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Solar Power - When/How Does it Backfeed the Grid?


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Here's a question for some of you green energy folks who're familiar with solar power - How/when does the system backfeed the grid?

We know that running a generator when it's still hooked up to the grid can be bad but what about a bunch of solar panels on the roof - if the power goes out and the solar panels are engaged to power the house, and there's no way to isolate the system from the grid, will they endanger a lineman working on a pole somewhere the same way kicking on a generator connected to a panel that's not isolated from the grid could kill that lineman?

How about under normal circumstances when the system is powering your house and there's nothing wrong with the grid? We've all heard about how when folks have those systems their meters sometimes run backwards? How exactly does that work? Is there some kind of sensing device that knows when power from the solar system is not sufficient and which then allows power from the grid to feed the system? How exactly does that work?

Chad, I know that you used to inspect those systems for a firm that manufactures them. Can you elaborate?



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In the old days, these problems would inspire unique solutions for each system. Today, the inverters that convert photovoltaic DC to AC are listed as "utility interactive." They need to see the signal from the utility and synchronize to its frequency. When it isn't there they shut down.

There is no danger to the utility from a utility-interactive inverter. For the same reason, a backfed circuit breaker from a utility-interactive inverter does not require a locking mechanism to hold it in place. If you pulled it out of the panel, the breaker jaws would no longer be live; the inverter will shut down. With other backfed breakers, you would have a tiger by the tail.

Very large arrays can cause issues with a utility because of increased short-circuit current. Small ones don't have that problem.

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