In this article we will discuss about:- 1. Classification of Transmission Lines 2. Regulation of Transmission Line 3. Efficiency.
Classification of Transmission Lines:
For discussing the performance of the overhead transmission lines they are classified as short, medium and long transmission lines. The overhead transmission lines are classified depending upon the manner in which capacitance is taken into account.
Transmission lines having length lesser than 80 km and operating voltage lower than 20 kV fall in the first category i.e. short transmission lines. Due to smaller distance and lower line voltage, the capacitance effects are extremely small and, therefore, can be neglected.
Hence the performance of short transmission lines depends upon the resistance and inductance of the line. Though in an actual line, the resistance and inductance are distributed over the whole length, but in case of short lines, the total resistance and inductance are assumed to be lumped at one place.
In case of single phase circuits, the total loop resistance and inductance are to be taken into account, whereas, in case of 3-phase circuits only resistance and inductance to neutral i.e. per phase is required to be taken into account. The effect of generators and transformers can be taken into account by adding their impedances to the impedance of the line.
Transmission lines having length between 80 km and 200 km and line voltage between 20 kV and 100 kV fall into second category i.e. medium transmission lines. Owing to appreciable length and voltage of the line the charging current is appreciable and, therefore, capacitance effect cannot be ignored. Though the capacitance is uniformly distributed over the entire length of the line, yet the capacitance may be assumed to be concentrated at one or more points.
The transmission lines having length above 200 km and line voltage above 100 kV fall into third class i.e., long transmission lines. In these lines impedance and admittance are to be considered uniformly distributed (not lumped) and, therefore, for their solution rigorous methods are employed.
It is to be emphasised here that exact solution of any transmission line must be based on the fact that the parameters or constants of any line are not lumped but are distributed uniformly throughout the length of the line. However, results obtained by assuming the constants as lumped for short and medium transmission lines are reasonably accurate.
Regulation of Transmission Line:
When the load is supplied, there is a voltage drop in the line due to resistance and inductance of the line and, therefore, receiving-end voltage VR is usually less than sending-end voltage VS. The voltage drop i.e., difference of sending-end voltage and receiving- end voltage expressed as a percentage of receiving-end voltage is called the regulation.
When the load is thrown off i.e., when the line is supplying no load, the receiving-end voltage becomes equal to sending-end voltage and therefore, regulation can be defined as below:
Regulation is defined as the change in voltage at the receiving (or load) end when the full-load is thrown off, the sending-end (or supply) voltage and supply frequency remaining unchanged. It is usually expressed as a percentage of receiving-end voltage.
Mathematically percentage voltage regulation of a transmission line is given by the expression:
where Vs is the voltage at sending or supply end and VR is the voltage at the receiving or load end. It is to be noted here that Vs and VR are the magnitudes of voltages.
The lower the voltage regulation, better it is, because low voltage regulation means little variation in receiving-end voltage due to variation in load current.
Knowledge of voltage regulation helps in maintaining the voltage at the load terminals within prescribed limits by employing suitable voltage control equipment.
Efficiency of Transmission Line:
When the load is supplied there are line losses due to ohmic resistance of line conductors and power delivered at the load end of a transmission line is less than the power supplied at the sending end.
Efficiency of a transmission line is defined as below:
Efficiency of a transmission line is defined as the ratio of power delivered at the receiving end to the power sent from the sending end.
Mathematically transmission efficiency is given by the expression:
where VR, IR and cos ɸR are the receiving-end voltage, current and power factor (all phase values) while Vs, Is and cos ɸs are the sending-end voltage, current and power factor (all phase values).