Generator reverse power protection on ships is required, to protect against failure of the Prime Mover.
Under normal operating conditions the ship’s generator is turned by the Prime Mover, which allows the generator to operate, and produce power.
Under conditions where the Prime Mover is not turning one of the ship’s generator, the generator will effectively act as a motor.
This condition is called ‘generator motoring’, and needs to be avoided.
The reason that we don’t want the generator acting as a motor, is that it presents a load onto the ships electrical system, and draws active power from the system.
To protect the ships electrical system against ‘generator motoring’, generator reverse power protection is incorporated into the ships electrical system.
This protection is in the form of a ‘reverse power relay’, which is incorporated into the ships electrical system.
Reverse Power Relays can be either of the mechanical or electronic types.
Mechanical types consist of an aluminium disc, set between an upper and lower electromagnet.
The upper electromagnet coil is the Voltage (Potential) coil, whilst the lower electromagnet is the current coil winding.
Eddy currents are produced by the operation of the electromagnets, which try to exert torque on the aluminium disc.
The aluminium disc is mounted on low friction bearings, but a mechanical ‘stop’, prevents it from turning clockwise due to the torque produced by the Eddy currents.
However, the disc is designed to freely rotate in the reverse direction, during a reverse power fault.
The action of the aluminium disc turning in a reverse direction, causes it to close the normally open switch contacts, therefore triggering the disconnection of the ‘motoring Generator’.