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August 22, 2012

What are the advantages of high voltage transmission and its limitations.

Advantages and Limitations of high voltage transmission line: 

The choice of transmission systems and rated voltages for a transmission line is made from HV AC (upto 220 kV) EHV AC (400 kV – 750 kV) UHVAC (above 760 kV AC) depending upon technical and economic consideration.

Advantages : 

  • High power transferability of AC lines P α V2. 
  • Line losses decrease with increase of transmission voltage and improvement of power factor for same power transfer. 
  • Bulk power transfer from large group of generating stations upto main transmission network. 


  • Higher voltage gives lesser current, lesser I2R losses.
  • Short circuit levels: In case of very long lines of above 500 km, intermediate switching sub-stations are necessary to install the shunt reactors for compensation. 
  • Right of way: In some cases of big cities, industrial localities, it is impossible to acquire right of way for EHV AC lines. 
  • Line insulation: The creep age distance (leakage distance) determined on the basis of required impulse with stand level. 
  • Corona: The critical value of voltage stress depends upon pressure, temperature, humidity, pollution level in air.

Explain important functions served by the damper winding in a synchronous motor.

Important functions of damper winding in a synchronous motor: 
  • The oscillatory motion of the rotor about the operating point is considerably reduced in amplitude and rotor quickly returns to the steady position. 
  • Providing starting torque like an induction motor.
  • At starting, the rotor achieves a speed close to synchronous speed and the rotor and stator fields lock into each other as soon as field excitation is switched on. 

What do you understand by the term cogeneration? Give its significance.

Cogeneration is a high efficiency energy system that produces both electricity (or mechanical energy) and valuable heat from single fuel source. Cogeneration is some times known as combined heat and not only generates power but also provides heat for industrial purposes. For example a hospital cogeneration plant would produce some of power and all hot water needed for its laundry and hot water system from the waste heat it generates. 

Significance : 
  • It reduces energy costs and green house gas emissions typically by upto two-third. In addition to reduction in cost, cogeneration also increases resource utilization. It offers major economic and environmental facilities because it turns otherwise wasted heat into a useful energy source. 
  • Cogeneration is a proven and reliable technology currently operating at over 100 sites across Australia. 
  • There is significant potential for cogeneration plants fired by other fuels, including biomass (e.g., plant waste from sugar or cotton harvesting), or biogas (e.g., methane produced by sewage works or piggeries). 

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