SMA Off-Grid Systems - Manual (Page 11)

SMA Solar Technology AG 3 Information on Off-Grid Systems and System Description
Installation - Quick Reference Guide Off-Grid-IS-en-30 11
3 Information on Off-Grid Systems and System Description
3.1 Off-Grid System Functions
Off-grid systems with Sunny Island inverters are self-sufficient utility grids that are being fed with energy from several AC
sources in the stand-alone grid (e.g., PV inverter), from an external AC voltage source (e.g., diesel generator), and/or
with DC charge controllers (e.g., Sunny Island Charger). The Sunny Island forms the stand-alone grid as a voltage source
and provides active and reactive power. The Sunny Island regulates the balance between the energy fed-in and energy
used and has a management system with battery and generator management and load control.
In order to be able to distribute the output power of the off-grid system to the loads, several Sunny Island inverters can
be connected modularly in an off-grid system.
Battery Management
Battery management of the Sunny Island inverter is based on precise determination of the state of charge. By combining
the three most common methods for recording the state of charge, the Sunny Island reaches a measuring accuracy of
more than 95%. This way, overcharge and deep discharge of the battery are avoided.
A further feature of battery management is the extremely gentle charging control. It automatically selects the optimum
charging strategy for the battery type and the situation in which it is used. This means that overcharging can be reliably
prevented and that the battery can be fully charged regularly. The available charge energy is used optimally at all times
(see Technical Information "Battery Management" at www.SMA-Solar.com).
Generator Management
The Sunny Island inverter generator management allows for uninterruptible connection of the stand-alone grid to the
generator and uninterruptible isolation from the generator. The generator management controls the generator via a start
and stop signal. A generator current control ensures that the generator always remains at the optimum operating point.
The generator management allows the use of generators that have a low output power in proportion to the nominal load
(see Technical Document "Sunny Island - Generator Whitepaper" at www.SMA-Solar.com)
Load Control
The load control enables control of the AC sources in stand-alone grids, control of a generator, and the specific
disconnection of loads.
The AC sources in the stand-alone grid are limited in their power output by the stand-alone grid frequency. In case of
excess energy, the load control increases the power frequency. This limits the output power of the PV inverters, for
example.
If there is not enough energy available for all loads or the battery is to be preserved, load control can request energy
from a generator by means of the generator management. The generator management starts the generator and the
off-grid system is supplied with sufficient energy.
If there is no generator present in the off-grid system or the energy is not sufficient despite the generator being available,
load control turns the loads off using load shedding. All loads are shed simultaneously with one-stage load shedding.
A load shedding contactor sheds the noncritical loads during the first stage with two-stage load shedding. The remaining
loads are shed during the second stage only when the state of charge declines further. This can further increase the
availability of the off-grid system for critical loads.