Outback Radian AC Coupling Note

Application Note
©2019OutBackPower,Arlington,WA98223FB‐MM‐2/22/2019Page4of12  
Theory of Operation – With Grid Outage
During a grid outage when the sun is shining, the Radian inverter becomes an AC source to which the
GTI can synchronize. This allows the PV power to flow to the backup panel’s connected loads, as well
as charge the batteries if the GTI is generating more power than can be absorbed by the loads.
Figure 4 shows all possible current flows, the paths of which can change depending on several different
factors. If the PV generation can satisfy the backup panel and battery charging loads, then PV power
flows in those two directions. If the backup panel load demand exceeds the GTI power generation, then
the Radian inverter will stop charging the battery (if Absorb or Float charging is active) and invert DC
power from the battery bank and contribute current to the backed up loads in parallel with the GTI.
Figure 4 AC Coupled current flows with sufficient load demand
and frequency shift curtailment
If the batteries become fully charged and the load demand falls below the GTI power production, then the
excess power from that production will flow back through the Radian inverter in an unregulated charge
back onto the batteries. When the battery bank voltage raises more than 0.4V volts above the active
charging voltage target then the Radian inverter begins to shift its output frequency above 60 Hz until the
battery voltage starts to level or drop off, but not above 64.5 Hz. The inverter’s frequency shift to a
higher frequency will also be enabled if the AC charge current coming back onto the battery bank
exceeds the AC Charge Current limit setting.
Grid
Loss