Join 60,000+ competitive exam aspirants
or with email
Your Cart
Subtotal₹0
Total₹0
Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
Join 60,000+ competitive exam aspirants
Reverse running of alternator is due to
Over voltage
Over current
Failure of prime mover
None of above
Failure of prime mover
Quick Summary: Reverse running (motoring) of an alternator occurs when the prime mover driving it loses power, causing the alternator to act as a synchronous motor drawing active power from the grid. This is a critical fault condition that requires protection using a Reverse Power Relay.
Reverse running (motoring) of an alternator occurs when the prime mover driving it loses power, causing the alternator to act as a synchronous motor drawing active power from the grid. This is a critical fault condition that requires protection using a Reverse Power Relay.
P=3VLILcosϕ — Active power flow equation where reversal of P implies change in direction of current
Pin=XsEVsinδ — Power angle equation showing motor operation when δ becomes negative
Normally, an alternator acts as a generator converting mechanical power to electrical power. If the prime mover (e.g., steam turbine or engine) fails or is shut down while the alternator remains connected to the busbar, the machine takes electrical power from the grid to maintain its synchronous speed. This condition is characterized by the reversal of real power flow (P<0).
Reverse power protection is essential for steam turbines to prevent mechanical damage from overheating.
The alternator consumes active power while remaining connected to the grid.
A directional power relay (Reverse Power Relay) is used to detect this condition.
The machine continues to rotate at synchronous speed despite acting as a motor.
Prevents turbine damage due to windage losses and heat.
Ensures grid stability by isolating faulty prime mover units.
Requires high-sensitivity relays as the motoring power is usually very small (1-5% of rated power).
May lead to nuisance tripping during power swings.
Thermal power plants
Hydroelectric plants
Diesel generator synchronization panels
Reverse power is typically monitored by a 32-element relay.
Failure of the prime mover is the most common cause of motoring in large utility alternators.
C is correct — Failure of prime mover results in the alternator drawing power from the grid and running as a synchronous motor.
Always remember that while alternators are designed for generation, they are physically reversible and can operate as motors if the prime mover torque drops to zero or below.