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How do solar microinverters prevent back-powering an electrical grid during a power company power failure?

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Microinverters typically will shut down when they sense that the main power grid is down. This is known as anti-islanding protection and is meant to protect power grid workers from working on a section that they think is down, but is actually still being powered by an inverter.

Sensing of the grid state can be passive, detecting things like voltage, frequency or zero-crossing, or it can be active, applying a signal to the grid and seeing if it reacts as expected.

On loss of grid the inverter should shut off rapidly, within a few seconds. On sensing the grid coming back there should be a delay of a minute or two before reconnecting the inverter to try to make sure the grid is stable first.

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At the start of a black-out, the inverter powers the grid, not just the house, but the entire neighborhood, creating "an island of light in a sea of darkness". That's unintentional and is dangerous to the linemen repairing the power line. The inverter must detect the loss of grid power and shut down, which is called, aptly enough, "anti-islanding".

The inverter senses the frequency. If it disappears, in means that the grid is gone.

However, if the load includes an induction motor, it will keep on spinning and creating the line frequency. But the sign of the slip will change, meaning that the frequency will drop. The inverter sees that and knows the grid is gone.

Once it senses that the grid is gone, it shuts down its output.

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As of 2025, the other answers are correct. Inverters sense the grid somehow and refuse to output power if it's missing.

There is a move to give the option for future inverters to be able to power an unpowered grid, to make them more versatile. It's only a software change. It remains to be seen how that facility will be presented to the user and controlled.

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    \$\begingroup\$ Wouldn't supplying power to an unpowered grid be a significant safety issue? The answer by @GodJihyo touches on this. \$\endgroup\$ Commented Aug 6 at 17:09
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    \$\begingroup\$ @MichaelRichardson Absolutely, which it why it remains to be seen how that facility will be presented to the user and controlled. \$\endgroup\$ Commented Aug 6 at 19:22
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    \$\begingroup\$ Powering an unpowered grid won't work. When the power comes back again, the phase of the inverter is not in sync with the phase of the grid. They will fight each other. The way to allow power in an unpowered grid is with battery inverter/chargers like Victron MultiPlus. You can connect inverters to its output, and if done so, the output maintains AC voltage, and the inverter sees that. When the power goes back up again, the MultiPlus will slowly start to sync its output to the grid, and when that's done, the MultiPlus will give you the grid voltage back again. \$\endgroup\$ Commented Aug 7 at 7:27
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    \$\begingroup\$ Powering an unpowered grid absolutely can work. There's lots of active research into "grid-forming" inverters which are capable of running a grid without rotating machines. Some work by simply mimicking the physics of conventional synchronous machines in software, others are exploring more novel control algorithms. nrel.gov/grid/grid-forming-inverter-controls \$\endgroup\$ Commented Aug 7 at 15:02
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    \$\begingroup\$ Well, yes, as long as you have a transfer switch that isolates your house from the grid. Otherwise, besides the obvious safety issues, there's the issue of your inverter attempting to power your entire neighborhood. \$\endgroup\$ Commented Aug 7 at 17:49

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