Latest Holyrood trip under investigation

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Andrew Robinson
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Underfrequency load shedding different than rolling blackouts

The cause of Monday’s brief power outage that affected 15,000 electricity consumers in communities across the island remains under investigation, according to a Nalcor Energy vice-president of corporate development

On its website Monday, Newfoundland Power cited under-

frequency load shedding as the reason for the outage, which lasted less than half an hour. Dawn Dalley told The Telegram Tuesday the public should not confuse such events with rolling black-outs.

A generating unit at the Holyrood power plant tripped and went offline. Why that unit tripped remains unknown, though Dalley said Tuesday it should not take long to find out.

“We just have to get some technicians in to do some testing. The unit itself is back on today.”

Underfrequency load shedding functions as an automatic response to such incidences, serving to protect the power system. Dalley said these events are unique to power systems isolated from the North American grid, but will not be an issue for Newfoundland and Labrador once its system links to the North American grid.

“The engine that is the North American grid pumps back the frequency in milliseconds,” said Dalley.

The grid in Newfoundland operates at a 60-hertz frequency. That frequency is maintained by a balanced system where generating supply equals demand.

When a generating unit trips, the system is no longer balanced. Therefore, the number of megawatts lost from supply must be accounted for on the demand side. Dalley believes 60 MW was lost in the case of the generating unit that tripped Monday.

“The system balances itself,” said Dalley. “It looks at it and goes, Holyrood just tripped.”

Customers are automatically taken off the grid and feeders are dropped. The system then looks to jump-start additional generation capacity to maintain the frequency of 60 hertz.

“Within 18 minutes (Monday) night, every customer was back,” said Dalley, noting those customers came back gradually and not all at once.

Rolling blackouts involve taking feeders off for a certain length of time before reactivating them, after which different feeders are deactivated.

“You’re rolling because there’s an extended period of time when you have a generation shortage,” said Dalley. “There was no generation shortage (Monday), we just experienced a trip in the system, so in order to get the generation back up and get the system back in balance, you had to take customers off and then take them back on.”

Over the last five years, Dalley said Newfoundland has experienced anywhere from two to seven underfrequency load shedding incidences annually.

As of late Tuesday afternoon, 96 per cent of the island’s generation capacity was available, with the load forecast over the next few days ranging from 1,000 MW and 1,375 MW. Newfoundland and Labrador Hydro has close to 1,700 MW of power available, according to Dalley.

Organizations: North American, Newfoundland Power, Newfoundland and Labrador Hydro

Geographic location: Holyrood, Newfoundland and Labrador

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