Advanced solar microinverters lay groundwork for smart grid

BY RAGHU BELUR
Cofounder, Vice President of Products and Strategic Initiatives
Enphase Energy, www.enphase.com

In the span of a few years, the photovoltaic (PV) microinverter has evolved from an intriguing and potentially disruptive technological advancement to a market-leading product category in the residential and commercial solar installation market. Millions of devices have been deployed and have already helped generate terawatt-hours of electricity from hundreds of thousands of rooftop and ground-mount PV systems.

Although microinverters in and of themselves are not a new idea, the key to their recent success is a scalable electronic topology rooted in a silicon-based digital architecture. An adaptive ASIC-based approach yields improved efficiency and reliability, plus reduced cost. Combined with a bidirectional communications hub and sophisticated monitoring software platform, microinverters offer several advantages over the more established string inverter systems.   

With a traditional inverter, any shading on a single solar panel will limit the entire system output. It is a least common denominator type system. With microinverters power loss is confined to the panel(s) with reduced output, without inhibiting the performance of the other modules.

Since their introduction, the leading microinverter systems have gone through multiple generations, with their California Energy Commission (CEC) rated efficiencies rising from less than 95% to 96.5% at 208 V or 240 V (AC) and static maximum power point tracking (MPPT) efficiencies now averaging 99.4%. The peak output power of a top-of-the-line microinverter has increased to 250 W (ac), compared to the 175 W of the first-generation devices. The latest output rating allows the units to be compatible with 60-cell PV modules with up to 300 W (dc) nameplate capacity.

Systems undergo testing based on tens of thousands of grid conditions collected from industry sources.

 As the efficiencies have increased, so has the quality and reliability of the devices, which are now subjected to at least 1 M hours of various accelerated life-cycle testing and regularly come with 25-year warranty packages. Before any new design is released into production, it must go through a rigorous set of engineering, system, and component tests. Multi-environmental overstress testing helps assess the limits of temperature, humidity, and vibration that the units can take.

Fig. 1: Solar installations are made faster fourth-generation microinverter technology.

The test systems “learn” from a growing library of tens of thousands of grid conditions collected from industry sources and active systems across the planet. This library is leveraged during the product development process to simulate grid fluctuations, identify possible stress points, and make the units more robust in a myriad of adverse conditions.

Other hardware enhancements include the introduction of standardized “integrated ground” capability in the microinverter system. Bundling the equipment grounding into the cable and the inverter itself and isolating the dc circuit from ground within the microinverter allows solar system integrators to design and deploy ungrounded PV systems that are easier and less costly to install. Systems are also safer from an electrical shock and fire standpoint than standard “grounded” systems.

To meet NEC 690.35 requirements for ungrounded photovoltaic power systems, these installations must be able to operate with an ungrounded PV source and have output circuits that are properly equipped with disconnects, overcurrent protection, and ground-fault protection.

As the amount of solar power on the grid increases, inverter manufacturers need to provide innovative technological solutions that will allow their products to play a larger role in the arena of Advanced Grid Functionality — known more commonly as the “smart grid.” Wind and solar are inherently intermittent power sources, and their impact on the electrical infrastructure becomes more challenging as their role as generating assets grows. The utility companies are insisting — and utilities commissions are mandating — that the inverters incorporate a range of automated control capabilities that can help smooth out potential grid fluctuations. The inverters must become even smarter.

Fig. 2: In this Enphase microinverter, the dc circuit is isolated and not grounded, simplifying installation and reducing material costs.

Microinverters already play a key role as the operating system of photovoltaic arrays and will be expected to provide comprehensive integration and communication capabilities. Fault ride-through, ramp-rate and reactive power control, power curtailment, voltage support through VARs, and other advanced inverter features will help the utilities deal with momentary disturbances, supporting their efforts to operate the grid and making it possible to deploy much more solar in a safe and reliable way. Industry and utility stakeholders are working together to update the necessary national standards (such as UL/ANSI 1741 and IEEE 1547) and regulatory compliance testing protocols that will help ensure the transition to an advanced, stable grid.

Although the AGF/smart-grid era is in its early days, microinverter-based systems have already proved their efficacy in a state where solar accounts for a substantial and growing portion of electrical generation – Hawaii. One major island utility was experiencing greater voltage fluctuations than in the past and needed to create a wider voltage window to compensate for the variability because of the increasing number of solar installations in its service area. The utility requested that the inverters expand their frequency trip limits from 59.6 Hz to as low as 57 Hz, and insisted that these changes be retrofitted onto PV systems that had been installed in the preceding years. At a network operations center in the San Francisco Bay area, technicians were able to meet the utility’s new requirements and upgrade the software remotely through the bidirectional communications link on more than 400 PV systems. In addition, the company’s proprietary software platform provided the utility with an automated audit trail of all systems affected by the changes.

One example of the latest in microinverters is the Enphase fourth-generation unit, the M250. It has undergone an unprecedented one million hours product testing and features a reduced parts count, an integrated ground that removes the need for a copper grounding wire or WEEB, plug-and-play Engage cable connectors, and 96.5% CEC efficiency.

The advent of software-defined smart PV microinverters designed to work effectively with the advanced utility grid of tomorrow is only part of the application story. Companies like Enphase are also engaged with partners to integrate microinverter systems with energy storage devices and as key energy-efficiency components of home automation platforms.