MODELLING AND PERFORMANCE OF TRANSMISSION LINES

 1. Length wise classification of transmission line:

  • Short transmission line (l < 80km)
  • Medium transmissio line (80km < l < 250km)
  • Long transmission line (l > 250km)
Short transmission line:
                 Transmission lines which are having length less than 80 km and operating voltage lower than 20 kV are called short transmission line.

Medium transmission line:
                 Transmission lines which are having length between 80 km to 250 km and line voltage between 20 kV to 100 kV are called medium transmission line.

Long transmission line:
                 Transmission lines which are having length above 250 kilometre and line voltage above 100 kilo volt are called long transmission line.

2. Regulation of transmission line:
                 Regulation of a transmission line is defined as the change in voltage at the receiving end from no load to full load, the sending end voltage remaining the same. It is usually expressed as a percentage of receiving end voltage.

3. Efficiency of a transmission line:

               Efficiency of a transmission line is defined as the ratio of power delivered to the power sent.

4. Nominal 'T'method:
                 In medium transmission lines the total capacitance of each conductor is concentrated at the centre of the line and half the line impedance is lumped on its either side. This is called as nominal 'T' method. Nominal 'T'method is a method which is used for obtaining a performance calculation of medium transmission lines.

5. Nominal 'π' method: 
                 In medium transmission lines one half of the total capacitance of each conductor is lumped at both ends. This is called as nominal 'π' method. Nominal 'π' method is a method used for obtaining the performance calculation of a medium transmission line.

6. Ferranti effect:
                 In long transmission lines and cables receiving end voltage is greater than sending end voltage during light load or no load operation. Under no load or light load the capacitance associated with the line generate more reactive power than the reactive power which is absorbed, hence VR >VS .This effect is known as ferranti effect. 

7. Surge impedance:

                 The square root of the ratio of series impedance to the shunt admittance is called us surge impedance of the line.

8. Use of power circle diagram:

                 To determine the maximum power that can be transmitted over the line both at the receiving and the sending end.

9. Factors limiting power transfer capability:

  • Thermal loading limit 
  • Stability limit 
  • Voltage drop limit

10. Corona:
                 When the potential difference is increased, potential gradient is set up. If the potential gradient is above 30 kV/cm, the conductor gets ionised.  The phenomenon of faint violet glow, hissing noise and production of ozone gas is known as corona.

Factors affect corona
  • Atmosphere 
  • Conductor size 
  • Spacing between the conductors 
  • Line voltage
Critical disruptive voltage:
                 Potential difference between conductors, at which the electric field intensity at the surface of the conductor exceeds the critical value (30kV/cm) and occurs corona is known as critical disruptive voltage.


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