Classification of Overhead Transmission lines

We know the parameters R, L, C greatly affect the performance of transmission line. Therefore, it is necessary for us to find out the line losses, transmission efficiency, voltage drop, etc. Resistance is more responsible for the power loss in the line and thus determines the transmission efficiency of the line.

These three constants i.e. R, L, &C are distributed uniformly along the line length. The resistance (R) and inductance (L) form the series impedance and the capacitance existing between the conductors for 1Ф or from conductor to neutral for 3Ф line forms a shunt path. Thus the capacitance effect creates complications in the calculation of transmission line.

The overhead transmission line is classified as below:

Short transmission lines:

  • In this, the capacitance effects are small and hence neglected. This is because the length of the overhead line is small up toabout50km and the line voltage is also comparatively low (less than 20KV). This line is thus considered as short transmission line.
  • Due to the small length of line and less line voltage, the capacitance effects are small. And therefore not taken into account.
  • Only resistance and inductancee. the series parameters are considered.


Medium transmission lines:

  • In this, the length of overhead line is about 50 to 150km and the line voltage is between 20 KV to 100 KV. This line is considered as medium transmission line.
  • Due to enough length and the line voltage, the capacitance effects are considered here.
  • For the calculation of line to become easier, the capacitance is divided and lumped (combined) in the form of condensers shunted across the line at one or more points


Long transmission lines:

  • When the length of an overhead line is more than 150km and the line voltage is more than 100KV, then this is considered as the long transmission line.
  • In this the line constants are considered to be uniformly distributed along the whole length of line.

Important Terms: –

While studying the performance of a transmission line, it is desirable to determine its voltage regulation and transmission efficiency. We shall explain these two terms in turn.

(I) Voltage regulation. When a transmission line is carrying current, there is a voltage drop inthe line due to resistance and inductance of the line. The result is that receiving end voltage (VR) of the line is generally less than the sending end voltage (VS). This voltage drops (Vs−V R) in the line is expressed as a percentage of receiving end voltage V and is called voltage regulation.

The difference in voltage at the receiving end of a transmission line **between conditions of no load and full load is called voltage regulation and is expressed as a percentage of the receiving end voltage.

(II) Transmission efficiency. The power obtained at the receiving end of a transmission line isgenerally less than the sending end power due to losses in the line resistance.

The ratio of receiving end power to the sending end power of a transmission line is known as the transmission efficiency of the line

Advantages & Disadvantages of Overhead Transmission Line: –
Overhead Transmission lines are cheaper as the insulation cost is lesser and the conductor material cost is lesser too. They also have better heat dissipation.

However, they have significant disadvantages.  Overhead lines are vulnerable to lightning strikes which can cause interruption.  Overhead lines use bare conductors and can cause damage if they break.  They are considered to be unsightly as they mar the scenery of the landscape.  The maintenance cost of overhead lines is more and the voltage drop in overhead lines is more.

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