AC Coupling in Utility-Interactive and Stand-Alone Applications: Page 12 of 16

Since 2008, the islanders have reaped the benefits of a modern 3-phase electricity grid, 95% of which is supplied by renewable energy sources. This ac-coupled hybrid system integrates hydroelectric, wind and solar generation. Generator operation is limited to times when the combined renewable generation sources do not fully meet the load demand. Although gridquality power is now available 24 hours a day, electricity costs for the islanders have fallen by more than 60%.

The central element of the stand-alone grid is the SMA MCB-12, which serves as the distribution hub for four Sunny Island clusters. Each cluster is rated at 15 kWac. Three hydro turbines with a total generation capacity of 110 kW, four small wind turbines with a total capacity of 24 kW, and a 32 kW PV plant provide a diverse supply of electricity. Two 64 kW diesel generators serve as backup. Each cluster’s battery bank has a storage capacity of 2,242 Ah at 48 Vdc nominal and can meet the island’s load for approximately 24 hours.

During normal operation, the master cluster controls the entire grid and ensures that the distribution system’s energy balance is maintained at all times. Excess energy from the renewable sources is stored in each inverter cluster’s battery bank. When the battery is fully charged, the master cluster reduces the power output of the Sunny Boy and Windy Boy inverters using frequency-shift power control. It also activates remotely controlled diversion loads such as water-heating tanks located in public buildings.

The master cluster starts the diesel generator when the system’s battery state of charge falls below 60%. In this case, the diesel generator sets the network’s power frequency and the Sunny Island clusters are synchronized to the generator’s voltage and frequency reference. The overload capacity of the Sunny Island inverter/chargers makes an important contribution to the system’s operation. When large loads are cycled, the load on the generator does not immediately change because the Sunny Islands compensate for load fluctuations. In this application, the Sunny Island system can supply 144 kW of battery power to the grid for 3 seconds. In its role as grid manager, the master cluster weighs the alternatives of operating the diesel generator with the highest possible efficiency while delivering the appropriate charge current to the system’s battery banks. As a result, the generator runs less frequently, runs more efficiently under partial loading, and is not subjected to short start-and-stop cycles.

Integrator Perspectives on AC Coupling

On one hand, equipment manufacturers are able to provide valuable technical insights on the use of their products in ac-coupled applications. On the other hand, the diverse nature of these sometimes complex systems makes the lessons integrators have learned equally important in many instances. We surveyed several solar professionals with ac-coupled system design and installation experience who graciously devoted some time to sharing their experiences from the field.

TOMMY JACOBY, PRINCIPAL, JACOBY SOLAR CONSULTING

When compared to dc-coupled PV systems, ac-coupled systems may provide a more efficient means for utilizing array output if the majority of a site’s ac loads (including energy exported to the grid in utility-interactive systems) is utilized during peak solar production hours. Assuming that losses from conductors are equal in the two systems, and that the majority of the ac-load consumption corresponds with PV production, ac- and dc-coupled system efficiencies can be compared as shown below. This simplified comparison uses sample inverter and charge controller efficiencies. More accurate comparisons can be developed using equipment-specific efficiency figures. In addition, this example does not include any losses associated with array power passing through or over the battery bank, which would result in a more favorable power output advantage for ac-coupled systems.

AC-coupled PV system. PV production x ac-coupled inverter efficiency = available ac power; 10,000 Wdc x 0.95 = 9,500 Wac

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