For want of a nail, the horseshoe was lost
For want of a horseshoe, the horse was lost
For want of a horse, the rider was lost.
The great ice storm of January 1998 still ranks among the most disruptive events in the modern history of electricity supply, affecting huge areas of eastern Canada and the northeast United States. The Canadian Province of Québec was particularly impacted, with over a million customers left without power – some for two months. This article, based on exclusive interviews and photos taken in the days immediately after the disaster, discusses some of the lessons learned from the event. As evidenced by similar ice storms affecting Europe as well as the U.S. Midwest, these types of events can no longer be seen only as freaks of nature that occur once in a generation. Today, with climate change, this type of weather phenomenon may well become a recurring problem for electric utilities worldwide.
The toppling of hundreds of transmission towers across the region affected by the Great Ice Storm of ‘98 was attributed mostly to catastrophic ice build up on conductors – well above the design limits of most lines, especially given the accompanying high winds. Yet, the fact was that not all lines toppled. In fact, some transmission structures remained standing even though virtually every tower of a duplicate neighboring line had already collapsed. Several years ago, an expert transmission engineer who was closely involved in subsequent probes into the ice storm provided INMR with his personal views on the great ice storm of ‘98 and offered his explanation of why hundreds of towers came crumbling down.
“If I have one mantra in my professional life,” explained H. Brian White. “And it is that the wire system on a transmission line cannot be allowed to break. That includes the insulators at strain or dead-end positions.” White made a living investigating permanent outage failures on transmission lines worldwide. Yet he had special a glint in his eye when he talked about memories of what he discovered two decades ago when he and colleagues were among the first to arrive at the toppled hulk of a transmission tower – one of dozens which collapsed on a 735 kV line running south-east of Montreal. There, lying on the ground beside the toppled structure was a porcelain insulator string that showed clear evidence of complete mechanical failure. He noted that, in spite of porcelain being a brittle material, such insulators are extremely durable. The condition of this insulator string, he remarked, just could not be accounted for simply by its fall to the ground. Said White, “if a quad bundle drops and you find a broken insulator string, it’s clearly not due to the fall which at worst might cause only a chip. No, the existence of such a severely-damaged insulator provides clear evidence of failure before falling to the ground.”
White, who passed away not long ago, travelled the world acting as a specialist consultant on tower design to electric utilities from Argentina to Japan and lectured in China. He explained what he thought was the real reason behind the multiple cascade failures from January 1998. “If a support structure on a transmission line is crushed by heavy ice loads, it collapses and that’s about it. Nothing more happens. The line may lose a tower, but that’s comparatively minor. Similarly, if a suspension insulator fails, the conductor drops. While this is a serious problem, it’s still nothing compared to the transmission engineer’s worst nightmare – a cascade failure.”
White contrasted these problems with what can happen if there is a failure of the overhead wire system on a transmission line. If this critical system is interrupted, he said, there will be longitudinal loads that will almost inevitably lead to cascading. He noted that certain type of structures, for example guyed V towers, may be able to resist this up to a point. But with heavy ice loads on the conductors, such a cascade would become virtually impossible to stop, no matter what the tower type.
White cited the example of the elaborate dead system for a 735 kV line consisting of four parallel chains of 36 insulators each and yoked together. He stated that if one insulator in any of these chains fails mechanically, tremendous shock is transferred to the entire assembly – an energy release that he estimates is the equivalent of dropping a good sized car a distance of several meters.
So what did White think caused the interruption in the wire system on lines that suffered cascade failures? “Low quality insulators,” he said without a moment’s pause. “Plain and simple. Like in any chain, electrical transmission systems are only as dependable as their weakest link.”
According to White, a proportion of insulators in-service on the affected networks were sub-standard, relying on a formulation of cement containing an aluminium oxide additive intended to significantly reduce cure times. But, he insisted this reduction was matched by a similar reduction in mechanical performance. In fact, he claimed that there were clear indications from testing that these insulators were beginning to lose mechanical strength even while still in the warehouse. White then produced a photo of a failed double V-string on a 735 kV line using these same inferior insulators and that he stated demonstrates that they were failing at only half their rated load. “All the available evidence from this disaster,” stressed White, “supports one simple conclusion: bad insulators in any dead-end assembly just cannot be tolerated.”
So how can such a cascade failure be stopped, once initiated? Some utilities have considered the notion of specially reinforced structures, designed to absorb the shock in such a way as to interrupt the cascade. However, White was sceptical about such a solution. “I personally do not believe that this will work in practice,” he said. “The only way to stop the cascade, in my opinion, is to prevent the wire system from moving. So, even if the reinforced tower somehow survives, the cascade will still continue moving on past it.”
Looking back on this whole experience, White became philosophical. “I have made my living from studying failures,” he sighed. “Here was a situation of bad insulators being in service at an especially vulnerable location on the network. If you examine these types of situations properly to find out what really happened, then you progress. Otherwise, you go nowhere.”