Outback Power Technologies
The Outback FX (FX, VFX, GVFX or GTFX) single-phase inverter/charger series and Radian series support AC coupling. Unlike their competing inverters, Outback Power inverter/chargers do not utilize frequency-shifting. The FX series inverter/chargers can be stacked in series and parallel in AC-coupled systems. When a single FX series inverter/charger with a 120 Vac output is AC-coupled to a 240 Vac string inverter, an Outback autotransformer can be used to step down the string inverter to 120 Vac.
The Outback Radian GS8048 is a grid-interactive 120/240 Vac inverter/charger that can be stacked in parallel. The Radian normally functions as the battery-based inverter in an AC-coupled minigrid, but has a Utility-Interactive mode that can feed excess power to the grid, but at a lower efficiency than a grid-tie inverter. The Outback inverter/charger is recommended to handle 125% of the maximum output of the string inverter.
Both Outback Radian and FX inverter/chargers do not have frequency-shift battery management capabilities. The Radian supports diversion loads and blackout relays, and the FX only supports blackout relays.
Outback Power maintains that AC coupling has trouble with systems involving backup-generators. For backup-generator-based system charging, the battery-based inverter will shift its frequency to synchronize with the generator and disconnect from the string inverter – at which point the generator becomes the voltage/frequency source for the system. This creates two problems. Firstly, most motor generators do not have sufficient voltage/frequency regulation for the grid-tie inverter to stably operate, so PV generation is lost. Secondly, the generator has to power all AC loads and charge the batteries because the string inverter is offline, which will require a larger generator that in AC-coupled systems.
Schneider Electric recommends pairing their TX grid-direct inverters with their XW battery-based inverters to avoid complications between inverters from different manufacturers in AC-coupled systems. Unlike Magnum Energy, Schneider does not support systems in which their battery-based inverter is AC-coupled to a grid-tie inverter of a different manufacturer. As a result, they only discuss the interaction between the TX and XW inverters in their contribution to this article.
Schneider XW inverter/chargers with firmware version 1.07 (downloadable here) include the AC-coupling feature. The Schneider XW uses frequency-shifting and on/off cycling to prevent overcharging similar to the Magnum inverters. The Schneider XW inverter is recommended to handle at least 100% of the grid-tie inverter power rating.
When one of CivicSolar's customers asked for help designing an AC coupled system for a client, all available solutions were analyzed - even coupling Enphase micro-inverters to a Xantrex XW 6048 inverter. While all the other components - Xantrex, Outback, Enphase and Magnum, are very high quality equipment, none of them offered as highly evolved a system as the SMA Sunny Island 5048 (SI) and Sunny Boy US (SB) inverter pair.
The unique advantage of the native SMA setup is that the grid tied and battery inverters employ direct rs485 communications to regulate the grid tied inverter's output, rather than bringing it to a hard stop and then restarting it 'cold' with a five minute delay. No relays or diversion loads are specifically required to protect the batteries or send the AC charger into an on/off mode.
The customer wanted to minimize cost by only installing one SMA SI 5048 - requiring an autotransformer to generate the necessary 240Vac to sync with the SB inverter. An Outback 6kVa Autoformer was used, but SMA also offers a Smartformer product that could have done the job as well.
The information and views expressed in this blog post are solely those of the author and not necessarily those of RenewableEnergyWorld.com or the companies that advertise on this Web site and other publications. This blog was posted directly by the author and was not reviewed for accuracy, spelling or grammar.
To add your comments you must sign-in or create a free account.