Electrical Power Transmission is the conveyance of bulk electricity from the power generation plant to the electrical substation. Electric conductors that carry out this job are known as power transmission lines. The transmission system is a part of the supply system, which essentially comprises three parts, these are namely:
- Generation system
- Transmission system
- Distribution system
The generation system is a power plant that generates electricity using 3-phase generators. The generated power is then stepped up using transformers and transmitted to the substation, where it is distributed to consumers.
Power can be transmitted by AC (alternating current) or via DC (Direct current). Conventionally the, electric power is transmitted in the form of AC. However, the recent trend is to use DC for transmission purposes.
Primary vs. Secondary Transmission
During its journey, power is not stepped down all at once but is carried out twice (sometimes thrice – though that is rare).
Primary transmission refers to the power transmitted from the unit transformer to the receiving station, usually at 500 or 765 kV. It is sent using 3 phase 3-wire overhead power system.
Secondary transmission refers to the power delivered from receiving station to the substation.
Transmission line components
Transmission line components are essential elements that ensure the functioning and working of a transmission line. These components are responsible for transmitting energy from one point to another. Design Engineers are always interested in developing components that enable power transmission with minimal loss and maximum efficiency. The successful operation of transmission lines greatly depends on mechanical design and the appropriate selection of components. The list of transmission line components includes conductors, pole or pylon supports, insulators, cross arms, lightning arresters, and anti-climbing wires.
Transmission line Conductors
One of the most essential components of a transmission line is the conductor. It is the element that carries the electrical energy and is responsible for transmission from sending to receiving end station. Conductors are made of materials such as aluminum, copper, or steel and are chosen based on their conductivity, strength, and cost-effectiveness.
The conductor used for the transmission line should possess the following features:
- High electrical conductivity to ensure efficient conduction of electric current
- Cost-effectiveness to ensure economic availability for long transmission line distances
- Low specific gravity to provide lighter transmission line conductors.
- High tensile strength to fight against mechanical stresses and tensions
One of the most popular conductors in practice is ACSR.
ACSR, an abbreviation for Aluminium conductor steel-reinforced, is an economical, affordable, stranded conductor used in overhead transmission lines. The outer stands of ACSR are made of aluminum which is lightweight and a good conductor of electricity. The inner strands are made of steel. They provide mechanical strength to the conductor.
Transmission line Insulators
Besides the conductor, the insulator is another essential component of a transmission line. The insulator separates the line conductor from the supporting structure and the ground. Insulators are generally assembled from glass, porcelain, or polymer. The choice of a particular insulator is based on its ability to withstand voltage, temperature, and environmental conditions. Transmission line insulators are generally of three types:
- Pin Insulator – Used below 33 kV
- Suspension Insulator – Used above 33 kV
- Strain Insulator – A configuration used when tension in lines is very high
Transmission Line Support Poles
Transmission line poles are another crucial component of a transmission system. They provide physical support to the conductor and the insulator while electrically isolating them. Transmission line supporting structures are made of materials such as steel. Poles are selected based on their strength, durability, and cost-effectiveness. Desired features of poles are:
- Robust mechanical strength to support conductor weight and fight wind loads
- Easy to maintain
- Extended lifespan
- Lightweight so as not to compromise mechanical stability
- Available at economically competitive price
- Convenient accessibility to perform maintenance when required
Transmission line Lightning arresters
Lightning arresters are vital protection components of a transmission line. They protect the transmission line from lightning strikes and overvoltages. Lightning arresters are usually installed on transmission towers designed to discharge the high voltage surge caused by lightning strikes.
Ground Wire
On the top of overhead transmission lines is the ground wire intended to protect the line. Ground wire provides a dedicated path for the flow of fault currents to the ground. It is installed as a safety measure to protect the transmission line and equipment in case of a fault or lightning strike.
Types of Transmission Lines Based on Length
Transmission lines can be classified based on their length into three different categories. That includes short transmission lines, medium transmission lines, and long transmission lines. This categorization is based on the physical length of the transmission line.
- Short transmission lines: Length < 80 km or 50 miles
- Medium transmission lines: 80 km < Length < 250 km OR 50 miles < Length < 155 miles
- Long transmission lines: Length > 250 km or 160 miles
FAQs on Transmission Lines
Question: What is the highest AC transmission voltage ever used?
The Ekibastuz–Kokshetau high-voltage line was commissioned in 1985 at the voltage level of 1150 kV. Later, the line was downgraded to 500 kV, and it now operates at the same voltage.
Question: Is there any plan for AC transmission line voltage greater than 1150 kV?
Yes, not only planned, but construction is already ongoing on the 1.20 MV or 1200 kV Ultra-High Voltage transmission line in India’s Wardha-Aurangabad transmission line project.
Question: What is the longest overhead transmission line system network in the world?
The Ameralik Span is the longest span of an electrical overhead power line in the world spans 5376 meters that are 17,638 ft. The network is situated near Nuuk on Greenland and crosses Ameralik Fjord.
Question: What is the longest underground power transmission network ever installed?
The longest underground power transmission network in Australia is 170 kilometers or 106 miles long.
Question: What is the highest capacity transmission network ever installed?
The highest capacity system title is 12 GW Zhundong–Wannan. It is a 3293 km long line with an operating voltage of 1100 kV HVDC and was completed in end of 2018.