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.
Types 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
A short transmission line is classified as a transmission line with:
- A length less than 80km (50 miles)
- Voltage level less than 69 kV
- Capacitance effect is negligible
- Only resistance and inductance are taken in calculation capacitance is neglected.
Medium Transmission Line
A medium transmission line is classified as a transmission line with:
- A length more than 80 km (50 miles) but less than 250 km (150 miles)
- Operational voltage level is from 69 kV to approx 133 kV
- Capacitance effect is present
- Distributed capacitance form is used for calculation purpose.
Long Transmission Line
A long transmission line is classified as a transmission line with:
- A length more than 250 km (150 miles)
- Voltage level is above 133 kV
- Line constants are considered as distributed over the length of the line.
Efficiency of Transmission Line
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.
Voltage Regulation of Transmission Line
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:
- For lagging load
- 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.
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.
End Condenser Method in Medium Transmission Line
In this method capacitance is assumed to be lumped at receiving end. One phase is shown below.
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
Nominal T Method in Medium Transmission Line
In the nominal T method the 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.
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
Nominal π Method in Medium Transmission Line
In the nominal pi 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.
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.
