Many times when designing systems we end up with “in-between” sizes as we match the customer’s available roof space, power requirements, inverters and budget. If the grid-tied, battery backup system design calls for 10kw and the inverters come in 8kw increments (like the Radians) the system will either be under or oversized. This is the perfect application for “AC Coupling.”
By connecting the grid-tied inverter (we typically use SMA) directly to the emergency load subpanel and back feeding it, we can increase our system output. When coupled correctly, the grid-tied inverter “sees” the grid through the Outback when the grid is available.
When the grid goes down, the Outback’s pure sinewave output and super fast automatic transfer switch tricks the grid-tied inverter into thinking it is still online. The additional power is available for loads while off-grid and for charging batteries.
In cases where an Outback system is already installed and the customer wants to expand AC coupling, this configuration can work well. Many times the inverter and/or charge controllers are maxed out leaving no room for expansion. AC coupling allows additional kw to be added seamlessly into the system.
Design parameters must be followed so the current range for the specific Outback inverter being used is not exceeded. In this case, a SMA-2000HF was used. Caution must be taken not to overcharge the battery bank as the grid-tied inverter output is unregulated. This means it has no way on its own to shut off to prevent overcharging. This is remedied with a relay in cases where the charging current will exceed the battery bank charge limits.
It is also possible to AC couple the smaller, 3.6kw Outback FlexPower 1 but it needs the PSX-240 transformer to operate. In this application the system size was 5.25kw and the backup loads were small so only a FP-1 was needed. It was AC coupled with a SMA-2000HF as well.
Feel free to contact me with your questions and comments.
Jeff Shaw
Owner, Gulf South Solar
jeff@gulfsouthsolar.com
