No discussion of PV systems would be complete without a look at the newest inverter technologies that installers and inspectors will encounter. These new technologies include microinverters and AC PV modules.
Most grid-tied inverters are “string inverters”—they operate with a string of series-connected PV modules. These inverters range in power from 700 watts up to 1 megawatt. DC maximum system voltages can be as low as about 125 volts.
Enphase’s microinverter, which hit the market in 2008, is a tiny inverter—about the size of a video cassette—that is designed to work with a single PV module and operate at a maximum of about 70 VDC (for one Enphase model). The inverter is electrically connected directly to the PV module using the existing conductors and connectors attached to both the module and the inverter.
Microinverters are grid-tied inverters with DC ground-fault protection (National Electrical Code Section 690.5) in the current offering. Enphase’s microinverter internally grounds the positive DC module conductor. That internal grounding bond (via the DC ground-fault protection circuits; NEC 690.5) requires that the inverter have a DC grounding electrode terminal and that the terminal be on the outside of the Enphase microinverter case. Other types and brands of microinverters may ground differently or switch to an ungrounded configuration using modules with the new “PV Cable” required by NEC 690.35 for such systems.
With both AC input and output connections, microinverters have been UL-listed to allow multiple inverters to be connected on the same cable and circuit. With a power output in the 175 to 210 W range (depending on the model), the rated AC output current at 240 V will range from 0.73 to 0.88 A. On 14 AWG cable with a 15 A overcurrent device, the rated current for that circuit is limited to a maximum of 12 A. This rating will allow up to 16 inverters for the 175 W version and up to 13 inverters for the 210 W version to be installed on the same AC output cable.
Take a standard DC PV module and connect a microinverter to it, fasten the microinverter to the back of the module and cover the exposed DC conductors so none of them are accessible, and secure a listing to UL1741 for a pre-assembled module/inverter device, and you have an AC PV module. At least one AC PV module is available—Akeena Solar’s Andalay AC PV module. Since the DC wiring between the module and inverter is no longer accessible and has become an integral part of the product, NEC DC requirements no longer apply to the AC PV module.
Akeena’s module has a unique frame that serves as the module mount. Because each module is connected with stainless steel threaded rods to the adjacent modules, only one equipment grounding connection is needed for the array. The AC PV module has no requirement for DC grounding electrode terminals, and the single AC equipment-grounding conductor from the module/frame rack may be all that is needed in many installations. In ground-mounted arrays, Section 690.47(D) requires an additional grounding conductor from the array frame to a grounding electrode.
Should the microinverter or module fail, the microinverter’s DC connection to the PV module will need to be disconnected for replacement. With today’s inverter designs, the maximum voltage will be about 70 V and the current may be in the 3 to 8 A range—posing risk of damage (and creating a possible safety hazard) to the connector. While some inspectors may request a costly and impractical load-break rated disconnect, the code-compliant solution is really quite simple: Covering the module with a thick blanket or other opaque material per NEC 690.18 will drop the DC output voltage and current (and the AC current from that inverter, but not the other inverters connected to the same cable) to near zero, allowing the module/inverter DC connectors to be opened safely. Opening this connection with the module covered will likely be safer than opening the same connectors on a module in a high-voltage string of modules. The AC PV module has no accessible DC connections, so this issue is avoided.
Each microinverter or AC PV module has an AC input/output cable to allow multiple parallel inverter connections. Under bright sunlight conditions, this cable may carry currents ranging from 0.7 A at 240 V from the first module/inverter in the set to as much as 12 A at 240 V through the last connector of a set with multiple devices. Servicing a single AC PV module or utility-interactive microinverter could be accomplished by covering the module to reduce the DC (and hence the AC) current to zero. However, only covering the module in question in the set would still allow current from the other modules/inverters to flow through the cable. Although inverter anti-islanding circuits minimize the hazard somewhat, since they shut down very rapidly and reduce arcing when the AC connector is opened. Opening these 240 VAC connections under load could damage the connector and pose a shock hazard.