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
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.