Classification of overhead transmission line

Based on the length, the transmission line is classified as:

1.) Short Transmission Line: Length < 80 km

2.) Medium Transmission Line:  80 km < Length < 200 km

It is further classified into end condenser method, nominal π method, and nominal T method.

3.) Long Transmission Line: Length > 200 km

End condenser method

  • In the end condenser method, the capacitance of the transmission line is assumed to be concentrated at the line's receiving end (the load end). Hence, The voltage across the load is equal to the voltage across the lumped line capacitance.
  • This simplifies the analysis of the line because it allows engineers to treat the line as a combination of a series impedance and a shunt capacitance at the load end.

Limitations:

  • The end condenser method is an approximation and is less accurate for very long transmission lines where the effects of distributed capacitance cannot be ignored.
  • It assumes that all the capacitance is concentrated at one point, which simplifies the calculation but may not fully represent the behavior of the line in practice.

Transmission and distribution system

The transmission and distribution systems are classified into two types namely, Overhead cable system and Underground cable system.

In the overhead system of transmission and distribution, the overhead line wires supported by poles are used for power transportation; while in the underground system, insulated underground cables are employed for the transportation of electric power.

Difference between overhead lines and underground cables:

Parameter Overhead cables Underground cables
Type of conductor Overhead lines use bare conductors. Underground cables use insulated conductors.
Voltage rating Overhead lines have a higher voltage rating, near about 765 kV. Underground cables have comparatively lower voltage ratings. It is typically limited to 66 kV.
Current carrying capacity The current carrying capacity of overhead lines is comparatively more. Underground cables have a lesser current-carrying capacity.
Fault detection and clearance In the case of overhead lines, the detection and clearance of faults is easy. It is very difficult to detect and clear the faults in underground cables.
Line cost Overhead lines are relatively less expensive. Underground cables are comparatively expensive.
Corona discharge The corona discharge occurs in the overhead lines. No corona discharge takes place in the underground cables.
Proximity effect Overhead lines remain unaffected by the proximity effect. The proximity effect influences the underground cables.

Methods of speed control of an induction motor

The speed of the induction motor can be controlled by any of the following methods:

1.) Cascade control method

  • In this method of speed control, two motors are used. Both are mounted on the same shaft so that both run at the same speed.
  • One motor is fed from a 3-phase supply and the other motor is fed from the induced emf in the first motor via slip-rings.

2.) V/f control or frequency control

  • This method is used to control the speed of the induction motor from the stator side.
  • The basic idea of a V/f control is to maintain the stator flux constant. To operate the machine under nominal conditions, the stator flux must be nominal.
  • This control method is commonly applied to blower fans and centrifugal pumps.

3.) Pole changing method

  • Pole Changing Method is one of the main methods of speed control of an induction motor. 
  • This method of controlling the speed by pole changing is used mainly for the cage motor only because the cage rotor automatically develops many poles, which are equal to the poles of the stator winding.

4.) Supply voltage method

  • The speed control of a three-phase induction motor is obtained by changing the supply voltage until the torque required by the load is developed at the desired speed.
  • The torque developed by the induction motor is directly proportional to the square of the supply voltage and the current is proportional to the voltage.
  • Therefore, the stator voltage control method is suitable for applications where the load torque decreases with the speed, as in the case of a fan load.