Transmission Line: Representation and Performance Parameters

Transmission Line Representation, Classification, and Performance Parameters

An electrical power system is classified into three parts;

  • Generation
  • Transmission
  • Distribution

Electric power is generated in different types of power plants. And these plants are far from the load. Hence, it is necessary to send this power to the remote areas of all regions.

Also, the power generated at an 11-15 kV voltage level. If you transfer the power at this voltage level, there will be a huge loss in a transmission line. Therefore, to reduce the transmission loss and cost, the power is transmitted at a higher voltage level.

A conductive line that is used to transmit the electrical power from a power plant to other places of the system is known as the “Transmission Line”.

Representation of Transmission Line

To represent the transmission line, a three-phase star-connected generator uses to connect with a load via the transmission line.

The transmission line has resistance, inductance, and capacitance. The capacitance of the transmission line is negligible if you consider a small length transmission line.

Hence, the graphical representation of the transmission line contains only resistance and inductance connected in series.

The representation of the transmission line is as shown in the below figure.

Representation of Transmission System
Representation of Transmission System

The generator is represented by an impedance connected in series with the generated EMF of each other.

In a balanced three-phase network, the sum of all current is zero. Hence, the current passing through the wire connection between the star points of load and neutral of the system is zero.

Therefore, the three-phase balanced system can represent and analyzed by a single-phase system. The equivalent circuit of a three-phase network is represented as a single-phase network as shown in the below figure.

Single-phase Equivalent Representation of Three-phase Transmission System
Single-phase Equivalent Representation of Three-phase Transmission System

Classification of Transmission Line

According to the length of line, the transmission lines are classified as;

  • Short Transmission Line
  • Medium Transmission Line
  • Long Transmission Line

Short Transmission Line

A transmission line having a length of fewer than 80 km (50 miles) is considered a short transmission line. And generally, the operating voltage of this line is less than 20 kV.

The capacitance of the line is neglected due to the small length and low operating voltage. Therefore, the performance of a short transmission line is only depending on the resistance and inductance.

Hence, the representation of a short transmission line is done using series-connected resistance and inductance.

Medium Transmission Line

A transmission line having a length between 80 km and 240 km is considered a medium transmission line. And generally, the operating voltage of a medium transmission line is 20 kV to 100 kV.

In a medium transmission line, the charging current is present and we cannot neglect it like a short transmission system. Therefore, we have to consider the capacitance due to sufficient length and operating voltage level.

So, the medium transmission line is represented by resistance, inductance, and capacitance. There are three methods to represent the medium transmission line;

  • End Condenser method
  • Nominal T method
  • Nominal π method

Long Transmission Line

A transmission line having a length of more than 240 km is considered a long transmission line. And generally, this line has an operating voltage level above 100 kV.

The model of a long transmission line is a uniformly distributed model over the entire length of a line. We cannot consider a lumped model like a short and medium transmission line.

Line constants are uniformly distributed over the entire length of a line. And the resistance and inductance are a series of an element. The representation model of a long transmission line is as shown below figure.

Representation of Long Transmission Line
Representation of Long Transmission Line

Performance Parameters

To analyze the performance of the transmission line, we have to consider the most important performance parameters like;

  • Voltage Regulation
  • Transmission Efficiency

Voltage Regulation

Due to line parameters (resistance, inductance, and capacitance), a current-carrying transmission line has a voltage drop. Therefore, the receiving end voltage is less than the sending end voltage.

The difference between sending end voltage and receiving end voltage expressed as a percentage of receiving end is called the voltage regulation.

Mathematically, the percentage voltage regulation of the line is expressed as the below equation.

    \[ \% \, Voltage \, Regulation = \frac{V_S - V_R}{V_R} \times 100 \]

Where;

VR = Receiving end voltage
VS = Sending end voltage

The value of percentage voltage regulation of the transmission line should be as low as possible.

Transmission Efficiency

Due to the impedance of the transmission line, some amount of loss occurs. This causes some amount of power at receiving end is less than the power input at the sending end.

The transmission efficiency is defined as the ratio of receiving end power to the sending end power. And mathematically, it is equating as;

    \[ Transmission \, Efficiency = \frac{Receiving \, end \, Power}{sending \, end \, Power} \times 100 \]

    \[ \eta_T = \frac{V_R I_R cos(\phi_R)}{V_S I_S cos(\phi_S)} \times 100 \]

Where;

VR = Receiving end voltage
VS = Sending end voltage
cos фR = Power factor at receiving end
IR = Receiving end current
IS = Sending end current
Cos фs = Power factor at sending end

 390 total views,  1 views today

3 Comments

    1. Derivation and Examples will cover in next articles. It will given for individual type of Transmission Line.

Leave a Reply

Your email address will not be published. Required fields are marked *