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Classifications of Overhead Transmission Lines

Classifications of Overhead Transmission Lines

An electrical energy transmitted from generating power stations to substations or grid through overhead transmission lines. A transmission lines has three constants resistance R, inductance L and capacitance C. These constants are distributed uniformly along the whole length of the transmission line.

The line resistance cause voltage drop (IR) and power loss (I2R) in the line, the inductance also cause voltage drop due to inductive reactance (IXL). The resistance and inductance form the series impedance. The capacitance produces charging current (2∏fcv) in the line, which quadrature with the voltage. This constant existing between conductor and neutral for single phase and or three phase line forms shunt path through out the length of the line. The capacitance effects make the calculations complex. The capacitance effects are more predominant in case of underground cables. The overhead transmission lines are classified as.

  • Short transmission lines
  • Medium Transmission lines
  • Long transmission lines

Short transmission Lines

Transmission lines having length about 50km and operating voltage is comparatively low about 20KV, it is usually considered as a short transmission lines. Due to smaller distance and lower operating voltage, the capacitance effects are extremely small and hence can be neglected. Hence, performance of short transmission lines depends upon resistance and inductance only. Therefore, while studying the performance of such lines, only resistance and inductance of the transmission line are taken into account. In short transmission lines, Constants are assumed to be lumped at one phase.

Medium Transmission Lines

Transmission lines having length is about 50 to 150km and operating voltage is more than 20KV and less than 100KV, it is considered as medium transmission line. Due sufficient length and operating voltage of the line, capacitance effects are taken into account. The capacitance is distributed uniformly over the entire length of the line. It is assumed to be shunted across the line at one or more points.

Long Transmission Lines

Transmission lines having length more than 100km and operating voltage more than 100KV (that is very high), it is considered as a long transmission lines. In case of long transmission lines constants are considered uniformly distributed over the entire length of the transmission line.







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Advantages of High Transmission Voltage

Advantages of High Transmission Voltage

Electrical energy is generated at generating power station at voltage level 11KV. The generating power stations are for away from the consumer premises or load centre. The transmission lines are used to transmit electrical energy from generating station to load centre. If we transmit electric power at 11KV, then the line losses will be increased. So, we increased voltage level to reduce the line losses. In this article, we will discuss about the advantages of high transmission voltage.

The electric power delivered in a 3-phase system is given by the relation.

V= Line voltage in voltage

I = Line current in ampere

CosΦ = Power factor of the load

advantages of high transmission voltage

Volume of conductor material required for the line:


The following are the advantages of high transmission voltage.

Reduces Volume of Conductor Material

The volume of conductor required for transmission line is inversely proportional to the square of transmission voltage and power factor. In other words we can say that greater the transmission voltage, lesser conductor material required. This decreases the cost of transmission line.

This also reduces the cost of supporting structure. Because the towers with high voltage level has less tension or stress. There is also saving in supporting structure material.

Higher Transmission Efficiency

The current in the transmission line is inversely proportional to the transmission voltage. If the transmission voltage increases at same power, the current is decreases.

Greater the transmission voltage, smaller the current. Hence, power loss reduces and the transmission efficiency is improved.

In this way, we can say that the transmission line efficiency greatly depends on the transmission voltage.

Better Voltage Regulation

Transmission of electric power at high voltage, reduces the line current which result reduction in voltage drop in the transmission line. Hence, it improves the voltage regulation of the transmission line. Further, longer transmission lines can be used.



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