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Examoogle • User • info@examoogle.com • EE-2024-8821
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Failure of excitation of alternator will be sensed by
Undercurrent relay
Over current relay
Both A and B
None of above
Undercurrent relay
Quick Summary: Failure of excitation in an alternator causes a loss of reactive power generation and a drop in excitation current. An undercurrent relay, typically monitoring the field current $I_f$, detects this drop below a predefined threshold to initiate protective action.
Failure of excitation in an alternator causes a loss of reactive power generation and a drop in excitation current. An undercurrent relay, typically monitoring the field current If, detects this drop below a predefined threshold to initiate protective action.
Pgen=XsEVsin(δ) — Power output dependency on excitation EMF E
If<If,min — Trip condition for undercurrent relay
When the field excitation is lost, the alternator acts as an induction generator drawing lagging reactive power from the grid. This leads to a decrease in the excitation circuit current. The undercurrent relay is connected in series with the excitation field circuit to sense this deviation from the nominal operating range.
Loss of excitation leads to the alternator operating as an induction generator.
This causes a significant increase in reactive power demand from the system.
The alternator may experience overheating due to high stator currents while under-excited.
Protection schemes often use field undercurrent relays to prevent damage during loss of excitation.
Simple and low-cost implementation
Direct monitoring of the field circuit parameters
Cannot detect field circuit faults inside the generator if excitation is partially maintained
May require coordination with other protective relays
Synchronous generators
Large industrial alternators
Option B (Overcurrent relay) is incorrect because an overcurrent relay typically responds to faults causing high current flows in the armature, not the drop in field current.
The relay is set to operate when the field current falls below roughly 80-90% of the rated no-load excitation current.
A is correct — The failure of excitation leads to a reduction in the field current, which is accurately sensed by an undercurrent relay placed in the field circuit.
In competitive exams, remember that loss of excitation makes the machine behave as an induction generator, which specifically requires detection by field undercurrent or impedance-based reverse reactive power relays.