SMA SI 4548-US - Manual (Page 136)

14 Connecting External Sources SMA America, LLC
136 SI4548-6048-US-BE-en-20 Operating Manual
Energy of the PV inverters or wind power inverters not being consumed in the off-grid system, is fed-in
by the Sunny Island via the internal transfer relay into the utility grid. The Sunny Island prevents an
overload of the internal transfer relay. For this reason, reverse power monitoring is used that, if
required, disconnects the connection to the utility grid if the reverse power limitation is exceeded or if
the relay is subject to overload.
If the battery is fully charged, the frequency limits the power output of the AC feeding-in generators
(PV inverter) in the stand-alone grid. If the utility grid is now reconnected, the frequency would be
lowered, if required, as the Sunny Island is synchronized with the grid. The AC feed-in generators
would then feed additional energy into the system and possibly overload the batteries. In order to
prevent this, the stand-alone grid frequency is temporarily increased, in line with the synchronization,
until the AC feed-in generators disconnect from the stand-alone grid as a result of the grid limits being
exceeded.
14.3 Generator and Utility Grid
In addition to the utility grid, a generator can also be integrated into an off-grid system as a secondary
protective measure. This is particularly useful in case of long-term grid failures, even if the battery
capacity is no longer sufficient to bridge the failure after a period of time.
The common solution in such cases is using a transfer switch which can be purchased as a manual or
automatic switch. By using such a switch, a diesel generator is connected to the AC2 terminal to which
the utility grid is normally connected, as displayed in the figure below:
To use such a switch, carry out the installation as follows.