Wiktor Wise
Electrical transmission lines can vary in length, from short pieces used in radio frequency engineering to very long distances of several thousand kilometres. The first long-distance alternating current (AC) line was built in 1884 and was 34 kilometres (21 miles) long. Today, transmission lines can be up to 2,000 kilometres long, as seen in China, where they transmit power of 7.2 GW. The distance that electricity can be transmitted is limited by the voltage and current, with higher voltages and lower currents allowing for longer distances. Transmission lines use either AC or direct current (DC), with DC cables not limited by length in the same way as AC cables, which can only efficiently transmit power up to 80 kilometres (50 miles) due to their capacitance.
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What You'll Learn
- Transmission lines use alternating current (AC) or direct current (DC)
- AC lines have been used since 1884, when a 34km line was built in Italy
- DC cables are not limited in length by capacitance
- Long-distance transmission is costly and requires trans-border lines
- Ultra-High-Voltage (UHV) is required for economic and efficient transmission over 1000km
Transmission lines use alternating current (AC) or direct current (DC)
Transmission lines can use either alternating current (AC) or direct current (DC). The first long-distance AC line was built in 1884 in Turin, Italy, and proved the feasibility of AC electric power transmission over long distances. The first commercial AC distribution system entered service in 1885 in Rome, Italy, for public lighting.
AC electricity is converted to DC electricity for transmission and then converted back to AC electricity for distribution to customers on the AC power grid. A converter station at each end of the line is required to convert power from AC to DC and back so that it can be used by the public. DC cables are not limited in length by their capacitance, whereas long underground AC cables have significant capacitance, which reduces their ability to provide useful power beyond 50 miles (80 kilometres).
The benefits of using DC power include better energy efficiency over long distances and less land usage. DC power also requires fewer conductors and incurs less power loss than equivalent AC lines. Additionally, DC power can be used to transmit power between AC transmission systems that are not synchronized, allowing for the transfer of power between grid systems running at different frequencies.
However, there are also disadvantages to using DC power. The required converter stations are expensive and have limited overload capacity. At smaller transmission distances, the losses in the converter stations may be bigger than in an AC transmission line for the same distance. Additionally, it is easier and cheaper to change voltage under AC than under DC. Transformers are simple, cheap, and reliable devices that can swap voltage for current and vice versa, but they only work on AC.
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AC lines have been used since 1884, when a 34km line was built in Italy
AC lines have been used since 1884, when a 34km (21-mile) line was built in Italy. This first long-distance AC line was constructed for the 1884 International Exhibition of Electricity in Turin, Italy. The system was powered by a 2 kV, 130 Hz Siemens & Halske alternator and featured Gaulard transformers with primary windings connected in series, powering incandescent lamps. This early demonstration of AC power transmission proved its feasibility over long distances.
The first commercial AC distribution system began operating in 1885 in Rome, Italy, providing public lighting. This system was soon followed by the first British AC system, serving the Grosvenor Gallery. These early AC systems played a pivotal role in the eventual widespread adoption of AC power transmission.
The use of AC power transmission continued to evolve in the late 1880s and early 1890s, with smaller electric companies merging into larger corporations. These larger entities, such as General Electric and Westinghouse Electric in the US, further developed AC systems. The technical advantages of AC power, particularly in terms of economies of scale with large generating plants and long-distance transmission, gradually enabled the connection of various loads.
Today, efficient long-distance transmission of electric power typically requires high voltages to reduce losses from strong currents. Transmission lines can utilise alternating current (AC) or direct current (DC), depending on the specific requirements and distances involved. For very long distances, DC technology is often preferred due to its greater efficiency. However, the majority of North American transmission lines rely on high-voltage three-phase AC.
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DC cables are not limited in length by capacitance
Electric power transmission involves the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. Transmission lines can use either alternating current (AC) or direct current (DC).
DC cables are not limited in length by their capacitance. This is because DC cables do not have a charging current, which is defined as the current that flows through the shunt capacitance of a transmission line. Charging currents generate heat losses in the cable, which can be as high as the thermal rating of the cable for long transmission lines. This phenomenon limits the maximum transmission line length for AC cables.
In contrast, the absence of charging current in DC cables means that they can theoretically transmit power over very long distances. For example, UHVDC transmission lines with DC voltages of ±800 kV have been used to transmit power over distances of up to 2000 km in China. This makes UHVDC transmission a preferred solution for transmitting bulk power over large distances of several thousand kilometres.
