Ice storm 98: Engineering considerations
Quebec’s hydro-electricity distribution system: Engineering and design considerationsHydro-Québec has played a key role in Quebec’s economy since the provincial government acquired and consolidated smaller power companies then nationalized the utility in the early 1960s. Thanks partly to its low operating costs (a 1995 study by Moody’s found that Hydro- Québec had the most competitive cost structure among 29 major power companies supplying the north-eastern U.S.) and generous government subsidies, Quebecers used electricity for 41 per cent of their energy consumption in 1996 compared with the national average of 23.8 per cent. Some critics claim that the province’s dependence on electricity contributed greatly to the grief caused by the storm. (Proponents of Hydro-Québec maintain that for the most part natural gas fireplaces and gas and oil furnaces still require electricity to operate. And while gas stoves are good for cooking, heating a house with one is unsafe.)
More than 120,000 kilometers of transmission and distribution lines – enough to circle the globe three times – were pulled down by the weight of the ice and by falling trees and branches. Over 900 pylons and 128 transmission lines were affected by the storm. Damage was so extensive that in such places as the Triangle of Darkness (see Figure 5) it wasn’t a case of repairing the damage, but rebuilding much of the power grid from the ground up. (Utility officials estimated February 2, 1998 that as much as 40 per cent of the power transmission infrastructure in the area bordered by Saint-Hyacinthe, Saint-Jean-sur-Richelieu and Granby was damaged or destroyed.) What took more than 50 years to create was destroyed in less than one week.
It was widely reported during the storm that customers may have been victims of a series of long-term engineering misjudgments by Hydro-Québec. First, the utility pioneered the use of large capacity 735 kilovolt transmission lines to carry power from massive hydroelectric developments at James Bay, Manicouagan and Churchill Falls to cities in the south of Quebec (see Figure 1). At the time of the storm, two-thirds of the province’s hydroelectric power had to travel anywhere from 500 to 1,200 kilometers in order to reach the end-user. (Primary feeder lines are usually just 1.5 to eight kilometers in length and in rural areas where demand for electricity is usually lighter, feeders are sometimes as long as 16 or 20 kilometers.) Only three utilities on the continent use such high capacity lines to carry power long distances: Hydro-Québec with over 9,600 kilometres, American Electric Power with about 2,800 kilometres of 765 kilovolt lines, and New York Power Authority with about 250 kilometres of 765 kilovolt lines. The latter is really just an extension of Hydro-Québec.
At the time of the storm, some energy analysts contend that big lines concentrate risk, and most of Montreal’s power comes from just six of these lines (five form the ring of power around Montreal, see Figure 5). The crisis in the city, Canada’s second-largest metropolitan area, started when the Drummondville line was pulled down January 7 after a series of nine pylons collapsed. In the 30-plus years since the 735 kilovolt lines went into service (1965) not one of the pylons that supported them had fallen. But as a result of the 1998 storm, a reported 150 fell in a twisted heap. Analysts suggest that this means the pylons were under-designed for the ice loads experienced. However it appears that the pylons affected during the storm weren’t limited to one design or era. Hydro-Québec has noted that its pylons were constructed to hold more than three times the 12 millimetres of ice required by Canadian standards. (A storm dumping 15 millimeters of freezing rain is projected to be a Once in 100 years event while 100 millimetres ravaged parts of the Quebec during Ice Storm 98.) Furthermore, the utility had said that in recent years it had installed at key points anti-cascading pylons – transmission towers designed to prevent the collapse of more than ten pylons in a row.
As a result of the failure of numerous pylons, four of the five 735 kilovolt transmission links forming the ring of power were severed. Two lines coming into that loop also collapsed, reducing power to the greater Montreal area by almost 60 per cent. By January 9, 1998 Montreal’s entire supply of electricity was reduced to a single high-voltage line running from Laval via the Saraguay substation to the Hampstead substation. Along with the failure of the Drummondville line, the breaking of Montreal’s electrical ‘backbone’ was the second big blow to Hydro-Québec and its customers. At the time, service to the entire region was via feeder lines radiating outward from this ring. No other North American utility used such large lines for an urban backbone. Furthermore, the Montreal ring had only a single line in its southern portion. That made the risk absolute. The catastrophic failure of this line and its substations created the crisis in the Triangle of Darkness. The grid was crippled.
