What is a transmission power line? Why DC is not used for power transmission

What is a transmission power line and why do we need transmission lines

The transmission line is a metallic conductor system that is used to transfer electrical energy from one point to another using electric current flow. A transmission line can be described as an electric conductor which would be two or more separated by a non-conductive insulator. The purpose of the transmission line is to connect a source to a load for energy transfer. Mostly the overhead transmission line consists of three conductors or bundles of conductors which has the three phases of the power system. During the transmission of power, energy loss would happen and this loss is depended on the electrical and physical property of the transmission system.

What are the types of faults that occur in a transmission line

  • Transient fault
  • Persistent fault
  • Symmetric fault
  • Asymmetric fault

How can we reduce the transmission line loss and why transmission is done at high voltages

Transmission of electric power is done for long-distance, so in order to reduce the energy loss during the transmission, the electric power is transmitted in high voltage. So high voltage transmission lines are used for energy transport over long distances. Higher voltage transmission lines require larger structures with longer insulator chains in order to have larger air spaces and the necessary insulation. While transferring the power at the high voltage the current would be decreased. So the same amount of power can be transmitted by reducing the loss in high voltage transmission. Smaller conductors can be used because of the reduction of current. High voltage will reduce the current and thus the resistive losses in the conductor.

What are the three power lines

The three power lines are transmission lines, the transmission lines have three conductors or bundles of conductors which has the three phases of the power system. These power lines are used to transmit power from the power plant to the substation and it is done is high voltage to reduce the loss.

Are transmission lines AC or DC

The power transmission is mostly done in AC. In AC transmission we can vary the voltage level while in DC we won’t be able to do that. Transmission is done in AC and DC but mostly we use AC transmission because the voltage can be varied in an AC transmission with the help of a transformer, but to vary the DC voltage is really complex process and that’s why power is transmitted in AC, at high voltages so that the power loss can be decreased.

Why DC is not used for transmission

By using AC we can step up the power from the generation station to any voltage level and can be transmitted to the long-distance load center. When the voltage is stepped up the current will be reduced so the conductor size can be reduced and the cost is reduced. The voltage drop is reduced so long-distance transmission is possible.

How transmission lines are classified

Balanced transmission line

In this type it has two wire balanced lines, with both conductors carry current, one conductor carries the signal and the other conductor is the return path and this kind of transmission is called balanced signal transmission. In balanced transmission, both conductors are in a balanced line carrying signal currents. The two currents travel in the opposite direction but have the same magnitude.


The two currents are equal in magnitude with respect to the electrical ground but travel in the opposite direction. The current that flows in the opposite direction in a balanced pair are called metallic circuit currents and the current that flows in the same direction in a balanced pair are called longitudinal currents. This type of transmission has the advantage that noise interference is induced equally in both wires.

Unbalanced transmission line

In this type one wire is at the ground potential, whereas the other wire is at the signal potential and this type of transmission is called single-ended or unbalanced signal transmission. With this, the ground wire may also be the reference for other signal carrying wires. Due to this, the ground wire needs to accompany the signal wires. The unbalanced transmission line has very little immunity to common-mode signals. The potential difference on each signal wire is measured from that wire to a common ground reference, balanced transmission lines can be connected to the unbalanced ones by the help of baluns.

Short transmission line

If the length of the transmission line is 50KM and the line voltage is low, the line voltage could be 20Kv or less than that then it is considered as the short transmission line

Medium transmission line

If the length of the transmission line is 50-150km and the line voltage is 20Kv -100Kv then they are medium transmission lines

Long transmission line

If the length of the transmission line is 150Km and the line voltage is about 100Kv then the transmission line is a long transmission line.

What is conductor loss and dielectric heating loss

Conductor loss is the loss that occurs due to the finite resistance of the transmission line. The dielectric heating loss is due to the difference of potential between the two conductors of the transmission line.

What is the use of transmission line

The major purpose of the transmission power line is the transmission of electrical power from the power plants to the substations and from there to the customers.

What are the characteristics of transmission lines

There are certain parameters for an electrical transmission line that would affect its ability to do the transmission of power they are

Resistance – Every wire has resistance and the amount of resistance is related to the type of materials used, the transmission line also has a resistance which is called loop resistance.

