I got low priority queue because of power outage

Leaverbuster makes no distinction between accidental or intentional disconnections by design. You're not only expected to play the entire game, but also to take all reasonable precautions against disconnects.

Losing power may have been outside your control (though not if you were, say, playing League during a massive thunderstorm), but the leaverbuster system has a lot of built-in leniency for such events. If you're on a 20-minute penalty, you must have left several games before this one.