.reverse power relay .rpr
The Reverse Power Trip Relay is a crucial protective device used in electrical systems, particularly for generators. It operates based on the principles of Faraday’s law of induction and Lenz’s law.
Working Principle
- Eddy Currents: Loops of electrical current induced within a conductor due to Faraday’s law of induction.
- Lenz’s Law: An eddy current creates a magnetic field that opposes the magnetic field that created it.
Components and Structure
- Aluminum Disc: A lightweight, non-magnetic disc mounted on a spindle with low-friction bearings.
- Magnetic Field Coils: Both voltage and current coils produce magnetic fields that pass through the disc.
- Trip Contacts: Located on the spindle.
- Stopper: The disc rests against this under normal power flow conditions.
Operation
Normal Power Flow:
- The disc bears against the stopper.
- Trip contacts on the spindle remain open.
Reverse Power Condition:
- Eddy Current Generation: Magnetic fields from the coils pass through the disc, causing eddy currents.
- Torque Production: The interaction between the eddy currents and magnetic fields produces a torque effect on the disc.
- Disc Rotation: When power reverses, the disc rotates away from the stopper.
- Contact Closure: As the disc rotates, the contacts close, triggering the circuit breaker to trip.
Safety Feature: To prevent unnecessary tripping due to power surges during synchronization, the relay incorporates a time delay of 5 seconds.
Reverse Power Relay (RPR) detailed Operation:
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
- Voltage coil:
- More turns, higher inductance
- Induced current lags by ~90°
- Produces weaker magnetic field
- 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