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SILICON
CHIP
www.siliconchip.com.au
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Leo Simpson, B.Bus., FAICD
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2 Silicon Chip
Publisher’s Letter
Anti-islanding in grid-tied inverters
is a big drawback
In the past, I have touched on the frustration of homeowners with solar panel installations who have no
electricity during blackouts. They have this wonderful
shiny panel installation which is prevented from generating power during black-outs by the “anti-islanding”
feature of grid-tied inverters! This disadvantage was
greatly magnified for many people in the aftermath of
the severe weather in Sydney at the end of April.
Many thousands of people were without power for more than a week. Just
imagine that: no power at all for a whole week – nothing for light, cooking,
heating, computers, TV and other entertainment or cordless phones – you
could not even charge your mobile phone! And this was in Sydney suburbs,
not somewhere out in the sticks!
No-one was to blame for this situation as the storms downed many thousands
of trees and the electricity linesmen were flat out reconnecting whole districts.
Even as I write, the clean-up of felled trees is still going on and is likely to
continue for another month or so.
Now we all know why this “anti-islanding” feature is incorporated into
grid-tied inverters. It is there to protect linesmen who may be working on the
system when there is a power outage. On the face of it, this is a good idea. But is
it really necessary to also prevent the home-owner from having any electricity
at all when there is a blackout? There are other ways of protecting linesmen.
The most obvious method would be to use the anti-islanding feature of the
inverter to switch a contactor, so that the home-owner’s system was disconnected from the grid but still leave the inverter itself to generate power. Sure,
the home-owner would not get any benefit from feeding power into the grid but
at least he (or she) would still have power while the Sun was shining. While
there would still be no power available at night, most home-owners would
be happy to work around this, knowing that food in their refrigerators was
not going to spoil and many other power-using tasks such as clothes washing
could be done during the day.
No doubt some people would argue that relying on a contactor to isolate the
home-owner’s system could be a recipe for a fatality. But surely a contactor
could be arranged to “fail-safe” so that if it did not work, the system would be
isolated anyway. I am sure that it would possible to arrange for redundancy in
the monitoring and switching to make sure it was always safe and reliable. Of
course, it would be necessary for the grid-tied inverter to still be able to monitor
for the presence of power on the grid, so that the system would automatically
switch back when grid power was restored.
If you concede that this idea has merit, then it is only a small step to allow
solar systems which are grid-connected (no longer “grid-tied”) to have battery
storage so that home-owners can generate their own power during blackouts
at night. The release of the Tesla PowerWall lithium battery system (see article
on page 25 in this issue) makes this a practical scheme.
The solar panels would charge the PowerWall during hours of sunlight and
feed the excess power into the grid. Then if there is blackout, the system automatically isolates itself from the grid and the home-owner can enjoy electricity
as normal. The PowerWall would also have the benefit of smoothing the peaks
in electricity demand from the grid, as it could supply at least some power
after the Sun goes down.
Of course, the Tesla PowerWall might also persuade some electricity customers to disconnect themselves entirely from the grid and thereby avoid paying
daily service charges.
Leo Simpson
siliconchip.com.au
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