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SILICON
CHIP
www.siliconchip.com.au
Publisher & Editor-in-Chief
Leo Simpson, B.Bus., FAICD
Production Manager
Greg Swain, B.Sc. (Hons.)
Technical Editor
John Clarke, B.E.(Elec.)
Technical Staff
Ross Tester
Jim Rowe, B.A., B.Sc
Nicholas Vinen
Photography
Ross Tester
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Phone (02) 9939 3295
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glyn<at>siliconchip.com.au
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David Maddison B.App.Sc. (Hons 1),
PhD, Grad.Dip.Entr.Innov.
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Dave Thompson
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2 Silicon Chip
Publisher’s Letter
Generating your own electricity
during blackouts
The Publisher’s Letter in the June issue certainly tickled
the fancy of a number of readers, as they postulated how
the “anti-islanding” feature of grid-tied inverters could be
circumvented. We have published a few of their letters in
the Mailbag pages this month. Of course, the reason for
wanting to circumvent the “anti-islanding” feature is to
trick grid-tied inverters into generating power when the
grid is blacked out. But the grid-tied inverter “knows”
when the grid is up and only generates power at that time, provided of course, that
the solar panels are in sunlight.
Well, the anti-islanding feature is pretty much bullet-proof, as it is meant to
be, so that the inverter cannot feed power to an otherwise dead grid and possibly
be a hazard to people working on the power lines. It cannot be tricked by simply
connecting it to the output of standard DC-to-AC inverter with a sinewave output.
Part of the anti-islanding feature is to measure the impedance of the grid and also
test whether its frequency can be “pulled” slightly high or low. When the grid is
connected, this isn’t possible.
Some readers think that perhaps the anti-islanding feature of a grid-tied inverter
is provided by a little module which can be disabled. We think that is highly unlikely and instead, it is part of the overall software. The only way to get around it
would be to get into the software and modify those lines of code which provide that
feature. Sounds simple but I will bet that even getting into the software would take
some doing. All of which means that those grid-tied inverters which are available
quite cheaply via the internet are pretty much useless for this purpose, unless you
are a software guru.
Still, the fact that the topic appears to be of considerable interest has us thinking
as well. Why not produce an inverter which will run from the same solar panels
as a grid-tied inverter? This would have to cope with the same high input voltages
as the grid-tied inverter but be completely independent and would generate power
when the grid-tied inverter was effectively disabled.
Really, that is not too hard and we just happen to have the basis of a such a
design already in our project archive. Which one is it? The answer is the 230VAC
Induction Motor Speed Controller that we featured in 2012. Apart from some initial
teething problems which led to the H-bridge module and current sensing resistor
failing in a rather noisy fashion on some pump loads, it has now proved to be quite
a reliable design, especially when driving 3-phase induction motors.
So how does that help us? In effect, the Induction Motor Speed Controller contains a complete high-voltage DC-to-AC inverter, albeit one in which the output
frequency can be shifted over quite a wide range to enable induction motors to
be controlled. As it stands, it can accept around 340V DC (eg, from a solar panel
array) and it will produce around 2kW at 230VAC. It would be relatively simple
to configure as a free-standing general-purpose inverter.
Unfortunately, producing such an inverter is only part of the story if you want
it to power your household. You would have to able to completely isolate your
household wiring from the grid and then decide which circuits would be powered
and so on. That would all need to be done by a licensed electrician and the whole
exercise is not likely to be simple or cheap.
However, there is now a better solution: a hybrid grid-tied inverter which has
battery back-up. This enables you to control the amount of power you export to the
grid and instead use it to charge batteries which can power the inverter when the
sun goes down and more importantly, let you generate power when the grid is down.
So that clearly is the answer but it means that all those thousands of existing
solar panel installations can only run at night or when the grid is blacked out by
having the inverter changed to a hybrid type. At the moment though, that is a really expensive solution.
Leo Simpson
siliconchip.com.au
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