3.2 Transmission Infrastructure

Transmission lines transport electricity long distances as efficiently as possible.

Introduction to Transmission Infrastructure

Canada has migrated from what was once predominantly localized generation, to the current model of large-scale generation at centralized locations, thus requiring transmission infrastructure.

The primary elements of the transmission infrastructure include transmission towers, rights-of-way, conductors, insulators, and ground wires.

Transmission Towers

Transmission towers carry the conductors or transmission lines and are typically built from steel, wood, or composite materials.

• Tower heights range from 25 to 100 metres depending on the transmission voltage level, and the distance between towers ranges from 250 metres to 500 metres.

• Towers are built to be tall because high voltage requires high clearance from the ground and ample separation from other conductors. Height is also required to safely straddle rivers, roads, bridges, railways, and distribution lines.

• Given their height and their often-remote locations, building and maintaining these towers can be challenging. Many transmission companies utilize crane-size bucket trucks, specialized helicopters, and even drones to accomplish this.

• Where adverse weather prevents flight-based access for maintenance purposes, other devices such as snowmobiles and off-road vehicles are used.

Right-of-Way

A right-of-way is a path where transmission towers and lines are placed. It is typically 30 to 100 metres wide, but can be wider to accommodate multiple tower lines, and to allow utility personnel faster access to the lines for inspection, maintenance, and repair.

Corridors

Rights-of-way corridors require a significant amount of land, and routes must be approved through established government processes.

Utilities endeavour to use as direct a route as is feasible, with due regard to environmental and other values and land uses.

Clearance

Transmission lines themselves are not insulated, and therefore tree and vegetation growth in rights-of-way must be carefully controlled.

Any point of contact with—or even close proximity to—a high-voltage conductor can cause an arc, or discharge of electricity from the line, which can damage the infrastructure and start fires.

The higher the transmission voltage, the greater the clearance required and the wider the right-of-way.

With an increasing frequency and scope of forest fires in some regions, transmission rights-of-way also provide the important supplemental benefit of acting as a fire break.

Three-Phase Power

In Canada, transmission lines are designed and constructed using a three-phase model. Each transmission line consists of three bundled lines or conductors (referred to as either phases A, B, and C, or phases red, white, and blue).

The advantages of three-phase circuits include:

• They can transmit more power at the same voltage and amperage than one- or two-phase models.

• They use smaller and less expensive conductors and associated infrastructure.

• It is easier to maintain a balanced flow of power across three-phase circuits.

• They are less prone to overheating and to line losses of electricity.

Ohm’s Law

To ensure the efficiency of electricity transmission, it is important to understand how voltage, current, and resistance are related.

Ohm’s Law is a formula used to calculate the relationship between voltage, current, and resistance in an electrical circuit. Named for German physicist Georg Ohm (1789 – 1854), Ohm’s law addresses the key quantities at work in circuits:

E = I × R

When spelled out, it means voltage = current × resistance, or volts = amps × ohms, or V = A × Ω.

To students of electronics, Ohm’s Law (E = IR) is as fundamentally important as Einstein’s Relativity equation (E = mc2) is to physics.

Transmission Conductors (Lines)

Transmission conductors are simply the lines across which electricity moves on its journey from generation stations to the local distribution grid. Voltages on these lines are typically in the range of 100 to 760 kilovolts.

Conductor material

Transmission lines are most often made with a steel core with an outer layer of aluminum. The aluminum layer is where the electricity is carried, while the steel core provides strength.

Conductor size

Conductor size depends on voltage level, the amount of power being transmitted across it, and the distance covered.

Conductors are sized to strike the right balance between being large enough to operate efficiently, while also minimizing weight and material costs.

Design considerations

High-voltage transmission across well-designed lines lowers both the current and resistance within the conductors, thereby reducing line losses.

Distances between towers is another important design consideration that must account for factors such as temperature extremes, icing, and wind, and the key variables of line sag, swing, and gallop.

Sag, Swing, and Gallop

While exposed to the elements, power lines can undergo stresses that alter their length and movement. These factors are taken into consideration during the design of the transmission grid.

Sag

Transmission lines expand and contract with changing temperatures, lengthening in the heat and shortening in the cold. Engineers therefore prescribe the right amount of sag between each tower to accommodate this.

The required sag depends on both external considerations, such as weather, and internal considerations, such as the amount of electricity travelling across the line. Greater loads cause more sag.

Swing and Gallop

In windy conditions, transmission lines will swing side to side and may also “gallop” in more severe weather conditions (a wave-like, up-and-down motion). These considerations must also be factored into determinations of height, width, and spacing clearance requirements.

Weights are often added to transmission lines to suppress both swing and gallop and to help keep the overall structure steady in the wind. Large and brightly coloured “marker balls” are also hung from transmission lines for visibility from the ground and for aircraft.

Lines that are weighted down by heavy ice build-up are especially vulnerable to wind damage, and this has been a major factor in large-scale outages resulting from ice storms.

Insulators and Ground Wires

Insulators and ground wires provide support and protection for transmission lines.

Insulators or insulating supports are used to attach transmission lines to transmission towers. They are made of either glass, porcelain, or polymers (a plastic-like material). They support the weight of the lines without allowing current to flow from the lines to the tower and into the ground. Insulators must have high mechanical strength to support the long length of lines in high wind and ice conditions, and they must have high electrical resistance to minimize current leakage. Insulators must also shed rainwater and be selfcleaning of contamination to maintain their insulating qualities.

Ground wires or earth wires are bare steel lines located at the top of the transmission tower. They serve to shield the line by intercepting potentially highly damaging lightning strikes before they can hit the currentcarrying transmission lines below.

Transmission Issues and Impacts

Transmission is important to bridge the gap between generation and distribution, to move electricity efficiently across long distances, and to connect provincial, regional, and national grids. However, there are some disadvantages to this transmission system.

• Transmission lines traverse long corridors, creating environmental impacts and potential conflicts with other land uses.

• Large towers and other infrastructure need to be built and maintained within these corridors.

• High voltage is dangerous, and separation is needed in both tower height and right-of-way width.

Knowledge Check

• Control centres: Restore, divert, and interrupt power transmission remotely

• Transmission towers: Carry the conductors or transmission lines over long distances

• Rights-of-way: Allow utility personnel faster access to lines for maintenance and repair

• Transmission conductors: Carry electricity from the generation station to the distribution grid

• Insulators: Attach transmission lines to transmission towers

• Ground wires: Shield the line by intercepting lightning strikes