Under Excitation Protection in Generators is a critical safeguard mechanism designed to prevent damage to synchronous generators (alternators) when they operate in a state of insufficient excitation current. This condition occurs when the generator absorbs excessive reactive power from the power system (acting like a reactive load) instead of supplying it, which can lead to instability, overheating, and catastrophic failures.
Why Under Excitation Protection is Needed ?
Synchronous generators require a DC excitation current in their rotor windings to maintain a magnetic field. This field interacts with the stator windings to generate voltage and supply reactive power (Q) to the grid. When excitation current drops too low:
The generator cannot maintain its terminal voltage.
It starts absorbing reactive power (like an inductor) instead of supplying it.
This destabilizes the generator and the grid, risking voltage collapse or loss of synchronism.
Causes of Under Excitation
1. Excitation System Failures: Faults in the AVR (Automatic Voltage Regulator), exciter, or DC power supply.
2. Incorrect AVR Settings: Improper voltage regulation or control logic errors.
3. Grid Disturbances: Sudden load changes, weak grid connections, or faults that reduce grid voltage.
4. Overloading: Operating the generator beyond its reactive power absorption capability.
How Under Excitation Protection Works?
Protective relays monitor key parameters and trip the generator if unsafe conditions persist. Common methods include:
1. R-X (Impedance) Diagram Method
– Plots the generator’s impedance ((Z = V/I)) on an R-X plane.
– A characteristic curve defines the safe operating zone (e.g., blinder curves).
– If the impedance trajectory crosses into the under-excitation zone, the relay triggers an alarm or trip.
2. Reactive Power (Q) Monitoring
– Measures the reactive power absorbed by the generator ((Q < 0)).
– Trips the generator if (Q) exceeds predefined limits.
3. Voltage vs. Excitation Current (V/Hz) Monitoring
– Low excitation current combined with low voltage or frequency indicates under-excitation.
4. Power Angle (δ) Measurement
– Tracks the angle difference between the rotor’s internal voltage ((E)) and grid voltage ((V)).
– A collapsing power angle ((delta right arrow 0)) signals instability.