Molly Ayala
A transition line with electricity is an electrical transmission line that connects an overhead power line to an underground power cable. Transmission lines are used to carry electromagnetic signals with minimal reflections and power losses. They are designed to conduct electromagnetic waves in a contained manner, taking into account the wave nature of the transmission. Efficient long-distance transmission of electricity requires high voltages, which reduce losses produced by strong currents. The transition from overhead to underground power transmission can be facilitated by transition towers, which improve the convenience and technological effectiveness of the installation.
| Characteristics | Values |
|---|---|
| Definition | In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. |
| Use case | Transmission lines are used for purposes such as connecting radio transmitters and receivers with their antennas, distributing cable television signals, routing calls between telephone switching centers, and for computer network connections. |
| Types | Parallel line (ladder line, twisted pair), coaxial cable, and planar transmission lines (stripline and microstrip). |
| Impedance | Transmission lines have uniform cross-sectional dimensions along their length, giving them a uniform impedance, called the characteristic impedance, to prevent reflections. |
| Current | Transmission lines can carry either alternating current (AC) or direct current (DC). |
| Voltage | Transmission lines carry high-voltage 'primary' power between generators and substations. |
| Overhead lines | Overhead power lines are transmission lines that transfer electromagnetic waves of varying voltage levels to supply electrical power. |
| Underground lines | Underground transmission lines are less affected by weather but are more costly and take up more space. |
| Transition towers | Transition towers are used to transition between overhead power lines and underground power cables. |
| Hardware accessories | Various hardware accessories are fixed to power lines, poles, and towers to ensure the smooth transition of electricity from one place to another. |
| Clean energy | The transition to clean energy requires an upgrade to existing transmission lines and the construction of new high-voltage transmission lines. |
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What You'll Learn
Transmission line types: parallel, coaxial, and planar
A transmission line is a structure used in electric power transmission and distribution to transmit electrical energy over long distances. They are designed to carry electromagnetic signals with minimal reflections and power losses.
There are several types of transmission lines, including parallel lines, coaxial cables, and planar transmission lines.
Parallel Lines
Parallel transmission lines include the ladder line and twisted pair. They consist of multiple conductors suspended by towers or poles, with a focus on maintaining adequate clearance from the ground for safety and reliable support.
Coaxial Cables
Coaxial lines confine electromagnetic waves within the cable, allowing them to be bent and twisted without negative effects. They are commonly used for television and other signals with a bandwidth of multiple megahertz.
Planar Transmission Lines
Planar transmission lines have flat, ribbon-shaped conductors or dielectric (insulating) strips. They are used to interconnect components on printed circuits and integrated circuits, particularly at microwave frequencies. Planar transmission lines include various types such as stripline, suspended stripline, microstrip, coplanar waveguide, slotline, and imageline. Each type exhibits different characteristics, including dominant transmission modes, maximum frequency, and impedance range.
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Overhead vs underground transmission lines
Transmission lines are used to transmit electrical energy over long distances. Overhead power lines are a common method of electricity transmission, consisting of conductors suspended by towers or poles. The conductors are usually made of aluminium, although some copper wires are used in medium-voltage distribution and low-voltage connections. Overhead lines are generally the lowest-cost method of power transmission for large amounts of electricity, as the surrounding air provides good cooling, insulation along long passages, and allows for easy optical inspection.
However, there are some drawbacks to overhead power lines. They are susceptible to damage from trees and extreme weather, and they can be vulnerable to outages caused by vehicles colliding with poles. Additionally, maintaining adequate clearance between energised conductors and the ground is essential to prevent dangerous contact and provide reliable support for the conductors.
Underground power lines, on the other hand, are constructed by digging trenches and laying wires directly or placing them in conduits for protection. While underground lines are more expensive to build and maintain, they offer several advantages. They are protected from wind, wildfires, tree branches, and damage from most animals. They also reduce the risk of electrocution from downed lines and are aesthetically more pleasing.
The choice between overhead and underground transmission lines depends on various factors, including cost, terrain, and the specific requirements of the power distribution system. While underground lines offer enhanced protection and reliability, overhead lines remain the most common method of electricity transmission due to their lower cost and easier maintenance.
Furthermore, some technical challenges are associated with underground power lines. The parallel capacitance issue with AC transmission becomes more significant at high voltages when the lines are underground. This results in higher losses and requires additional insulation to reduce parasitic ground capacitance. Additionally, insulated conductors are necessary for underground lines, and there can be issues with cable ampacity derating.
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Alternating current (AC) vs direct current (DC)
A transmission line is a specialised cable or structure designed to conduct electromagnetic waves. They are used for various purposes, such as connecting radio transmitters and receivers, distributing cable television signals, and routing telephone calls.
Now, let's delve into the differences between alternating current (AC) and direct current (DC):
Alternating Current (AC):
AC is a type of current where the electric charge periodically changes direction. The voltage in AC circuits also reverses because of the changing current direction. AC power is typically generated by power plants and transmitted to homes, where it is used to power most appliances. One of the key advantages of AC is its ability to easily convert voltage levels using transformers, making it suitable for long-distance power transmission. AC is commonly used for powering electric motors, which are found in many large appliances. Additionally, AC is utilised in continent-wide networks to reduce the risk of blackouts by providing alternative routes for power flow.
