AC Coupling in Utility-Interactive and Stand-Alone Applications: Page 13 of 16

DC-coupled PV system. PV production x charge controller efficiency x battery-based inverter efficiency = available ac power; 10,000 Wdc x 0.95 x 0.95 = 9,025 Wac

When evaluating the cost of ac- versus dc-coupled system designs, account for all materials and installation-labor savings on the dc side of the system. In ac-coupled systems, the grid-direct inverter essentially replaces the dc-charge controller, but ac coupling may also eliminate the need for a source-circuit combiner box if PV source-circuit fusing is included in the ac-coupled grid-direct inverter.

Make sure to compare the power output from the ac-coupled portion of the PV system with the pass-through capability of the battery-based inverter when designing ac-coupled systems. This need applies to both grid-tied and off-grid applications. Additionally, be sure to match the grid-direct inverter voltage requirements with the battery-based inverter voltage output. For example, grid-direct inverters that have 240 Vac output cannot be ac coupled with a single battery-based inverter with 120 Vac output without the addition of a step-up/step-down transformer that lowers overall system efficiency. The SMA Sunny Island system requires an RS485 communication cable between the Sunny Boy and Sunny Island inverters. This enables full communication between the inverters and allows the battery-based Sunny Island inverters to modulate the power output of the string inverters based on ac load requirements and battery state of charge. While the communication cable can connect inverters over a distance of up to 3,937 feet if necessary, it is advantageous to group inverters as close together as possible.

KELLY KEILWITZ, PRINCIPAL, WHIDBEY SUN & WIND

Our most complex ac-coupled system integrates a 21 kW PV array and a XZERES 442 wind turbine with two SMA Sunny Island 5048-US inverters. The system utilizes a transfer-relay contactor to switch the PV output from the grid-connected main panel to the critical load panel powered by the Sunny Islands when the grid goes down. This strategy avoids an ac amperage pass-through bottleneck through the two Sunny Islands during normal system operation when the grid is up.

From my perspective, ac-coupled systems offer the following advantages:

  • The efficiency of grid-direct inverters is much higher than the efficiency of a dc-coupled battery-charging system.
  • PV array stringing options, combiners, and wiring are simpler and more flexible in ac-coupled systems than in dc-coupled systems.
  • The material and labor cost of installing an ac-coupled grid-direct inverter is comparable to the total cost of a charge controller, a dc-integration panel and the associated equipment required for dc-coupled systems.
  • Some inverter combinations, such as those used in SMA’s Sunny Island system, work in harmony to reduce griddirect– inverter output without fully disconnecting the grid-direct inverters. This allows for regulated battery charging when the system is in Off-Grid mode and generation exceeds the ac load.
  • Renewable energy production metering is simpler in ac-coupled systems because it is not necessary to account for grid and renewable energy generation that is consumed by critical loads or battery charging.

Some disadvantages of ac-coupled systems are also critical to consider:

  • When a system is in Off-Grid mode during utility outages, many ac-coupled–inverter combinations require an ac relay controlled by the battery-based inverter’s aux output to disconnect the grid-direct inverter when generation exceeds the energy consumed by the critical loads. This disconnect relay can cycle repeatedly, resulting in a loss of energy production during the 5-minute resynchronization waiting period every time the grid-direct inverter reconnects to the system.
  • The capacity of renewable generation sources is limited by the ac pass-through capacity of the battery-based inverter system. For example, the battery-based inverter capacity may need to be increased to handle the production of a large PV system even if the critical load requirements do not call for it. One work-around is a fairly complex contactor-relay setup that switches the grid-direct inverter output between the main electrical panel and the critical load subpanel.
  • While the frequency-shift regulation approach used in SMA’s Sunny Island system works well, the installation manual is not as clear or complete as it could be. That said, SMA’s documentation, experience, understanding and support for ac-coupled systems is likely better than that of other manufacturers. Expect to develop a close relationship with the inverter tech support people. Good cellphone coverage from the job site is a big advantage.
  • If the grid-direct inverter has a possibility of syncing with a backup generator, I instruct the system operator to disconnect the grid-direct inverter before running the generator. The operator can accomplish this with the installation of an either/or interlock switch.

DANA BRANDT, OWNER, ECOTECH ENERGY SYSTEMS

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