Transmission Line in Power System

What is Transmission Line?

Transmission line is the long conductor with special design (bundled) to carry bulk amount of generated power at very high voltage from one station to another as per variation of the voltage level.electrical transmission line

Type of Transmission Line

In transmission line determination of voltage drop, transmission efficiency, line loss etc. are important things to design. These values are affected by line parameter R, L and C of the transmission line. Length wise transmission lines are three types.

Short Transmission Line

  • Length is about 50 km.
  • Voltage level is up to 20 kV
  • Capacitance effect is negligible
  • Only resistance and inductance are taken in calculation capacitance is neglected.

Medium Transmission Line

  • Length is about 50km to 150km
  • Operational voltage level is from 20 kV to 100 kV
  • Capacitance effect is present
  • Distributed capacitance form is used for calculation purpose.

Long Transmission Line

  • Length is more than 150 km
  • Voltage level is above 100 kV
  • Line constants are considered as distributed over the length of the line.

What is the Transmission Efficiency?

Transmission efficiency is defined as the ration of receiving end power PR to the sending end power PS and it is expressed in percentage value.

cosθs is the sending end power factor.
cosθR is the receiving end power factor.
Vs is the sending end voltage per phase.
VR is the receiving end voltage per phase.

What is Transmission Line Voltage Regulation

Voltage regulation of transmission line is defined as the ratio of difference between sending and receiving end voltage to receiving end voltage of a transmission line between conditions of no load and full load. It is also expressed in percentage.

Where, Vs is the sending end voltage per phase and VR is the receiving end voltage per phase.

XL is the reactance per phase.
R is the resistance per phase.
cosθR is the receiving end power factor.
Effect of load power factor on regulation of transmission line:

  1. For lagging load
  2. For leading load

Now

  • Power factor is lagging or unity, and then VR is increased and goes to be positive.
  • Power factor is leading, and then VR is decreased and goes to be negative.

Effect of Load Power Factor on Efficiency of Transmission Line

We know efficiency of transmission line is

Now, for short transmission line, IR = IS = I
So, considering three phase short transmission line,

So,
Now it is clear that to transmit given amount of power, the load current is inversely proportional to receiving end power factor.
Again in case of medium and long transmission line,

Here it is clear that transmission efficiency depends on the receiving end power factor.

What is End Condenser Method in Medium Transmission Line?

In this method capacitance is assumed to be lumped at receiving end. One phase is shown below.
medium transmission line
Here IR is the receiving end load current per phase,
R is the resistance per phase,
XL is the inductive reactance per phase,
C is the capacitance per phase,
cosΦR is the receiving end lagging power factor,
VS is the sending end voltage.
Let us assume, as the reference phasor,

Load current at receiving end

The capacitive current

Now,


Now,

and

What is Nominal T Method in Medium Transmission Line?

Total capacitance of the line is assumed to be concentrated at the middle point of the line, and at both side half of line resistance and inductance is lumped.
t method in medium transmission line
Here,
IR is the receiving end load current per phase,
R is the resistance per phase,
XL is the inductive reactance per phase,
C is the capacitance per phase,
cosΦR is the receiving end lagging power factor,
VS is the sending end voltage.
V1 is the voltage across the capacitor.
Voltage across Capacitor C,

Capacitive current

Sending end current

Sending end voltage

What is Nominal π Method in Medium Transmission Line?

In this method total line capacitance is assumed to be lumped and divided into two halves to be connected across sending end and receiving end respectively. Total line resistance and inductance are assumed to be present in middle of the line.
pi method in medium transmission line
Here IR is the receiving end load current per phase,
R is the resistance per phase,
XL is the inductive reactance per phase,
C is the capacitance per phase,
cosΦR is the receiving end lagging power factor,
VS is the sending end voltage.
Let us assume, as the reference phasor,

Load current at receiving end

The capacitive current at load end

Line current

Sending end voltage,

Charging current at the sending end is

Sending End current is

What is Nominal T Method in Medium Transmission Line?

In this method the whole line capacitance is assumed to be concentrated at the middle point of the line and half the line resistance and reactance are lumped on its either side. Therefore in this arrangement full charging current flows over half the line.

Leave a Comment