What is a transmission line?

The performance of a power system is mostly determined by the producing stations and transmission lines in the system which can be studied with the help of transmission line parameters.

The transmission line is the cable that carries power that is responsible for transmitting or conveying power from the generating power station to the distribution network. Transmission line voltage drop and power losses, as well as transmission efficiency, are crucial in the operation of transmission lines.

A figure showing a 500 kV three-phase AC transmission line. It is noted that tower length is quite high to avoid line charging capacitance due to very high voltage.
CC Attribution-Share Alike 4.0 International | https://commons.wikimedia.org | Varistor60

What is the principle of the transmission line?

Two measures, percentage efficiency, and percentage regulation can be used to assess the transmission line's performance.

The proportion of power supplied at the receiving station to power sent from the sending station is known as transmission line efficiency.

 The higher the efficiency, the better is the transmission line's performance and the better is the electrical power transferred. A perfect transmission line would have a hundred percent efficiency (i.e. maximum electrical power and zero losses), which is impossible to achieve in practice.

Voltage regulation of a transmission system is the increase in voltage at the receiving station, expressed in percentage of full load voltage when the full load at a specific power factor is decremented while the sending station voltage remains constant. The better the transmission line's performance, the less is the voltage regulation.

A perfect or ideal transmission line has no regulation for maximum electrical power flow.

Types of transmission lines

Transmission lines are distinguished into the following varieties based on their length: short, medium, and long lines.

The operating voltage of a short line is between (0-20) kV and the length is between (0-80) km, having zero capacitance, having only two transmission line parameters.

The length of a medium line is between 80 and 200 kilometers, and the working voltage is between 20 and 100 kV, having negligible capacitance, having three transmission line parameters.

Long transmission lines have a length of more than 200 kilometers and a voltage of more than 100 kV, having considerable capacitance, having all four transmission line parameters.

Transmission line parameters

A transmission line's parameters are the following:

Resistance (R) parameter

The wire's resistance of the loop for a unit length is defined as resistance (R). It is measured in the unit of ohm/Km.

Inductance (L) parameter

The wire's inductance of the loop for a unit length is defined as inductance (L). It is measured in the unit of Henry/Km.

Capacitance (C) parameter

The wire's capacitance of the loop for a unit length is defined as capacitance (C). It is measured in the unit of Farad/Km. It is also a measure of dielectric strength between the transmission line and the earth.

Conductance (G) parameter

The wire's conductance for a unit length is defined as conductance (G). It is measured in the unit of mho/Km.

Resistances, inductance, capacitor capacitance, and conductance are four characteristic parameters that affect an electric transmission line's capacity to perform its role as part of a power system.

The transmission line conductor resistance parameter is the most common parameter of power loss in the line. From the perspective of a power system engineer, the inductance parameter is by far the most important line parameter that limits a line's transmission capacity. The inductance parameter of a circuit is the association of the voltage induced by changing flux and the current rate of change. The ratio of the charge stored on the wires to the potential difference defines the capacitance parameter that exists between them. The transmission line and earth are considered as a capacitor having a dielectric strength of air (dielectric strength of air is equal to epsilon). The conductance parameter exists in the middle of conductors or from conductors to the earth as the fourth parameter.

Applications of transmission line parameters

Transmission lines are utilized in microwave design to convey digital and analog signals, as well as frequency domain components.

Transmission lines are used for a variety of purposes, including the following:

  • Length of tower can be calculated by considering transmission line parameters
  • Transmission line for electricity
  • Lines of communication
  • Multi-Chip Module
  • Integrated Circuit Package Traces

Signal quality for a PCB and wires in a system is required for analog and digital circuits. The frequency response of transmission lines circuits can be studied in analog circuits.

Lumped and distributed transmission line parameters

This figure shows the distribution parameters of the transmission line.
Lumped and distributed transmission line

Lossless transmission lines, defined as R = G = 0, are referred to as ideal transmission lines. This is true if attenuation and skin effect are either non-existent or unimportant for the signal frequencies under consideration.

