For that to happen, the grid would have to have been operating at higher than its normal frequency. Assuming they use 50 Hz power and the grid running at 55 Hz, the clocks would run 10% fast. To gain 15 minutes, the grid would have to have been running that fast for over two hours (actually, 150 minutes). (I'd think that some alarm at the power stations would have been going off for that entire time.)
I just noticed that digital watches were affected as well. I have no idea how that could, in any way possible, be related to the grid.