Direct Current (DC):
In contrast, DC is characterised by its unidirectional flow. The electric charge in DC only flows in one direction, similar to the consistent flow of a river. DC is commonly obtained from batteries, solar cells, and electric vehicle batteries. It is also used for low-voltage applications, such as powering phones and computers. DC is generally easier to understand than AC since it provides a constant voltage or current. However, interrupting DC circuits, especially at high voltages, can be challenging due to the continuous voltage, which can pose safety risks.
AC vs DC:
The choice between AC and DC depends on the specific application. AC is widely used for power transmission and long-distance transmission due to its ease of transforming voltage levels. On the other hand, DC is preferred for energy storage, such as batteries, and is found in almost all electronics. DC is also used for rapid charging in public superchargers since it eliminates the need for AC-to-DC conversion. However, converting DC voltage levels often requires complex conversion processes.
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Impedance matching and power losses
In electrical engineering, a transmission line is a specialised cable or structure that conducts electromagnetic waves. Transmission lines are used for various purposes, including radio transmitters and receivers, cable television signals, and computer network connections. Transition towers are used to arrange transitions between high-voltage overhead lines and underground power cables.
Impedance matching is a critical technique in electrical engineering that involves adjusting the input or output impedance of a device to maximise power transfer and minimise signal reflection. In transmission lines, impedance matching is essential to prevent reflections that can cause frequency-dependent losses. The characteristic impedance of a transmission line must match the load impedance to ensure maximum power transfer and reduce losses due to standing waves.
When the impedances at each end of a transmission line match its characteristic impedance, signals can be transmitted without reflections. This is achieved by using impedance-matching devices, such as transformers, adjustable networks, or properly proportioned transmission lines. Practical impedance-matching devices provide optimal results within a specified frequency band.
Power losses in transmission lines can occur due to ohmic or resistive losses, especially at high frequencies. Additionally, dielectric losses become significant at high frequencies when the insulating material absorbs energy and converts it to heat. The total power loss depends on the frequency of the signal and is often specified in decibels per metre (dB/m).
To improve impedance mismatch, various techniques can be employed, such as inserting a matched attenuator before a mismatched load or using an LC network as a matching network. Mismatch losses can be minimised by ensuring the signal pad and transmission pad have the same width. By addressing impedance mismatch, power transfer can be maximised, and signal quality can be improved.
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Transition towers and line supports
Transition towers are used to arrange a transition between a high-voltage overhead electrical transmission line and an underground power cable. They are designed to increase the technological effectiveness of the installation and convenience of use of equipment for transitioning between an overhead line and a power cable.
Transition towers usually consist of a post made up of three joined sections. Crosspieces in three tiers are fixed to the two upper sections of the post for suspending linear conductors with the aid of tensioning insulator suspension means (strings). The crosspieces are fixed to the upper section, while the crosspiece is fixed to the section which is joined to the upper section. The sections and crosspieces are polygonal and in the form of truncated pyramids made of sheet steel. A platform is fixed horizontally to the lower section of the post for mounting the terminal cable equipment: terminal cable bushings and surge arresters.
The configuration of a transmission line tower depends on many factors, including the number and type of conductors, the length of the insulator assembly, the minimum clearances to be maintained between conductors and the tower, and the location of ground wires with respect to the outermost conductor. The height of the tower also depends on the voltage of the transmission line, with higher voltages requiring taller towers to ensure adequate spacing between the lines and other objects.
Transmission towers can be classified into several types, including:
- Monopole towers: These consist of a single, slender pole that supports power lines vertically. They are often used in urban or suburban areas where space is limited and can be aesthetically pleasing, sometimes disguised as flagpoles or trees.
- Hybrid towers: These combine the design features of multiple tower types to optimise performance and cost-effectiveness. For example, a hybrid tower may have a lattice structure at the base for added stability, transitioning into a tubular or monopole design above.
- Angle towers: These towers resist transverse loads induced at an angle, in addition to wind, ice, and broken conductor loads. They are used when the line deviation exceeds an angle greater than 2 degrees.
- Suspension towers: These use strain insulators to resist axial loading placed on the tower from the conductors.
- Dead-end towers: These support the weight of the connecting conductors and cater for the tension in the conductors, also using strain insulators. They are typically used at the end of a transmission line before it passes to a substation or underground line.
Underground cables are an alternative to transmission towers, offering lower visibility and less susceptibility to weather conditions. However, they are more expensive and faults are harder to locate and repair.
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Frequently asked questions
A transmission line is a specialised cable or structure that conducts electromagnetic waves to supply electrical power. Transmission lines carry high-voltage 'primary' power between generators and substations.
Transmission lines can be classified by their voltage levels and length. There are medium power lines that transmit between 66-132 kV and span 50-100 miles (80-160 km). Long power lines transmit 132 kV or above and cover distances of 100+ miles (160+ km). There are also subtransmission lines that operate at lower voltages, such as 66 kV and 33 kV.
A transition tower is a structure used to facilitate the transition of an overhead power line to an underground cable line. It consists of a metal rack with three sections and a traverse fixed to it, allowing for the suspension of linear conductors. Transition towers aim to increase the technological effectiveness and convenience of transitioning between different types of power transmission lines.
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