Some Special Considerations related to SMA SunnyIsland Inverters and the use
of 240/120 V Autotransformers
This memo relates to some issues discussed in a prior memo (“Issues Related to the Use of Lockable-Open
AC Disconnects with Grid-Tie Battery Backup PV Systems”)
Connecting SunnyIsland Inverters:
The SMA SunnyIsland inverter presently available in the United States is designed to be used
differently than the traditional battery-backup inverters. This may require some special
considerations in designing and inspecting a SunnyIsland/Sunny Boy installations that would not be
required for TRACE/Xantrex or OutBack battery-backup grid-tie PV systems.
With traditional battery-backup systems, the energy from the PV array is directed to the batteries –
and, if there is a surplus – will either be disconnected or be taken by the grid-tie inverter from the
batteries to be fed back to the utility. Thus, since the utility is not ever connected to the batteries
(DC), and the PV array is not connected directly to the grid, disconnecting the inverter also
disconnects the PV array completely from the utility.
The SMA Sunny Boy PV inverter is designed to produce grid voltage (120 or 240 V AC). In a nonbattery-backup grid-tie PV system using Sunny Boy inverters, the output of the Sunny Boy
inverter(s) is backfed directly to the utility.
The SMA SunnyIsland battery-fed inverter is designed to control (slightly modified) Sunny Island
grid-tie inverters during a utility outage, enabling them to continue to supply AC power, rather than
shut down (UL 1741), as they would if directly connected to the utility grid. It does this by
supplying “utility quality” AC power to the protected loads and the Sunny Boy inverters. The
Sunny Boy inverter senses this AC power from the SunnyIsland as if it were proper grid power, and
continues operating.
The modification to the Sunny Boy involves the communication between the SunnyIsland and the
Sunny Boy. When feeding the utility grid, it is unlikely that the Sunny Boys will “oversupply” the
grid in normal operation. However, when the grid is unavailable, power from the Sunny Boys can
only go to the protected loads and to the battery. When the battery is fully charged, the Sunny Boy
output must be reduced, since the excess power would have no place to go.
The way this is accomplished is by the SunnyIsland slightly changing its output frequency. The
Sunny Boy senses this, and reduces output – or shuts down, thus protecting the system.
SERVICE PANEL
Jim Easton
Line
AC
DISCONNECT
Load
INVERTER
LOAD PANEL
Sunny Boy
Inverter
1 of 3
PROTECTED
LOADS
PV Array
02/16/16
Some Special Considerations related to SMA SunnyIsland Inverters and the use
of 240/120 V Autotransformers
No Bypass Switch
Bypass Switch
SERVICE PANEL
AC
DISCONNECT
LOAD
PANEL
Sunny Boy
Load
INVERTER
PROTECTED
LOADS
PV ARRAY
Line
(Note that the orientation of the SunnyIsland has been reversed to simplify the diagram.)
Implementation of the bypass would require a 3PDT switch or a ganged double CB toggled with a
ganged triple CB for each phase. Somewhat expensive, but possibly justified in a critical situation.
Use of 240/120 V Autotransformers:
Frequently it is desired to use a 120V inverter for 240 V or, alternatively, to distribute loads
between two 120V inverters on a 240 V split-phase system. The most common and least expensive
way to do this with an off-grid system is to use a 240/120 V autotransformer across the phases with
the center tap connected to the neutral. Because of the magnetic coupling of the two halves of the
winding of the autotransformer, the transformer will distribute current so as to balance the voltages
of the two phases – provided its capacity is sufficient.
While such a transformer can be used on a grid-tie system – for example to allow a 120V
SunnyIsland to operate in conjunction with 240 V Sunny Boys, extreme care must be done in the
design and inspection to ensure that under no circumstances is the autotransformer ever connected
across the grid phases (240 V) and to neutral. If this were to happen, the autotransformer would
attempt to pass current sufficient to balance the two phase to neutral voltages of the utility grid: a
task far beyond its capacity. In the most benign case, a fuse or CB would open. However this
would remove the autotransformer from the circuit – thus negating any purpose it might have been
intended to serve.
Jim Easton
2 of 3
02/16/16
Some Special Considerations related to SMA SunnyIsland Inverters and the use
of 240/120 V Autotransformers
Designers and inspectors should verify that any 240/120 V autotransformer, if installed, is never
subjected to demands beyond its capacity, and never placed in such a position in a circuit where its
normal operation would lead to its disconnection by protective devices or its self-destruction.
Please let me know your comments.
Thank you.
Jim Easton, PE
4364 Bonita Rd., No. 166
Bonita, CA 91902-1421
Tel: 858-527-0240
Fax: 619-470-8616
Jeaston@ucsd.edu
www.JimEaston.com
Jim Easton
3 of 3
02/16/16