Basic Principle

A reverse power relay detects when power flows in the reverse direction from the load back to the generator, which can damage the generator. It protects the system by disconnecting the generator when reverse power is detected.

Components

• Voltage coil (connected in parallel)
• Current coil (connected in series)
• Aluminum disc
• Trip contacts

Magnetic Field Generation

1. Voltage coil:

• More turns, higher inductance
• Induced current lags by ~90°
• Produces weaker magnetic field

2. Current coil:

• Fewer turns, lower inductance
• Induced current lags less
• Produces stronger magnetic field

The magnetic fields from both coils are approximately 90° out of phase.

Disc Movement

• Eddy currents are induced in the aluminum disc by the magnetic fields
• Torque is generated on the disc (Lorentz force)
• Under normal power flow, disc rotation is restricted by stoppers
• During reverse power, disc rotates in opposite direction

Trip Mechanism

• Disc rotation closes trip contacts when reverse power occurs
• This activates the protection system to disconnect the generator

Setting the Reverse Power Relay

• Typically set to 20-50% of the prime mover’s motoring power
• Motoring power: Power required to drive the prime mover at rated RPM
• Setting obtained from prime mover manufacturer specifications

Considerations for Ships

• Marine applications require careful calibration
• Settings may vary based on ship type and operating conditions
• Regular testing and maintenance are crucial for reliability