- Explain what happens to the output voltage of an a.c. generator from sudden application of a large load to a steady state condition.(10)
Effect of Sudden Application of a Large Load on an AC Generator Output Voltage
When a large load is suddenly applied to an AC generator, the output voltage does not remain constant immediately. The sequence of effects can be explained as follows:
1. Immediate Effect (Transient Period)
- Voltage dip occurs because:
- The sudden demand for current causes an increased armature reaction (demagnetising effect of armature flux opposing field flux).
- Internal voltage drops occur across the generator’s synchronous reactance.
- The generator cannot instantly increase excitation or mechanical input, so the terminal voltage falls sharply.
2. Electromechanical Response
- Governor action: The load increases shaft torque demand. Initially, the prime mover slows slightly until the governor increases fuel/steam supply to restore speed and frequency.
- Exciter/AVR response: The Automatic Voltage Regulator (AVR) senses the voltage dip and increases field current to restore the generator’s internal EMF.
3. Transition Towards Steady State
- The generator’s excitation system raises the field strength, increasing the internal generated voltage (E).
- The prime mover governor increases input power to match the new electrical load.
- The transient oscillations (voltage dips, overshoots, small fluctuations) are damped by system inertia and damper windings.
4. Steady-State Condition
- A new equilibrium is established:
- Terminal voltage is restored close to nominal (depending on AVR setting and load power factor).
- Generator frequency stabilises at the governed set point.
- Armature current is higher, supplying the new load.
- If the load power factor is poor (lagging), a slightly lower steady-state terminal voltage may persist due to increased reactive drop.
Summary of Voltage Behaviour
- Initial: Sudden voltage dip due to increased current, armature reaction, and synchronous reactance.
- Transition: AVR boosts excitation, governor supplies more power.
- Final steady state: Terminal voltage nearly restored, frequency stabilised, generator carrying higher current.