However, it is important to note that the actual cable technology limits the transmission voltage to ±500 kV, which results in higher transmission losses compared to higher voltages. As a result, the UHVDC transmission of electric bulk power over long distances may be limited to overhead lines in the near future.
In conclusion, DC cables are not limited in length by capacitance, and they can transmit power over much longer distances compared to AC cables. However, there are still technical challenges and limitations to be addressed, such as the voltage limitations and transmission losses associated with current cable technology.
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Long-distance transmission is costly and requires trans-border lines
The long-distance transport of electricity is costly, requiring the construction of trans-border transmission lines and dealing with efficiency losses in the grid. The cost of long-distance transmission lines is influenced by factors such as the need for high-voltage infrastructure, technical challenges, and the impact of distance on power losses.
High voltages are essential for efficient long-distance electricity transmission. This reduces losses caused by strong currents. Higher voltages keep the current low, minimising power losses. Ultra-High-Voltage (UHV) transmission lines, with voltages ranging from 1000 kV to 1200 kV, are required for economic and efficient transmission over distances exceeding 1000 km. This technology is currently employed in Russia, China, and Japan, but technical issues prevent optimal voltage levels from being achieved.
The use of Direct Current (DC) or Alternating Current (AC) transmission lines impacts the feasibility of long-distance transmission. DC cables are not limited in length by capacitance, unlike AC cables, which have significant capacitance that restricts their effective range to approximately 50 miles (80 kilometres). However, in the past, DC voltage could not be easily increased for long-distance transmission, necessitating the placement of generators near their loads.
Long-distance transmission projects, such as the DESERTEC initiative, face significant costs and technical challenges. The construction of transmission lines under the Mediterranean Sea, for instance, involves depths that exceed the capabilities of current HVDC technology. Additionally, the transmission capacity of the grid must be improved to accommodate the transmission of renewable energy over long distances to major demand centres.
To address the challenges of long-distance transmission, improvements and reconstructions of the existing electricity grid are necessary. This includes enhancing the transmission capacity and adopting technologies like UHVDC grids with higher voltage capacities. Furthermore, the adoption of local generation through solar panels can reduce the reliance on long-distance transmission by feeding electricity directly into the distribution side of the grid.
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Ultra-High-Voltage (UHV) is required for economic and efficient transmission over 1000km
The transmission of electricity over long distances has been a challenge since the early days of electrical power. The first long-distance AC line, built in 1884, was just 34km long. Since then, the challenge of transmitting electricity over long distances has been tackled with higher voltages, which reduce losses from strong currents. However, this has its limits, as underground lines are limited by their thermal capacity, and long underground AC cables have significant capacitance, which reduces their ability to provide useful power beyond 50 miles (80km).
For distances of 1000km and above, Ultra-High-Voltage (UHV) transmission is required for economic and efficient transmission. UHV transmission refers to power transmission lines operating at greater than 800kV (800,000 volts). The primary benefit of UHV transmission is the ability to transport electricity over long distances with minimal power loss. This is achieved by reducing the losses produced by strong currents.
UHV transmission also allows generating plants to be moved closer to fuel sources and away from population centres, improving local air quality. It also enables the construction of newer, cleaner, more efficient power generation plants. The integration of widely dispersed renewable power sources (hydro, wind, and solar) into a national distribution network is also made possible by UHV transmission.
UHV transmission is particularly relevant for countries with large distances between energy resources and consumers, such as China, Russia, and India. China, in particular, has aggressively invested in UHV transmission to fulfil the unprecedented growth in electricity consumption since 2004. In 2015, the State Grid Corporation of China proposed the Global Energy Interconnection, a long-term proposal to develop globally integrated smart grids and UHV transmission networks to connect over 80 countries.
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Frequently asked questions
Electrical transmission lines can travel from 1,000 kilometres up to several thousand kilometres.
Transmission lines use either alternating current (AC) or direct current (DC). AC lines can suffer from energy loss over long distances, so higher voltages are used to reduce current and minimise power loss.
Ordinary electrical cables can carry low-frequency AC, but DC cables are not limited in length by their capacitance.
A transmission line is a specialised cable or structure designed to conduct electromagnetic waves.
Long-distance electricity transmission is very costly and requires the construction of cross-border transmission lines. There are also efficiency losses, and the lines can degrade over time due to power loss converted into heat.
