Page 40 - North American Clean Energy March April 2015

For areas where power outages are frequent, the on/off-grid inverter can be installed in-
side the garage. Two input channels of the inverter can connect to two solar panels on
solar energy
the roof using a 4-conductor DC wire. The other 2 input channels can connect to a 36V
battery set. This way, the system can produce 600W AC power in the evening when the grid is down. This is a scalable design so a bigger system can be easily installed with
more solar panels, inverters, and batteries.

Figure 1.
Figure 1.
Figure 2.

How It Works
Figure 2.
On/Off-Grid Solar Power
Mini-Inverter
During normal operating conditions when the grid is on, the inverter works like a regular
By Dr. George S. Cheng
on-grid power inverter that meets all UL1741 and IEEE1547 standards including: over or
under voltage shutdown, over or under frequency shutdown, and anti-islanding. When the

grid is down, the inverter will shutdown within a fraction of a second. After waiting for a
ergy is wasted when grid is on. Can we combine both on-grid and of-grid functions in one Climate change in our world is becoming more catastrophic. Power grid brownouts and
few seconds, the inverter sends a test signal to the connected off-grid AC circuit to check:
inverter?
shutdowns are more frequent and severe, resulting in loss of production and personal
(1) if there is AC present in the off-grid circuit; and (2) if there is an AC load in the off-
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A New On/Off-Grid Power Mini-Inverter
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Figure 1 shows a 4-channel on/of-grid power inverter that inverts the DC power from designed to power AC loads, which cannot send power to the grid.
four DC sources, such as photovoltaic solar panels or batteries, to AC power and supplies During the off-grid mode, the inverter is constantly detecting if the grid is back on. After
On the other hand, homeowners with on-grid solar systems often wonder why their sys-
the AC power either through its on-grid AC output port, or of-grid AC output port. Each grid power is detected, the inverter will wait for a few minutes to assure the grid is stable.
tems are unable to generate power during a power outage when the sun is shining and they
inverter has four input channels that can connect directly to four 250W-320W solar pan- It will then stop generating power to the off-grid circuit. After a 5-minute mandatory wait
need the power the most. he reason is simply because grid-interactive or on-grid solar
based on UL1741 standard, the inverter starts generating AC power to be sent to the grid.
els and produce 1150W AC peak power. Each input channel has its own control and MPPT inverters in the system are required to shutdown immediately when the grid is down. In
Now, the inverter works in its on-grid mode.
to eliminate partial shading problems and maximize power production.
fact, it is a key testing item in the UL1741 certiication process. When the grid is down and
In North America and some other parts of the world, split-phase AC standard is used. if an inverter is still sending power to the grid, it can cause ire and even fa-tal accidents to
A Grid-Flexible Solar Power System
In the U.S., the line-to-line (L1 to L2) output voltage is 240V, and the Line-to-Neutral (L1 repair workers.

to N, or L2 to N) is 120V. hus, the on/of-grid power inverter for the U.S. market must So, the dilemma is that on-grid inverters are designed to deliver power to the grid and
For ease of installation, multiple on-grid and on/off-grid power inverters of the same fam-
supply 240V AC to the grid from its on-grid AC output port, or 120V AC to the AC loads cannot run as of-grid inverters to power AC loads, and therefore cannot be used for solar
ily can daisy-chain using the attached AC wires and connectors. The inverters can be
from its of-grid AC output port. As shown in Figure 1, the on/of-grid power inverter has
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two AC output ports and wires on the top, of the inverter with MC-4 connectors. Since there is no high-voltage DC, the system is

where the left is the on-grid AC output and safe and easy to install.
the right is the of-grid AC output.
Figure 2 illustrates a 3.6KW grid-flexible solar power system that includes two 4-channel A highly secure safety system
on-grid solar inverters and one 4-channel on/off-grid solar inverter. All three inverters
How It Works
During normal operating conditions when
the grid is on, the inverter works like a
regular on-grid power inverter that meets

all UL1741 and IEEE1547 standards
including: over or under voltage shutdown,
over or under frequency shutdown, and
anti-islanding. When the grid is down, the

inverter will shutdown within a fraction of a
second. After waiting for a few seconds, the
inverter sends a test signal to the connected
of-grid AC circuit to check: (1) if there is AC

present in the of-grid circuit; and (2) if there
is an AC load in the of-grid circuit. If there
is no AC present and an AC load is detected,
it will start generating AC to power the load.

Now, the inverter works in its of-grid mode.
Greeneye Smart PV Switch
During the of-grid mode, the inverter is
constantly detecting if the grid is back on.
After grid power is detected, the inverter
• Failsafe safety system
• Individual shutdown feature per PV module
will wait for a few minutes to assure the
• Protection for ireighters or PV installers and electricians
grid is stable. It will then stop generating
power to the of-grid circuit. After a
5-minute mandatory wait based on UL1741
Ferraz Shawmut is Mersen
ep.mersen.com standard, the inverter starts generating
AC power to be sent to the grid. Now, the
inverter works in its on-grid mode.

40 nacleanenergy.com
MARCH/APRIL 2015