Aux 2 Unit 5 Q5

When a large load is suddenly applied to an AC generator operating at a steady state, the following sequence of events typically occurs with respect to the output voltage:

1. Initial Voltage Dip:

• Increased Load Demand: The sudden application of a large load demands more electrical power from the generator.
• Torque Imbalance: This creates a temporary imbalance between the driving torque (provided by the prime mover, like an engine) and the load torque (the electrical power demanded by the load).
• Speed Reduction: Due to this imbalance and the generator’s rotational inertia, the generator’s speed momentarily decreases.
• Voltage Drop: Since the output voltage of an AC generator is directly proportional to its rotational speed, the voltage also experiences a sudden dip.

2. Recovery and Stabilization:

• Governor Response: The engine’s governor senses the speed drop and increases fuel supply to the engine to compensate for the increased load and restore the generator’s speed to its set point.
• Voltage Regulator Response: The Automatic Voltage Regulator (AVR) detects the voltage dip and increases the field excitation current to boost the generator’s output voltage.
• Stabilization: As the engine speed recovers and the AVR adjusts the excitation, the output voltage gradually rises back towards its nominal value. Depending on the system’s response characteristics and the magnitude of the load change, the voltage may stabilize at the original level or slightly above it to account for the increased load.

3. Steady-State Condition:

• New Equilibrium: Once the engine speed and voltage regulator have adjusted to the new load demand, the system reaches a new steady-state condition.
• Stable Voltage: The output voltage is now stable at a level that can adequately supply the increased load.

Factors Affecting the Magnitude and Duration of Voltage Dip:

• Size of the Load: The larger the load change, the more significant the initial voltage dip will be.
• Generator Inertia: Generators with higher rotational inertia will experience a slower speed drop and a less severe voltage dip.
• Governor and AVR Response: The responsiveness and tuning of the governor and AVR systems play a crucial role in how quickly the voltage recovers and stabilizes.

Potential Issues:

• Excessive Voltage Dip: If the voltage dip is too severe or prolonged, it can cause problems for sensitive loads connected to the generator, potentially leading to equipment malfunctions or shutdowns.  
• Under-Frequency: If the engine governor cannot fully compensate for the increased load, the generator speed may remain below its rated value, resulting in an under-frequency condition, which can also affect the operation of connected equipment.
• Over-Voltage: If the AVR overcompensates, it can lead to a temporary over-voltage condition after the initial dip, potentially damaging sensitive equipment.

In summary, the sudden application of a large load to an AC generator causes an initial voltage dip due to a temporary torque imbalance and speed reduction. The system’s governor and voltage regulator then respond to restore the speed and voltage to a stable level, establishing a new steady-state condition capable of supplying the increased load.