31 March 2019

Advantages of 25 kV, 50 Hz AC Supply System

The advantages of 25 kV, 50 Hz AC supply system is shown as below.

Light overhead catenary

  • As the system voltage is high, the line current for a given traction demand is very less. This will result in cross section of over head conductor is reduced. 
  • As the size of conductor is reduced, line conductor support and foundation are also light. 
  • The high voltage needs higher insulation of over head equipment this will be compensated by reduction in the size of conductors.

Less number of sub stations

  • As the line current is less in the 25 - kV supply system, this will result in less voltage drop and improves voltage regulation of the line which makes larger spacing between sub stations. 
  • The distance between two sub stations due to 25 kV is approximately 50 – 80 km as against 5 – 15 km for 1500 V DC and 15 – 30 km for 3000 V DC system. 
  • As the number of sub stations is reduced, the capital cost of track electrification is also reduced.

Flexibility for location of sub station

  • The sub station can be located near the high voltage grid which is close to main railway track. 
  • These sub stations are fed from high voltage grid which results in saving in cost of special transmission line for substation.

Low cost of installations

  • The cost of fixed installation for 25 kV AC system as compared to DC system.

Co – efficient of Adhesion

  • The DC locomotive has co – efficient of adhesion about 27% as compared to rectifier locomotive which is closely 45%. 
  • This will result in lighter AC locomotive can takes the same load as heavier straight DC locomotive.

Higher starting efficiency


  • As the AC locomotive has higher string efficiency as compared to straight DC locomotive. 
  • The supply voltage at starting in the DC locomotive is reduced by means of ohmic resistors but it is changed by using tap changer in AC locomotives.
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