Working and Advantages of Reverse Power Relay and Preferential Trips

In series of marine electro-technology blogs, here we are going to discuss about the basic working principle & importance of Reverse Power Relay & Preferential Trip arrangements provided on MSB.


Reverse Power Relay construction & working
Reverse Power Relay

Reverse Power Relay is a directional protective relay that prevents/protect the generator from motoring effect (going to reverse direction). It is used where generator runs in parallel with other utility or generator. The relay monitors the power supply from the generator and in case the generator output falls below a preset value, it quickly activates the trip and disconnects the generator.


The relay is made up of lightweight non-magnetic Aluminium disc between two soft laminated iron core electromagnets. The upper magnet is wound with Voltage Coil (PT) which is supplied from one phase & artificial neutral of generator output. The other output magnet is wound with Current Coil (CT) connected to the same phase as the voltage in the upper electromagnet.


Since Voltage Coil has more number of turns, so it has move inductive value and more induced current that lag in the coil by an angle of 90°. The current coil has less number of turns so less number of turns, so it has less inductive valve & less induced current that lag less.

As we all know that current carrying conductor produces the magnetic field. So both upper and lower section produces magnetic fields. But Induced current in PT lags more than CT so magnetic field produced in upper section will be weaker than lower section & both magnetic fields will have a difference of 90°

When both fields pass through the Aluminium disc, it produces eddy current. As a result of the formation of eddy current torque is generated that tries to rotate the disc. Under normal power flow, the trip contact on the disc are open and rotation is restricted by stoppers but if a reverse power starts to flow the disc is rotated in opposite direction, moves away from the stoppers in the direction of trip contact that activates the trip.

Why is Reverse Power Relay required?

When two or more power units are running in parallel and if reverse power flow occurs, the same unit will start drawing power from the main bus bar. it can cause overloading of the other power supply unit and hence leads for the preferential trip or may lead to total power failure(Blackout). At the same time the faulty unit will draw power from main bus bar and go for motoring effect and RPM will soot up which leads to over speed trip or in worst case some mechanical failure to the prime mover.

When does reverse power flow…..?

  • When the prime mover of a generator is not supplying sufficient torque to keep the generator rotor spinning at the same frequency as the bus goes to which the generator is supposed to be connected, the generator will start behaving like a motor and instead of supplying power it will draw power from bus bar.
  • During synchronization, it might be possible to have the synchroscope rotates slow (anticlockwise direction) and then close the breakup. under this condition. The generator would then be drawing current from the bus for instead of supplying current through the bus (which occurs when the breaker is closed with the synchroscope rotates in the fast anticlockwise direction).
  • Faulty Governer of the prime mover.
  • Loss of excitation in the alternator.

How do you test reverse power trip?

Reverse power trip can be tested by load shifting with the help of Governor control. when the Load has shifted sufficiently from the generator to be offloaded (Nearly 10% of the max rated ), reverse power relay will open the ACB of the same generator. this relay can be tested by simulation using boost test push button on the relay to see if it gives a trip signal.


What is the use of preferential trip?

The preferential trip is a kind of electrical arrangement on the ship which is designed to disconnect the non-essential circuit .i.e non-essential load from the main bus-bar in case of partial failure or overload of the main supply.

The non-essential circuit or loads on the ship are air conditioning unit, exhaust ventilation fans, & galley equipment which can be disconnected momentarily. The main advantage of the preferential trip is to supply uninterrupted power for propulsion and safe navigation under overloading condition of the power unit. 


Preferential Trip Construction & Working
Preferential Trip

The preferential trip circuit consists of an Electromagnetic coil and a dashpot arrangement to provide some delay to disconnect the non-essential circuits. Along with this, there is also an alarm system provided, which function as soon as overload is detected and trips start operating. There are some mechanical linkages provided in the circuit which instantaneously operates the circuit for preferential trips.

The dashpot arrangement consists of a small Piston with the small orifice and which is placed inside a small cylinder assembly. The Piston moves up against the viscous fluid silicon and the time delay is governed by the orifice in the Piston


The current passes through the electromagnetic coil and the linkage are kept from the contacting using a spring arrangement. As soon as the current value increases the limit, the electromagnetic coil pulls the linkage up against the spring force and operates the instantaneous circuit and the alarm system. The lower linkage completes the circuit for the preferential trip circuit.

The current passes through the coil in the preferential trip circuit which pulls the piston in the dashpot arrangement. The movement of the piston is governed by the diameter of the orifice and the time delay made by the same. The preferential trip operates at 5,10, & 15 seconds and the load is isolated accordingly. If the overload still persists, then blackout will occur.

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He is alumni of Marine Engineering & Research Institute(MERI) Mumbai. He has total sailing experience of 18 months. Presently working with Shipping Corporation and India. He is a member of Nielsons Educational Forum. and pursuing his Master’s from Narottam Morarjee Institute Of Shipping. Specialty : Tanker's Equipment and Electrical

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