AC Coupling in Utility-Interactive and Stand-Alone Applications: Page 10 of 16

In both utility-interactive and stand-alone systems, ac coupling offers a scalable system platform based on grid-quality alternating current. Due to its modular design, suitable applications for ac coupling range from small off-grid or grid-tied residential systems to large stand-alone microgrids developed for village electrification projects. AC-coupled systems are more scalable than dc-coupled systems and can seamlessly integrate diverse charging sources, including PV arrays, wind and hydro turbines, and ac backup generators. In addition, ac-coupled systems can be more easily expanded than dc-coupled systems as ac load and generation capacity is added.

The components that make up an ac-coupled system are themselves another benefit of ac coupling. BOS components for an ac-coupled system are considerably less expensive and more readily available than dc-powered alternatives and simplify everything from system design to ongoing maintenance. AC coupling offers increased planning flexibility, since long distances between the power supply, batteries and loads do not pose the same limitations that they do in high-current dc systems. In ac-coupled microgrids, the connection of additional ac-power sources and loads is possible at almost any point in the system, allowing for subsequent expansion even years after the initial installation.

SMA is no stranger to ac coupling. In fact, it researched the feasibility of this approach to battery-based systems throughout the 1990s with the Institut fu¨r Solare Energieversorgungstechnik with funding from the German Ministry of Finance and Technology. By 2005 it had commercialized ac-coupling technology with the development of a fully integrated system platform that utilized grid-direct and battery-based inverters and controls. Today, SMA continues to expand its offerings for ac-coupled systems with the launch of new products like its Multicluster Box off-grid ac distribution solution.

INTEGRATED EQUIPMENT 
One of the strengths of SMA’s offerings for ac-coupled applications lies in the highly integrated nature of the products. SMA Sunny Island battery-based inverters and Sunny Boy and Windy Boy grid-direct inverters can be used in conjunction with one another and with backup generators to form a highly integrated stand-alone ac power grid.

To complement the existing Sunny Island 5 kW 5048-US inverter/charger, SMA will soon add two new models to its battery-based inverter family: the 4.5 kW 4548-US and the 6 kW 6048-US. SMA Sunny Island inverters feature peak dc-toac conversion efficiencies of above 95% and offer high-surge capabilities for starting and supporting large inductive loads such as motors and water pumps. For example, the Sunny Island 5048-US model can provide 6.5 kW output for 30 minutes, 8.4 kW for 1 minute and 12 kW for 30 seconds at 25°C. US Sunny Island inverter models have single-phase 120 Vac output. Multiple-inverter systems can be configured for single, splitphase or 3-phase Vac.Sunny Island inverters. SMA’s ac-coupled solution is driven by the bidirectional Sunny Island inverter/charger, which is equipped with sophisticated ac grid-management functions and highly developed battery management that includes full system monitoring. The Sunny Island continuously tracks the batteries’ state of charge, and as system manager makes ongoing decisions based on those states. For example, during operational periods where the batteries are discharged and there is little solar generation, the Sunny Island can remotely and automatically start a backup generator, or even switch off consumer loads using a built-in load-shedding relay parameter. The Sunny Island also determines the optimum strategy for charging the batteries, and in doing so increases their service life.

Sunny Boy inverters. SMA manufactures a wide range of griddirect string inverters that can be used in ac-coupled applications. With rated power outputs of 700 W to 10 kW, these inverters can be fully integrated with and controlled by SMA Sunny Island inverters in ac-coupled systems. In addition, SMA manufactures Windy Boy grid-direct inverters that process power from wind turbines; these can also be integrated with an ac-coupled system platform.

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