Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

Combating ice deposits on overhead transmission lines has been a big challenge in cold climate regions for the last eighty years. With an expanding electric system, it has been a difficult task to prevent or remove ice from long lines with single or bundled conductors. Considerable research has been carried out and large-scale technologies have been developed to address this problem. Based on worldwide power utility experience, two different strategies regarding ice accretion on overhead lines have been adopted. To prevent failure, power utilities try to build overhead lines that are capable of withstanding large icing events (with a low probability of occurrence). This commonly requires strengthened towers and costly lines. In the past, when these reinforced lines had to face extreme icing events, (ice sleeve overload), where collapse of transmission lines was possible, "de-icing" methods were used as the first strategy to protect them. These de-icing methods are generally put into operation when a certain amount of ice has accumulated on the conductor in order to shed the ice sleeve as soon as possible. These methods require specific ice detection systems able to follow the ice storm in local areas as the ice load accumulates on conductor line sections in order to be able to intervene in time. The second strategy, "anti-icing", is the prevention of part of or the entire expected ice load. This strategy can be used on existing weaker lines that may not have been designed to current extreme ice loading standards or utilized on new lines to allow for a less expensive standard of construction, considering the low occurrence probability of extreme ice storms. To prevent severe ice load damage, ice accumulation on conductors must be avoided or considerably reduced. This could be achieved by using anti-icing methods to prevent or weaken ice adhesion or/and to use de-icing methods in the early stages of an icing storm to limit the size of the ice load on the conductors. In order to support these two strategies aiming at combating icing damage on overhead lines, a large number of anti-icing and de-icing methods have been developed. Some of these methods have been well documented in specific reviews since the 1980s. To the best of our knowledge, the first review dedicated to these antiicing and de-icing methods applicable to overhead power lines was presented by Polhman and Landers (1982). A few years later, another paper presented a detailed review of two methods, rolling and heating by short-circuit current, which are used on the Manitoba Hydro power network (Hesse 1988). A classification of de-icing and anti-icing methods into four categories, passive, thermal, mechanical, and miscellaneous, based on the physical principle used in the method of ice removal, was proposed later (Laforte et al. 1998). More recently, another report on power line anti-icing and de-icing techniques was presented (CEA 2002). The methods listed were classified into six categories; passive techniques, active coatings and sheathings, active methods on bare conductors, methods using external thermal energy, methods using external mechanical energy, and finally, miscellaneous methods, with less potential for application to ground-wires and energized conductors. Another classification was proposed to illustrate the permanent or temporary character of a method, the need for line modification, and whether it is automated or manual (Volat et al. 2005). Thus, the existing potential methods presented in previous reviews can be divided into inline, limited-use, and permanent. Inline methods refer to those using Joule effect to melt the ice, using the energy from the line itself or using an external energy source, with no device or coating directly added to the energized conductor or ground wire (GW). Limited-use methods regroup all methods that are not permanently installed on the lines, but are used at specific locations, primarily for de-icing, or that can only be used once. Finally, permanent methods include all methods permanently installed on the conductors or GWs. It appears that no permanent method, above, is presently in widespread use. Most of them, such as coatings or devices that are added to energized conductors or GWs, are in fact in development or in the conceptual stage. The development of such methods is quite complex as they have to meet specific requirements to ensure good performance and life expectancy. This consideration is developed in detail in the next chapter.