Inductance – Due to this there will be an opposition to the flow of AC. A wire that carries current would have inductance. The line has current and the conductors are surrounded by magnetic flux, so the inductance will be present all along the line.

Capacitance – Two conducting wires are used to create transmission lines, and dielectric will be present between these wires

Conductance – Insulating materials are used to isolate the two wires of the transmission line, and this insulation material has a conductive effect.

What are the factors that must be considered while designing a transmission line

  • Type and size of conductors
  • Line regulation and control of voltage
  • Corona loss
  • Power flow capability
  • Efficiency of transmission

What are the types of conductors used in transmission lines

Copper wires were used for the transmission process but now aluminum conductors are used nowadays because they have many advantages over copper. An aluminum conductor is of low cost than copper and they are lightweight. Corona loss is also less for aluminum conductors.

How overhead transmission lines are constructed and components of overhead lines

Transmission lines are constructed with the help of conductor, insulator, and tower. An underground cable is made of conductor, insulation and it is buried. The three-phase conductor will carry electric current. Insulators will electrically isolate the conductor.

Major components of overhead lines

  • Supports
  • Cross arms and clamps
  • Insulators
  • Conductors
  • Guys and stays
  • Lightning arrestors
  • Fuses and isolating switches
  • Earth wire
  • Guard wires
  • Phase plates
  • Bird guards
  • Danger plates
  • Barbed wire
  • Miscellaneous plates (vibration damper)

What is gas-insulated transmission line (GIS)

This type of transmission line is used to transfer electricity at high power ratings over a long distance. In case where overhead transmission lines are not possible, we can use gas-insulated transmission lines. GIS has low power loss when compared to other types of transmission systems.

What is an ideal transmission line

An ideal transmission line doesn’t have any losses it’s conductors have zero resistance

  • Dielectric has zero conductance
  • Possible only with superconductors
  • Approximated by a short line

What are the types of transmission lines

Parallel wire line

Parallel wire lines are used when, balanced properties are needed, these lines have two metallic conductors and they are separated by an insulating material. The parallel wireline is divided into three types they are.

  • Open wire
  • Twin lead
  • Twisted pair

Open wire

The open wire transmission line has two wires and they are spaced from 2 to 6 inches apart by the insulating spacer. Their construction is very simple and they have certain disadvantages like high radiation loss and no shielding.

Twin lead

This type is the same as the open wire and the only difference is the uniform spacing which is done by embedding the two wires in a solid low loss dielectric. This type is vulnerable to interference

Twisted pair

In this type, two insulated wires are twisted together to form a flexible line without the use of spacers. It has a high dielectric loss, so it is not used for high-frequency applications.

Shielded and unshielded twisted pair

In shielded twisted pair a parallel twisted pair lines consisting of two conductors separated by a dielectric material. Whereas the unshielded pair has two copper wires and both the wires are separately insulated by using PVC.

Coaxial lines

This type of transmission line has a center conductor that is surrounded by a dielectric material. There are two types of coaxial lines they are rigid and flexible.

Rigid coaxial lines

The rigid coaxial lines have a central insulated wire which is mounted inside a tubular outer conductor. This type of coaxial lines are able to minimize the radiation losses, but they are expensive to construct

Flexible coaxial lines

These lines are very flexible and flexible wire is used in inner conductors and these wires are insulated. Metal braid is used to make the outer conductor so better flexibility.


A waveguide is classified as a transmission line and its transmission method is different from conventional methods. Waveguides are classified according to their cross-section or according to the construction material.

What are the losses in transmission lines

Radiation loss

This loss occurs when a high-frequency current or voltage waveform flows through the transmission line. Because of this the magnetic field will expand and collapse around the transmission lines at the rate of the input frequency. So at high frequency, the radiation causes the attenuation in the energy provided by the source towards the load.

Conductor heating

When the current flows through the transmission line the conductor will heat up. So when the lines heat up it would reduce the energy.

Dielectric loss

Transmission lines are composed of two parallel conducting wire and the flow of current is through the line and there will be a potential difference between the lines, this potential difference will cause the leakage current through the dielectric.

How to protect transmission lines

When the length of transmission lines increases then the chances of faults also increase. There are certain methods for transmission line protection they are time graded overcurrent protection, differential and distance protection