Rating of Alternator:

The rating of the alternator can be specified either in KVA along with power factor or in kW such that active power must be known for the selection of prime mover rating. Because the boiler and turbine size and fuel requirement in a thermal station or the turbine size and water requirements in a hydro stations depend on the power output and are independent of the load volt-amperes.

Ex: For 100 MW load at 0.8 power factor, the alternator rating is 125 MVA, while the size of the boiler, turbine and fuel requirements are decided by 100 MW alone.

  • While stating the KVA or MVA rating of alternator, the power factor (usually from 0.8 to 0.9 lagging) for which they are designed to operate under steady state conditions must be mentioned.
  • In modern alternator voltage is kept almost constant (with in ± 5% of rated voltage) by automatic voltage regulator.
  • The need for mentioning the power factor arises, because an alternator designed to operate at 0.9 power factor lagging at rated load, would required more field current when operated at 0.8 power factor lagging.
  • More field would result in over heating of the field system, which is not desirable.
  • If the alternator power factor given on the name plate details is not mentioned, it Can be taken as lagging.
  • Under rated power and rated voltage condition, reactive power flow handled by an alternator is limited by its armature heating for operating power factor near the rated value.
  • When the operating power factor is away from its desired value, the reactive power flow is limited by both field heating and armature heating.

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