Next Generation String Inverters

Competitive market conditions, technological advances and Code changes will continue to evolve string inverter products. In general, industry experts expect the string inverter products of the future to be smaller, more efficient and reliable. For example, Richard Scott, KACO product development manager, expects to eventually see 99% efficient production string inverters.

The increasing commercialization of transformerless inverter designs is one way that string inverters are expected to achieve higher efficiencies in a smaller package. If efficiency gains and cost reduction benefits overcome perceived safety concerns,  theorizes Solectria's Worden, then transformerless string inverters will gradually become more standard and accepted. Other inverter manufacturers agree and are planning new products for the North American market accordingly. SMA America Technical Sales Support Manager Verena Arps notes: We expect to see Code changes in the near future that will allow a full adoption of transformerless inverters in the US market. This technology provides additional benefits to the decentralized string inverter concept. The topology already used by SMA in Europe achieves 98% efficiency, ensuring high energy yields and resulting in the shortest possible payback.

One possible Code change that may influence string inverter design in the near future is a draft proposal of Article 690.11 that recommends dc arc-fault protection for PV systems. When dc PV arc signatures are defined and detector circuits are available, explains Wiles, the inverter will be a prime location for a series arc fault detector/interrupter that will shut off the inverter to deal with the series arc. Others are less confident in this analysis, including KACO's Scott, who argues: The proposed dc arc fault detector/interrupter change makes no sense and should not be put into the Code as written. The arc protection must be as close as possible to the array, the dc source. Locating dc arc fault protection at the inverter does not address parallel and ground arc faults, which will not be stopped by removing the inverter load.

Inverter manufacturers also expect California Energy Commission requirements to continue to refine string inverter offerings. According to Xantrex's Edmunds, Requirements in California for inverter meters to have traceable calibration will lead to more accurate power and energy metering for all applications.

REF: David Brearley / SolarPro magazine