The transmission line is the part of the power system, it is the medium for transmitting the electrical power generated at generating station to load centers. The transmission lines are classified based on the voltage level, length of the conductor, and types of current flow. They are mainly classified into two types,
- AC transmission line, and
- DC transmission line.
1. AC Transmission Line :
An ac transmission line uses an alternating current to transmit the power from generating stations to load centers. It is a set of conductors being run from one place to another supported on transmission towers. Such lines will have three constants resistance, inductance, and capacitance distributed uniformly along the whole length of the line.
The resistance and inductance form the series impedance and capacitance forms the shunt element throughout the length of the line. Depending upon the operating voltage and length, the overhead ac transmission lines are classified as,
- Short transmission lines
- Medium transmission lines
- Long transmission lines.
Short Transmission Line :
If the overhead transmission line is less than 50km, then this type of transmission line is referred to as Short Transmission Line. Generally, the short transmission lines carry operating voltages of less than 20kV.
In these lines, the effect of capacitance is neglected due to smaller length and low operating voltage. Hence, the resistance and inductance effects of the line are considered while determining the performance of the short transmission line as shown below.
Medium Transmission Line :
When the length of the overhead transmission line is in the range of 50-150km, and the operating voltage is greater than 20kV, then such lines are called medium transmission lines. As the length of the medium transmission lines is more, the charging current will be significant and hence the effect of capacitance will be taken into consideration.
The capacitance of the transmission lines forms the shunt element and is distributed throughout the length of the line. But, for analysis and calculations, it will be assumed that the capacitance is lumped and concentrated at different strategic places.
Based on the location of the capacitance at different places, the medium transmission lines have different configurations. These configurations show the different ways in which the effect of capacitance is taken into consideration. The three configurations based on the location of capacitance are,
- End condenser representation of medium transmission lines
- Nominal-T representation of medium transmission lines
- Nominal-π representation of medium transmission lines.
End Condensor Representation of Medium Transmission Line :
In this configuration, the total capacitance of the line is assumed to be concentrated at the receiving end side of the transmission line. The below shows the end condenser medium transmission line configuration.
Nominal-T Representation of Medium Transmission Line :
The nominal-T method is also called as middle condenser method and it is one of the localized capacitance methods used to calculate the performance of the transmission line. This method overcompensates (gives low values for) sending end voltages.
In the nominal-T method, the total capacitance of the line is assumed to be concentrated at the mid-point (center) of the entire transmission line. Since the total line capacitance is assumed at the center, the charging current flows through half of the line, and the series impedance (resistance and inductive reactance) of the line is divided into half on either side of the capacitance as shown above.
Nominal-π Representation of Medium Transmission Line :
The nominal-π method is also called as split condenser method. and it is one of the localized capacitance methods, using which the performance of the transmission line can be calculated efficiently. This method under compensates (gives higher values for) sending end voltages.
In the nominal-π method, the capacitance of the transmission line is divided into halves placed at the receiving end and sending end respectively as shown above.
Long Transmission Line :
The overhead transmission lines whose length is more than 150km are referred to as long transmission lines. The operating voltage of these lines is more than 100kV. For treatment of this type of line, their line constants are assumed to be distributed in a uniform manner along the length of the line, and also rigorous methods are used for their solution.
2. DC Transmission Line :
In ac transmission lines, for transmission of power over long distances at higher voltages, the cost of transmission line and loss increases. Also, the ac long transmission line suffers from problems like stability limits, voltage control, line compensation, interconnection of lines, ground impedance, etc due to an increase in voltage levels and distance.
The various problems associated with long-distance ac transmission have led to the development of HVDC (high voltage direct current) transmission nothing but a dc transmission line.
The use of dc power for long transmission lines has various advantages like no stability problem, absence of charging current, no skin effect, need for reactive compensation, bulk power transfer, economic power transmission, etc.
However, the power utilization and generation remain at ac, by using converters, a dc transmission can be implemented. It requires two converters one at sending end side and the other at receiving end side as shown above.
The converter at sending end side acts as the rectifier (converts ac to dc) while the converter at receiving end side acts as the inverter (converts dc to ac). But it is inefficient to use a dc transmission system for shorter and medium distances.
