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Power lines lead into downtown Calgary Wednesday, July 4, 2001.Jeff McIntosh

Six power-generating stations in Alberta failed Monday as a heat wave hit the province, causing rolling blackouts and pleas for consumers to cut back on energy use.

Alberta rarely forces power distributors to curtail demand – it last did so six years ago – but with an unusually high number of power generating facilities sitting idle, Alberta Electric System Operator (AESO) forced companies like Enmax Energy Corp. and Epcor Utilities Inc. to employ rolling blackouts Monday afternoon.

The blackouts ended after a few hours, although with heat remaining in the forecast, AESO has asked Albertans to reduce the amount of electricity they consume.

Alberta, rich in hydrocarbons like natural gas and coal used to create electricity, depends on about 35 power-generating facilities, and it is not unusual for one or two of those to be out of order at any given time, said Doug Simpson, director of market operations for AESO. "It is a very rare event," he said. "We had an unfortunate coincidence."

The rolling blackouts came on the first day of the year that Calgary hit 30 degrees. The Calgary Stampede said its Skyride was down in the afternoon, leaving people trapped on the attraction for an hour or so, but officials weren't sure if it was related to the power outage. Both Stampede officials and Enmax said the midway and rodeo grounds were not part of the blackouts.

Meanwhile, Edmontonians were sweating it out at 32, or 39 with the humidity, according to Environment Canada. In Lethbridge, where the mercury soared to 31, residents were asked to use as little power as possible or it could result in bigger blackouts.

The troubles occurred at four coal plants and two natural-gas plants, Mr. Simpson said. They all suffered unrelated problems, have different owners, and are located in different parts of the province. Wind power helped fill the void, and by late afternoon, three of the six troubled power units were ramping up their operations, Mr. Simpson said.

Enmax is building a new natural gas-fired power station east of Calgary, dubbed the Shepard Energy Centre. It is designed to produce about 800 megawatts and the company hopes to begin operations in 2015. By way of comparison, AESO asked Enmax to "shed" 40 megawatts Monday, said company spokeswoman Doris Kaufmann Woodcock.

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