SMA Sunny Boy Series - Safety; Intended Use; Safety Precautions

SMA Solar Technology America LLC 6 Frequency Shift Power Control (FSPC)
Technical Information SB-OffGrid-TI-US-en-19 7
6 Frequency Shift Power Control (FSPC)
If PV inverters are connected on the AC side in off-grid operation or back-up operation, the Sunny Island must be able to
limit their output power. This situation can occur, for example, when the battery of the Sunny Island is fully charged and
the PV power available from the PV array exceeds the power required by the connected loads.
To prevent the excess energy from overcharging the battery, the Sunny Island recognizes this situation and changes the
frequency at the AC output. This frequency change is monitored by the PV inverter. As soon as the power frequency
increases beyond the value specified by Fac-Start delta, the PV inverter limits its power accordingly.
Figure1: Operating principle of FSPC
The terms used have the following meanings:
• f
AC
refers to the base frequency of the stand-alone grid (here 60 Hz).
• f
AC
Delta- and f
AC
Delta+ are the maximum ranges relative to f
AC
in which the PV inverter is active.
•fAC Start Delta is the frequency increase relative to fAC at which frequency-based power control begins.
• f
AC
Limit Delta is the frequency increase relative to f
AC
at which frequency-based power control ends. The power
of the PV inverter at this point is 0 W.
If the limits f
AC
Delta- or f
AC
Delta+ are exceeded, the PV inverters disconnect from the utility grid.
When an external energy source, (e.g. a diesel generator) is operating in the stand-alone grid, this external energy source
determines the frequency and the PV inverters set to off-grid operation react to certain frequency changes brought about
by the external energy source.
When diesel generators are used as external energy sources, the output voltage frequency under load is 60 Hz. For this
reason, in most cases the PV inverters will deliver their entire power to the stand-alone grid, even when the diesel
generator is in operation.
If the current battery voltage is greater than the battery voltage setpoint and the stand-alone grid is also to be
synchronized with an external energy source, the Sunny Island temporarily increases the frequency and the PV inverters
disconnect via frequency shutdown (overfrequency). Afterwards, the Sunny Island synchronizes with the external energy
source.