Outback Radian AC Coupling Note

Application Note
©2019OutBackPower,Arlington,WA98223FB‐MM‐2/22/2019Page5of12  
Example: There is a grid outage and the Radian becomes the new AC source for the GTI which then
delivers 3400 watts to the load. The load then drops to 1,000 watts, meaning the other 2400 watts
(10 Aac at 240V) will come back through the Radian inverter to the battery bank. If the 10 Aac GTI
charging current is less than the AC Charging limit, then it will continue delivering charge current to the
battery bank. If greater than the AC Charging limit, the Radian output frequency will start to rise until the
GTI reduces its output if Freq/Watt compliant, or just go offline if non-Freq/Watt compliant. If the Radian
backfeed current to the battery bank stays below the AC Charger limit, but the battery voltage eventually
rises above the active voltage target (Absorb, Float or SellRE), then frequency shift will also be enabled.
There is a significant installed base of non-Freq/Watt legacy GTIs that will just simply go off line and wait
to reconnect for the time specified by the local jurisdiction (usually 5 minutes in N. America). Newer
Freq/Watt compliant inverters may actually “feather” back their output to allow some degree of charging
regulation to the battery bank. Depending on the difference between load demand current and GTI
output current, this feathering back of the GTI output may or may not be enough to prevent the frequency
shift from reaching its 64.5 Hz maximum and shut down the Freq/Watt compliant GTI as well.
IMPORTANT:
The GTI connect time must greater than or equal to the Radian connect time.
If the GTI connect time is shorter, the battery bank is full and grid power comes
back, then the Radian will try to feather back the GTI charging current with a
frequency shift that will never allow the Radian to reconnect to the grid. The cycle
will keep repeating until the GTI is no longer receiving power from the PV array.
Should the GTI go offline, then the Radian inverter is powering the loads as in the backup mode of
operation. See Figure 5. After the reconnect time period, the GTI will try to reconnect to the Radian
inverter’s output voltage where the cycle would repeat again until the load or battery charging demand
increases, or PV production goes down, or some combination thereof.
Figure 5 Off-grid current flow due to PV loss or GTI power
overcharging battery bank
Grid
Loss
PV
Loss
