Here are some of the reasons for this.
The nature of electricity
The laws of physics that govern how electricity behaves cannot be overlooked. Using electricity is easy, but transmitting it over long distances is much more complicated.
Let’s take the example of the phenomenon of reactive power. Essentially, reactive power is power that does not perform any useful work. It is an electrical phenomenon that is related to the very nature of an alternating-current (AC) system and that must be controlled.
The higher the voltage and the longer the transmission line, the harder it is to manage reactive power. Hydro-Québec’s transmission system, which carries electricity from the generating stations in the north to major load centers in the south in the form of an alternating current, therefore presents a significant challenge with regard to reactive compensation.
Also, when a line is placed underground, the closed environment in which the cables are placed amplifies reactive power. Consequently, it is standard practice all over the world to use overhead lines to transmit electricity at high voltage over long distances because the air surrounding the conductors acts as an insulator, thereby reducing the amount of reactive power.
The reactive power phenomenon makes undergrounding AC transmission lines less of an option. More equipment is required to carry a given amount of electricity, and the equipment itself must be oversized. It is therefore more complicated and more costly to build an underground AC transmission system than an overhead system.
In Québec, overhead transmission lines also have a specific advantage: in winter, the cold air dissipates the heat released by the conductors, which means that the same transmission line can carry more electricity when consumption is very high due to electric heating. An advantage of our northern climate!
Maintenance and lifespan of our equipment
The service life of an overhead steel line is roughly twice as long as that of an underground line, or about 80 to 100 years compared with 40 to 60 years. These are approximate numbers since the actual lifespans depend on the project and the specific technologies used. Not only that, but the service life of the insulating sheaths of underground cables is shorter, while repairs are more complex and take longer.
A flexible power system and new connections
Overhead transmission lines also have an advantage in this regard. It is much easier to modify the configuration of an existing overhead line to meet changing needs than to connect a new facility to an underground line. For instance, for a new direct connection to an underground line, at the very least, a splice vault needs to be built, whereas with an overhead system, all that is required is to build a tap line between the facility and the existing overhead line.
The cost of an underground line is determined by a set of variables that must be analyzed for each project. However, an underground line generally costs more to build than an overhead line given the high price of the insulated cable and the scope of the work to be carried out.
Undergrounding: One exception
It is common practice all over the world to underground high-voltage lines only in exceptional cases. Often, the decision to do so involves short distances, for instance in large urban centers where there’s a shortage of space or an overhead line is blocked by an impassable obstacle.
However, like all technologies, underground line technology is still evolving. Hydro-Québec is contributing to those efforts and incorporating technical breakthroughs into its processes.