Add battery back-up power option to existing grid-tied PV and solar systems: Page 5 of 14

August 05, 2013 //By Mark Mays
Add battery back-up power option to existing grid-tied PV and solar systems
Mark Mays of OutBack Power explains how to add a battery back-up power option to existing grid-tied PV and solar systems.
mean the difference between filling up a 5-gallon gas tank once a day versus once a week if the generator is tied into a battery-based backup system that is running the generator just a couple of hours a day. News photos and videos of the aftermath of extreme weather and other emergency events show people standing in long gas lines for hours, and even then not always successfully. Many of the gas stations that had gas couldn’t pump their storage tanks without available grid power!

3. In a BB inverter system augmented with generator power, the batteries can greatly extend generator runtime—the same fuel will go a lot further since the generator does not need to run 24/7.

Typical AC-Coupling solutions: Like any solution, the devil is in the details, and AC Coupling a GT and a BB inverter together is no different, especially if one wants a “one size fits all” solution for all applications. That “one size fits all” package usually includes one or more diversion loads, possibly a blackout relay or some other method to take the GT inverter offline to prevent it from overcharging the batteries on the BB inverter. And it requires a lot more upfront design to be sure all devices in the system can handle all possible conditions.
Some battery-based inverter manufacturers attempt to simplify their AC Coupling implementations by “dithering” or altering the frequency of their power to the GT inverter outside its operational window of 59.5 to 60.5 hertz. This will effectively shut it off when the batteries are full instead of using a blackout relay, thereby saving the cost of the relay. However, in many instances frequency dithering prohibits the use of a generator as the generator frequency is not always stable enough for the GT inverter to synchronize to. Even if it synchronizes, there is a risk of back-feeding and damaging the generator in low or no load

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