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Electric power transmission moves bulk electricity from a power station to distribution networks. Transformers step the voltage up to 110 kV or more so less current flows and less energy is lost as heat. The electricity travels along high-voltage overhead lines, almost always bare aluminum, then is stepped back down and delivered to the socket in your home.
Electric power transmission is a very large sector of industry, despite being a subset of electric power generation. Electric power transmission is the bulk movement of electrical energy from the site of its generation (such as a power station) to the sites of distribution. The interconnected power lines that we see stretching over barren land from the city to the horizon compose the transmission network. It is only due to modern electric power transmission that electricity has been easily transported to different geographical areas and topographies where it was once inconceivable for electricity to reach. Before we jump into the nuances of electric power transmission, let’s first take a look at the history behind it.
History
In the earliest days of electric power transmission, it only dealt with the transmission of current of a constant voltage, also known as Direct Current (DC). The trouble with DC was the opposite of what you might expect: with the technology of the time, there was no practical way to step its voltage up for the journey and back down again at the other end. So DC had to be sent out at roughly the same low voltage it was generated at, around 110 volts. At low voltage you need a large current to deliver a given amount of power, and large currents bleed energy away as heat in the wires. That made it highly ineffective and economically unfeasible to push DC more than a mile or two. It also meant the generation stations had to sit close to the load they served, which would have demanded a large number of small generating stations dotted across every city.
Fortunately, this problem was soon put to rest with the advent of Alternating Current (AC). This type of transmission took off after Lucien Gaulard and John Dixon Gibbs built their "secondary generator," an early transformer, in 1881. The first long-distance AC line was 34 km (21 mi) long and was built for the 1884 International Exhibition in Turin, Italy. It was powered by a Siemens & Halske alternator delivering 2 kV at 130 Hz. Several Gaulard secondary generators were strung along the line with their primary windings wired in series, and the lower-voltage output from their secondary windings fed the incandescent lamps that lit up the streets, a vivid demonstration of stepping voltage down close to where the power was actually used. This system was a solid foundation upon which AC transmission could prove itself for long-distance power delivery.
Overhead And Underground Transmission
The most common type of electrical power with which most of us are familiar is overhead transmission. These high-voltage overhead transmission lines do not come with any insulation. The conducting material used for these long-distance lines is almost always aluminum, usually in the form of aluminum-conductor steel-reinforced (ACSR) cable, in which aluminum strands carry the current and a steel core gives the conductor the tensile strength to span the towers without sagging. Even though copper is a better conductor than aluminum, aluminum is still the metal of choice. Aluminum conducts only about 61% as well as copper for the same cross-section, but pound for pound it actually carries more current, because it is roughly a third the weight. That lighter weight lets towers stand farther apart, and aluminum is several times cheaper than copper, so over the length of a transmission line the savings are enormous. Overhead transmission lines are manufactured by various companies around the world.
Today’s transmission-level voltage for overhead lines is 110 kV and above. Voltages of 66 kV and 33 kV are considered sub-transmission and are used for long lines when the loads are light. Voltages below 33 kV fall within the distribution part of the grid. However, there are some problems that accompany the usage and erection of these overhead transmission lines. Adverse weather conditions, such as high wind and low temperatures, can lead to serious power outages. Wind speeds as low as 43 km/h (27 mph) can set the conductors swinging far enough that neighboring wires encroach on each other’s safe clearance, which can trigger a flashover (an electric arc jumping the gap) and a loss of power.
Another way to overcome the shortcomings of overhead transmission lines is by using underground transmission lines. The best aspects of underground cables are that they can take up less line length than overhead lines. They are also not affected by the weather and have low to zero visibility. The major cost they incur appears at the front, i.e., excavation costs and insulation costs. Not only that, but the maintenance and repair of these lines are far greater, as it is hard to locate and reach the specific point that requires attention.
In metropolitan areas where this method of transmission is employed, they’re covered by a thick metal pipe and insulated with a dielectric medium. This proves to be an ingenious method on a number of fronts. If a fault were to occur that would damage the outer metal piping, it would cause the dielectric medium to leak out into the soil, thereby preventing further damage. Liquid nitrogen trucks are frequently employed to freeze portions of the pipe to enable the draining and repair of the damaged pipe section. The temperature of the pipe and soil are usually monitored constantly throughout the repair period. Underground lines are strictly limited by their thermal capacity, which permits less overload or re-rating than overhead lines. Long underground AC cables also have significant capacitance, which draws a large charging current and can sharply reduce their ability to deliver useful power to loads more than about 80 km (50 mi) away. For really long underground or undersea runs, engineers often switch to high-voltage direct current (HVDC) instead, which sidesteps the capacitance problem entirely.
This clearly shows that various kinds of transmission have their respective pros and cons. The bottom line is that the transmission of electricity from the power station to the power socket in your home is no small feat!
