Microinverters are not a new concept. Since the earliest days of the PV industry, solar engineers and entrepreneurs have sought to create grid-compatible AC power direct from a solar panel. Some of the first commercial microinverters appeared in the mid-1990s, including the SunSine AC Module developed by Ascension Technologies. In fact, one of the first ACPV systems, delivered to the U.S. Pentagon in 1994, is still functioning today.

Microinverters all but disappeared from the market in the early 2000s, due to their high cost and relatively low efficiency. But over the last five years, rapid advances in semiconductor technology and improvements in efficiency and reliability have made microinverters a viable alternative to central inverters. Today, ACPV systems account for more than 30% of all residential and small commercial (<100kW) installations in the U.S., according to IMS. And their share is expected to continue to grow.

In fact, many industry analysts agree that in the next five to ten years all solar panels will include some form of distributed electronics. These could include DC to DC optimizers, detached microinverters and a new category of AC modules with integrated microinverters.

Advantages of ACPV
Manufacturers say the advantages of solar microinverters include a simplified design, installation, and most compellingly, improved uptime over other systems as there is no single point of failure in the PV system. System owners easily comprehend the bottom-line benefits of module-level monitoring and maximum power point tracking. It’s actually hard to understand how the rooftop PV industry has grown to the point it is today without system owners having regular awareness of how their own systems are performing.

Microinverter monitoring systems help operators keep tabs on an entire portfolio of systems at the module—rather than the system—level. No more time-consuming truck rolls to diagnose a non-functioning array; module-level monitoring pinpoints the performance of every solar microinverter and module. Consumer views of monitoring systems provide PV owners peace of mind that their system is running properly. When presented as part of a module company or installer’s offering, monitoring systems are a nice added value service.

Detached Micros vs. AC Module Systems
The category of “ACPV systems” includes both detached microinverters and integrated AC modules, reflecting the two ways these systems are wired together in the field:

• Detached microinverter systems: Individual solar microinverters and PV modules are installed as a pair (making a DC connection in the field), both units typically within a foot of each other. The assembly is then wired in parallel with other assemblies creating an AC branch circuit.
• AC module systems: Microinverters are factory-installed onto PV modules and the integrated units are brought to the field as a certified assembly. AC modules are wired in parallel to other AC modules, creating an AC branch circuit.

It’s important for installers to understand the regulatory and practical differences between these two systems. The NFPA 70 National Electrical Code (NEC) defines an AC module as a complete, environmentally protected unit consisting of solar cells, an inverter and other components, exclusive of a tracking system, designed to produce AC power from the sunlight. AC modules must be evaluated by a Nationally Recognized Testing Laboratory (NRTL) using both UL 1741 and UL 1703. Detached solar microinverters are tested to UL 1741, but not UL 1703.

“Combinations of PV modules and microinverters combined/assembled in the field or at the dealer or distributor do not meet the intent, definition or requirements associated with true ACPV modules as defined in 690.2 and in 690.6,” wrote John Wiles in his article, “Microinverters and ACPV Modules are Different Beasts” (IAEI News, May/June 2012).

There are also differences in the way AC modules are installed vs. detached microinverters. For example, AC modules do not require a separate equipment grounding conductor (EGC) run to the inverter or the frame of the module, because the frame is grounded to the microinverter chassis and bonded to the integrated EGC within the interconnecting cable system. The entire EGC path (including the grounding between inverter and PV module) is evaluated as part of the listing certification by the NRTL.

For more details on the differences between detached microinverters and AC modules, installers and inspectors should consult NEC Article 690, Section 690.6. In addition, SolarTech has published the Microinverter and ACPV Module Application Guide. To download a copy or provide feedback, visit www.solartech.org.

By: Nancy Edwards, Marketing Director at SolarBridge Technologies