Brian Zinchuk is editor and owner of Pipeline Online
These transmission lines west of Medicine Hat were not humming with wind-generated power the morning of April 24, 2025. Photo by Spencer Zinchuk
Two major natural gas-fired power plants in Alberta going offline at the same time caused the Alberta’s electrical contingency reserves to fall to zero in the late afternoon on Tuesday, June 17.
According to the Alberta Electric System Operator’s (AESO) event log, Genessee Repower Units 1 and 2, each 466 megawatts, tripped off at the same time at 5:08 p.m. Those units are among the largest in Alberta, and together their loss mean the Alberta grid suddenly lost about 8.8 per cent of its generation at that moment in time.
The minute-by-minute reporting of AESO’s data showed zeros across the board for its dispatched contingency reserve across hundreds of generating units. According to North American Electric Reliability Corporation (NERC) guidelines, at least 4 per cent of demand is supposed to be available in a dispatched contingency reserve (DCR). If that goes to zero, and demand outstrips supply, the grid’s frequency can drop. If it goes too low for too long, individual components of the grid can automatically disconnect themselves, resulting in a cascading failure similar to what happened in Spain and Portugual a few weeks ago.
This screenshot of the top part of the Alberta Electric System Operator grid monitoring website shows zeros across the board for the Dispatched Contingency Reserve (DCR). AESO
At 5:25 p.m., Alberta needed a DCR of 485 megawatts, based on a demand of 10,561 megawatts, or a DRC of 4.6 per cent. AESO’s records were showing zero.
They literally opened up the dams to maximize hydro capacity and seven of Alberta’s 10 grid-scale batteries were thrown into the mix. The Brazeau Dam, by far Alberta’s largest hydro asset, was churning out 330 megawatts out of a 350 megawatt capacity. All hydro assets, along with all other forms of generation, were showing zero DCR. There was literally nothing left in the tank.
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By about 5:32 enough capacity came online to put some hydro capacity back on the DCR board. Five batteries reverted from producing to the grid to DCR status. eReserve3 Mercer Hill, for instance, produced its full 20 megawatt capacity from 5:15 to 5:30 p.m.
According to Dispatcho.app, which logs AESO data, nine simple cycle natural gas generating units were fired up in the 5 o’clock hour.
Wind output at 5:25 p.m. was 832 megawatts out of 5,688 capacity. Solar was 1,149 megawatts out of 1,826 megawatts capacity.
The issue was resolved before 6 p.m.
No grid alert was issued by AESO during this event. Alberta was pulling in 47 megawatts from British Columbia and 53 from Montana as of 5:25 p.m., far below the capacity of both interties. As Saskatchewan’s intertie with Alberta is available for emergency use, but is not operating day to day. As of 5:25 p.m., it was not being used. Overall power demand was comparatively low.
As this time of year does not typically see excessive demand due to either hot or cold weather, that’s when most major maintenance takes place – a time known as “shoulder season.” However, as summer is rapidly approaching, shoulder season is ending soon.
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