This blog is part of a 2-part series on the components of our electrical grid: the lines and cables, which transport electricity, and the substations, which process the power. What is on a power line?
Let’s begin with the basics on the electric grid. Traditionally, electricity has been generated at large generating stations, like coal, or gas-fired power plants. Then, transmission substations step up the voltage to high magnitudes for transmission. Due to the inverse relationship between voltage and current, high voltage means low current; and low current means low line losses, which means efficient system operation.
Transmission lines transport the electricity to load centres, where it encounters another substation and is stepped down to a lower voltage which is both economical and safe for distribution. Then, distribution lines, whether overhead lines or underground cables, bring electricity to your home, businesses and more. The lines are joined at substations, which bring together power lines and allow for control and safe operation of the electric grid.
Electric flow
Electricity travels via two mediums: via overhead lines and underground cables. Overhead lines, which are held up by wooden poles or metal towers, are easier and cheaper to construct. Therefore, they are often found in rural and/or less urbanised areas. Conversely, underground cables are laid under the ground and thus are more complex to put in but are common in cities and urbanised areas.
Cable specifics
Technically speaking, it matters whether the power flows via overhead lines or underground cables. This is because the capacitance of cable systems is much greater than for overhead lines, for the same conductor size, which increases the line charging current as well. This means cables require a much larger degree of reactive power compensation compared to overhead lines, and limits the feasible length of the cables in practice.
Therefore, power system engineers need to accordingly include power factor correction in their designs, corresponding to the chosen medium. For example, shunt reactors or series capacitors often are included to provide reactive power compensation. In my courses in the Master’s in Smart Electrical Networks and Systems, we learned how to calculate and size out these systems per different specifications.
Line specifics
Meanwhile, in overhead lines, the air is the insulation for the system. They are much easier and cheaper to construct but may often face permitting delays and critiques of visual pollution.
The structures on which overhead lines hang vary in shape and size. Key for design is the distance between the line and the ground, as adequate clearance must be maintained to prevent dangerous contact with the line and maintain resilience in the face of various weather events. Often, a grounded wire is strung at the tops of the towers to provide lightning protection. Insulators, made of porcelain, glass or polymers support the conductors against switching or lightning events, too.
If located near an airfield, towers may be shorter and wider, carrying more lines horizontally in parallel. They may also contain visibility marker balls, which are the colorful spherical balls placed on power lines to improve visibility for low-flying aircraft and protect the power lines.
Ever wonder…
Have you ever wondered why birds can sit on overhead power lines, when power lines are supposed to be dangerous given the high voltages? It is because birds are not good conductors; they sit on a single line, while electricity in the line continues flowing in the line since it is the path of least resistance.
However, if the bird were to simultaneously touch two wires, or a wire and another object, it would likely be in trouble. If there is a voltage difference between the two, it creates a path for electricity to travel; depending on the magnitude of the difference, electricity would travel through the bird and the bird could be electrocuted.
Curious how this can be solved? Stay tuned for the second part of this blog.
by Emilia Chojkiewicz, EIT InnoEnergy alumna