But as bad as the damage was to the transmission system, the storm was even harder on power distribution – the network of substations, poles and wires that delivers electricity to customers. At least 24,000 (and by some accounts as many as 37,000) wooden utility poles were pulled down, sheared in half or uprooted by ice. Over 30,000 cross-arms met a similar fate.
Required pole strength is determined by the weight of crossarms, insulators, wires, transformers and other equipment it must carry, as well as by ice and wind loadings. All these forces tend to break a pole at the ground line. Ice forms around the conductors and other equipment during a snow or ice storm. The weight of ice is .9 kilograms per litre. Thus, if ice about 2.5 centimeters thick forms around a conductor that is 30 metres long, more than 45 kilograms will be added to the weight carried by the poles. While these direct weights may be substantial, normal wood poles are more than capable of meeting the ordinary load challenge. However ice formation around conductors presents quite a surface to the wind. A 100 km/h wind, for example, blowing against an ice-coated wire will result in a force of more than 61 kilograms per conductor being applied to the top of the pole. If this pole suspended three conductors, the total force would be more than 180 kilograms.
In its own defense, Hydro-Québec maintained a the time that its transmission system has been designed and developed over the years according to reliability criteria that generally exceed recognized standards in North America "because of the exceptional climactic conditions and great importance of electricity in the province." Higher reliability criteria were introduced with the building of the James Bay transmission systems. In the early 1970s, for example, the standard of ice resistance for pylons was increased to 45 millimetres, a level considerably above the Canadian standard of 12 millimetres. Use of anti-cascading pylons, it says, is another example of how the utility has raised its construction standards to improve reliability of the system.
However Hydro-Québec conceded in a January 21, 1998 report to the provincial government that there is room for improvement of its grid. The utility submitted a five-year development plan to provincial cabinet, which was approved January 19, 1998. (Review of the plan by parliamentary committee had been scheduled long before the ice storm.) A cabinet order-in council exempted Hydro-Québec from seeking the usual legislative approval before implementing the blueprint. The plan raised the ire of critics, energy analysts and many Montrealers because it included the construction of a 315 kilovolt above-ground backup line through Duvernay-Anjou – a residential section in the northeast part of the city. The line, scheduled to be operational by early 1999, would have provided the island with an additional 1,000 megawatts of power.
The utility’s proposal for the line, originally made in 1989, was rejected in 1996 by the province’s environmental assessment board. The board concluded that it could not justify the construction of a new corridor through a residential area. And even if the link was badly needed, it said, the line should be underground for all but three of its nine kilometers. Energy analysts say the new link would be just as vulnerable to bad weather as the older corridors and suggest the utility’s decision was motivated solely by cost. Hydro-Québec estimated the aboveground line would have cost about $28 million while a buried link would cost anywhere from $130 million to $217 million.
Along with construction of the Duvernay-Anjou line, Hydro-Québec announced January 28, 1998 plans to invest a total of $815 million over three years to reinforce its transmission system, in part to prevent future catastrophes. Part of the funds would go toward providing an additional 5,000 megawatts of power to regions where current demand drew on about 16,000 megawatts of capacity. The additional 5,000 megawatts could be re-routed in the event of an emergency, it said. By the end of winter 1998/99, Hydro-Québec planned to put into service a new transmission line between the city of Sherbrooke and the 735 kilovolt St-Cesaire distribution station south of Montreal that could support an additional capacity of 500 megawatts. Hydro-Québec would also make provisions for temporary links with Ontario Hydro to provide an additional 400 megawatts if needed. By the winter of 1999/2000, the new line from Sherbrooke to the St-Cesaire station would be extended to the Hertel distribution station (see Figure 5) to link up with the 735 kilovolt ring of power. A new 1,000 megawatt transmission line would be built between Hull, near Ottawa, to St-Jovite in the Laurentians north of Montreal, which is linked to the utility’s generating complex in James Bay.
The intent of Hydro-Québec’s ambitious plan was to diversify lines supplying power to key areas, increase power flow, and establish more links to alternate sources of power (such as Ontario Hydro) should there be a repeat of the 1998 event.
