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Micro Grid-Tie
A possible system to feed micro solar power back to the mains grid - 23 May 2009

What is it?

After some discussion on the Electro-Tech Forum's 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.

The topology

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 projects.

    - end -

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