime.
That’s just the beginning. Developments like installation-friendly features such as built-in string combiners and disconnects are helpful for installers and can also help better manage PV system costs through reduced equipment and labor. Increasing serviceability by offering string inverters with power stages separable from the wiring compartment, or even replaceable components, is definitely an added perk for installers by reducing service time while simplifying future inverter replacement.
Another positive development for string inverters is incorporation of arc fault circuit interruption (AFCI). The National Electrical Code NEC 2011 requires detection and interruption of “arcing faults resulting from a failure in the intended continuity of a conductor, connection, module, or other system component” (NEC 2011 Section 690.11). String inverter manufacturers have responded by developing and integrating technology that detects and extinguishes an arc fault. This protects both installers and PV systems operators from excessive downtime while also preventing potential damage due to fire.
Other innovations, such as SunSpec Alliance’s MODBUS communication protocol suite, are also improving overall sustainability of PV systems. New MODBUS cards allow string inverters to communicate with third-party monitoring systems and can easily be integrated into many inverters, so energy yield can be maximized.
The Future
New utility requirements for advanced control are on the horizon and string inverters will play an important role, especially with the growth of the smart grid. For example, advanced power control functionality allows for a higher penetration of PV systems on existing distribution circuits. In the past, inverters have experienced harsh conditions. NEMA classifications and environmental segregation have led to enforcement of more reliable inverter designs to limit moisture and containment intrusion.
Although the solar industry has seen many improvements, it does still see some challenges that string inverters can address. For example, the National Fire Protection Association (NFPA) implemented a change in NEC 2011 with the 2014 Rapid Shutdown requirement, which has to do with the safety of emergency and fire safety services in relation to solar electronic equipment. Specifically, it requires PV source circuits installed on roofs be de-energized from all sources within 10 seconds of when the utility supply is de-energized. This presents a challenge for all inverter designers, including those that manufacture string inverters, because they now have to design units so that responders can safely and easily de-energize the system in a very short amount of time.
However, this is emblematic of the solar industry’s continuing evolution and sustainable growth. This challenge, and others to follow, are bound to make string inverters that much more robust, reliable and versatile, able to handle demand for higher energy yields and increased return on investment.
By Thomas Enzendorfer, Fronius USA