Sheerness Generating Station which supplys about 800 M/W of power got disconected from the grid due to line problems from Anderson to Sheerness. When these power lines went down they had to shut down.
And to answer the question why they plan major matainance during the spring, summer and fall and not in the winter is that they don't have to keep everything from freezing up. Plants like Sundance, Wabamun, and Keephills generaly have 1 unit down for maintance and they have a roving crew that move from plant to plant. Others like Sheerness have an outage about once a year (planed) If it is an unplaned outage like a boiler leak they have to depresurize and vent the boiler, repair the leak then refire the boiler, then warm the uint up and then bring it back online. You can't just flick a switch and you are making 400 meg/watts you have to bring it up slow.
If you want to check out who is making power here is a link
http://ets.powerpool.ab.ca/