Automatic Voltage Control and Generation Control in Power Systems

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Automatic Voltage Control

Basically consists of a main exciter field which excites the alternator field to control the output voltage. The exciter field is automatically controlled through an error equal to V reference - Vt. When the output voltage is higher than the required value, the BT is measured by a potential transformer and fed back to the input so that the error V reference - Vt is applied to the exciting field. This ensures that the generator output voltage is maintained at the desired level.


  • Potential Transformer: Provides a sample of terminal voltage
  • Differentiating Device: Gives actuating error e = V ref - Vt
  • Error Amplifier: Amplifies the error signal
  • SCR Power Amp: Transfer Function = (ke / 1 + TeS)

Automatic Generation Control

Generator output must be continuously regulated to meet the active power demands, as they change with varying loads. The machine speed will vary with frequency changes, and the state of generators must be regulated to match reactive power demand in large interconnected systems. Manual regulation is not feasible, so automatic generation and voltage regulation equipment are installed.

Need for Load Frequency Control

The active and reactive power demands change with load trends, causing frequency variations. This can lead to issues such as speed and EMF fluctuations in AC motors, excessive vibration in some steam turbines, and maloperation of power converters due to reduced harmonics. Synchronization of generators in parallel operation requires constant network frequency.


  • Maintain real frequency and desired power output in interconnected power systems
  • Control power exchange between control areas

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