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A possible system to feed micro solar power back to the mains grid
- 23 May 2009
What is it?
After some discussion on the
Alternative Energy forum
I drafted up this basic topology (circuit) to show a VERY minimalist way
to build a "Micro Grid-Tie" to efficiently feed a very SMALL amount of
power from a solar panel etc back into the mains grid. I had been thinking
about a "minimalist" system like this for a few weeks.
Warning1! Parts of the device are at mains voltages! It can be lethal!
Only attempt something like this if you are trained in mains voltage equipment.
Warning2! Some countries and states actively encourage you to feed your
home generated power back into the grid, BUT in many countries this is
still regulated by specific rules or even illegal!! You must check your
local regulations before attempting any grid-tie experiment.
My basic design goal was;
Absolute minimum parts to feed SMALL power back
Good efficiency for the energy that is transferred
Won't "dirty" the mains like square wave inverters
Reasonable safety against "islanding"
But its only HALF wave!!!
Well yes. I knew people would notice that. That actually gives slightly
higher efficiencies and VERY much reduces parts complexity, making it
easier to build and more reliable.
There is absolutely no need to feed micro power back to the grid in both
halves of the AC wave.
What's good about it?
1. Being half-wave the mains commutation requires just 1 switching device,
which can be a IGBT or even a cheap SCR. There is no need for a full
commutation bridge with 4 hi/lo side drivers.
2. It is very 'nice" with power fed back to the grid. The PIC or TL494
measures the instantaneous mains voltage and feeds in the proportional
current. So if the mains wave is 100v it gets 100mA, if at 200v it
gets 200mA. This removes any need to "generate a sine wave" and is
both fool proof and simple way to mimic the "perfect" resistive source.
3. It only feeds power back to the mains when the mains voltage is over
(say) 1/4, or about 70v. This is very simple but provides pretty good
insurance against islanding and against reverse connect to the negative
half of the wave. And of course does away with the need for
zero-cross detection. As 95+ % of the mains power occurs near the top
of the wave this limit at 70v won't affect the efficiency.
4. A basic monostable can be implemented so that after measuring
the 70v / "start" transition the device will only connect and run PWM for
10mS (for one half wave). This is a second level of protection against
islanding and reverse connect.
5. It is fully isolated between low voltage and high voltage sides,
needing only 2 optocouplers. A 3rd optocoupler could be added to detect
the input being over 13v for fully automatic micro solar use.
6. It draws no power from the mains at any time, apart from the 2mA
or less needed to run the PIC. Suitable for permanent connect of
micro solar to mains.
Am I going to build one?
I have a feeling I might, and if so I will post a full schematic and
the results. But it won't be for a while as I am busy with commercial
- end -
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