Equipment and structures to transmit electricity

Electricity is a function of voltage and current. Combining high voltage and low current generates the same amount of power as combining low voltage and high current. During the transmission of electricity, current produces heat. This heat represents the loss of electricity from the line. To minimize electricity losses in the transmission system, power is transmitted across lines with high voltage and low current.

After the electricity is generated, large "step-up" transformers at the power plants increase the voltage to carry electricity efficiently over long distances along high voltage transmission lines. The lines can be as high as 115,000 to 735,000 volts. At the other end where the electricity is used, "step-down" transformers in the distribution system lower the voltage to suitable levels for domestic, commercial and industrial users.

To step down electricity, a transformer takes in electricity at a higher voltage and lets it run through a coil of wire wound around a core of iron or steel. Because the current is alternating, the magnetism in the core is also alternating. Also around the core is an output wire with fewer coils. The magnetism changing back and forth makes a current in the wire. Having fewer coils means less voltage. So the voltage is "stepped down."

Substations are key transformation points in the transmission system. A substation at the power plant houses the step-up transformer that prepares electricity for long-distance transmission. Nearer to the delivery points, utility substations use step-down transformers to lower the voltage to feed the distribution system. Some substations also provide equipment for switching to connect power between different transmission systems.

Power can be transmitted along overhead power lines or underground cables. Most of Canada’s transmission system has overhead lines. More expensive underground lines are used in urban areas or for crossing bodies of water. Transmission lines are made of copper or aluminum because of their low resistance.

Overhead power lines have three main parts: supports, insulators and conductors.

There are many different types of transmission line supports used throughout the world, including wooden poles or steel towers. In Canada, steel towers are commonly used for higher voltage lines.

Transmission lines are connected to the towers by insulators made of porcelain, glass or other materials. These are designed to support the weight of the transmission conductors while separating live (energized) wires from the towers. Each insulator consists of a metal cap on top and a metal pin underneath separated by the insulating material. The number of insulators used varies with the voltage level and application.

Transmission lines may have smaller conductors called shield wires strung above them. These are connected directly to the transmission line towers and protect the main conductors from a direct lightning strike. The shield wires provide an easy or low resistance path to the ground through the transmission towers. If lighting strikes, it will hit the shield wires rather than the conductors.