The phase velocity of real lines is slightly dependent on the applied frequency, and the series resistance is not nearly zero. Attenuation and dispersion are the results of these non-idealities.

These effects can result in the frequency elements of a signal at the far end of the line differing significantly from those at the source. The input signals' rapid peak and fall times can be lowered and "rounded." It's worth noting that when R/L = G/C, there's a theoretical case where there's attenuation without dispersion. Normally, this has no practical significance.

Cable makers frequently present an attenuation versus frequency curve to demonstrate vulnerability to these effects:

This figure shows the graph plotted between attenuation and frequency for 100 m. coaxial cable having losses. It can be observed that as the frequency is increased, the attenuation value becomes more negative.
Frequency curve

Common Mistakes

Remember that, transmission lines, like other capital projects, present obstacles throughout construction.

Note that reactance is different from resistance throughout the length of the line. Due to the reactance property of the transmission line, power flow is possible. If the entire length of the line does not possess reactance property, then the line cannot be charged and charges become static.

Context and Applications

In each of the expert exams for undergraduate and graduate publications, this topic is huge and is mainly used in the following context:

  • Bachelor of Technology in the Electrical and Electronics Department
  • Bachelor of Science in Physics
  • Master of Science in Physics
  • Distribution system
  • Corona
  • Sag and tension

Practice Problems

Q1. The distributed transmission line parameters are _____________.

(a) resistance and shunt conductance

(b) resistance and inductance

(c) resistance and capacitor capacitance

(d) resistance, inductance, capacitance, and shunt conductance

Correct option: (d)

Explanation: These are the transmission line parameters.

Q2. In overhead lines, we generally use _____________.

(a) copper conductors

(b) all aluminum conductors

(c) ACSR conductors

(d) none of these

Correct option: (c)

Explanation: ACSR (Aluminum conductor steel reinforced) conductors have mostly replaced copper conductors for overhead lines due to their less cost and lightweight.

Q3. For a medium transmission line, A is _____________.

(a) equal to B

(b) equal to C

(c) equal to D

(d) None of these

Correct option: (c)

Explanation: A medium transmission line is symmetrical. Therefore, A=D.

Q4.  An EHV transmission line's insulation strength is mostly determined by ______________.

(a) load power factor

(b) switching overvoltage

(c) harmonics

(d) corona

Correct option: (b)

Explanation: The guiding factor in the design of insulation for EHV and UHV transmission lines is switching surges.

Q5.  Which of the parameters listed below is not a primary parameter?

(a) Resistance

(b) Attenuation constant

(c) Capacitor capacitance

(d) Conductance

Correct option: (b)

Explanation: Resistance, inductance, capacitor capacitance, and conductance are the basic properties of the line. The attenuation constant is the second parameter.

Want more help with your electrical engineering homework?

We've got you covered with step-by-step solutions to millions of textbook problems, subject matter experts on standby 24/7 when you're stumped, and more.
Check out a sample electrical engineering Q&A solution here!

*Response times may vary by subject and question complexity. Median response time is 34 minutes for paid subscribers and may be longer for promotional offers.

Search. Solve. Succeed!

Study smarter access to millions of step-by step textbook solutions, our Q&A library, and AI powered Math Solver. Plus, you get 30 questions to ask an expert each month.

Tagged in
EngineeringElectrical Engineering

Electric Circuits and Networks

Two Port Parameter

Transmission Line Parameter (T-parameter)

Search. Solve. Succeed!

Study smarter access to millions of step-by step textbook solutions, our Q&A library, and AI powered Math Solver. Plus, you get 30 questions to ask an expert each month.

Tagged in
EngineeringElectrical Engineering

Electric Circuits and Networks

Two Port Parameter

Transmission Line Parameter (T-parameter)