Question

Explain why it is necessary to have reverse power protection for alternators intended for operation,

(b) (i) Sketch a reverse power trip.

(ii) Explain briefly the, principle on which the, operation of this power trip is based and how tripping is activated.

Generators intended to operate in parallel must have ‘Reverse Power Protection’.

A reverse power relay monitors the direction of power flowing between the generator and the switchboard.

If a prime-mover failure occurred the generator would act as a motor.

The reverse power relay detects this fault and acts to trip the generator circuit-breaker.

Need for Reverse Power Protection

When two or more a.c. generators operate in parallel to supply the ship's load, then it is possible that some fault will develop on one generator set (e.g. loss of steam supply or fuel supply to prime mover), and that set will no longer contribute to supplying the ship's load.

The faulted set will not stop; it will continue to run in the same direction and at the same speed.

The generator will now be acting as a synchronous motor driving its own prime mover, and taking power from the bus-bars instead of supplying it as it should, i.e. a reversal of power flow occurs.

The faulted set must be stopped by tripping its circuit breaker so disconnecting it from the bus-bars because:

Damage may be caused to the prime mover, e.g. due to failure of the lubricating oil system.

The remaining healthy generators will probably be overloaded due to now having to supply all the ships load
and in addition having to supply power to the faulted set

Even if no damage is likely to the generator set, this is a fault condition and, as such, is undesirable.

Regulations state that where two or more generators are required to operate in parallel to supply the ships electrical load then a Reverse Power Protection device should be fitted to each generator.

In the event of reverse power, the fault is sensed by the reverse-power relay which then closes the contact RP which is connected into the ACB circuit as shown in Figure 1.

This de-energised the no-volt coil to release the latch and trips the ACB.

Reverse power occurs when two (or more) generators are running in parallel.

When there is a loss of prime-mover power, the driven generator acts as a motor and impose a heavy load on the remaining sets in parallel.

To obviate overload trip in the other generators, its ACB must be released.

Figure 1

Figure 3

The reverse power relay is similar in construction to the electricity supply meter.

The lightweight non-magnetic aluminum disc, mounted on a spindle which has low-friction bearings is positioned in a gap between 2 electromagnets. The upper electro-magnet has a voltage coil connected through a transformer between one phase and an artificial neutral of the alternator output.

The lower electromagnet has a current coil also supplied from the same phase through a transformer.

Magnetic fields are produced by the voltage and current coils.

Both fields pass through the aluminum disc and cause eddy currents.

The effect of the eddy currents is that a torque is produced in the disc.

With normal power flow, trip contacts on the disc spindle are open and the disc bears against a stop.

When power reverses, the disc rotates in the other direction, away from the stop and the contacts are closed so that the breaker trip circuit is energised.

A time delay of 5 seconds prevents reverse power tripping due to surges at synchronising.

Reverse Power

5 % of rated power

5 seconds