Alternator and Associated Systems' Protection

Protective devices are built into main alternator breakers to safeguard both the individual alternator and the distribution system against certain faults. Over-current protection is achieved by means of relays which cut power supplies to non-essential services on a preferential basis, as well as breaker overload current trips and instantaneous short-circuit current tripping. A reverse power trip is fitted where alternators are intended for parallel operation (in some vessels they are not), unless equivalent protection is provided by other means. Parallel operation of alternators also requires an under-voltage release for the breaker.

Over-current Protection

Various methods are used to detect over-current in a circuit. They have an inverse current-time characteristic, i.e. the higher the current, the faster will it operate. A few of these:

Magnetic

The solenoid drives an iron core to operate a 'trip' switch. The core movement is'slugged' by either an oil dashpot or a mechanical delay (clockwork action).

Thermal

A thermal relay utilizes the bending action (or rather the coefficient of expansion) of a bimetallic bar to trip the circuit breaker. The time taken to heat the bimetal gives the necessary time lag. Thermal relays are commonly fitted in moulded case circuit breakers (MCCBs) and in miniature circuit breakers (MCBs) for overload protection.

Electronic

An electronic over-current relay usually converts the current into a proportional voltage. This is then compared with a set voltage level within the transistorized monitoring unit. The time delay is obtained by the time taken to charge a capacitor. This type of relay usually has separate adjustments for current trip level and for trip time. The amplifiers within the electronic relay require a power supply (usually 110, 220 or 440V a.c).

Both the magnetic and electronic relays can be set to give an almost instantaneous trip (typically 0.05 sec.) to clear a short-circuit fault.

Over current protection current transformers (CTs) generally drive relays. The CT secondary usually has a 5A or 1A rating for full load current in its primary winding. Injecting test currents to check their current trip levels and time lags can help to test all over-current relays.

Primary injection is where current is fed through the normal load circuit. This requires a large injection test set. The test set is essentially a transformer and controller rather like awelding set, i.e. it gives a low voltage - high current output.

Secondary injection feeds current directly into the over-current relay. This requires only a small current (5 - 10A). Secondary injection does not prove the CT performance but is like a most common, reliable method for testing an over-current relay.

The setting up of an over-current relay is obviously critical to its protective duty. It is carried out in strict accordance with the manufacturer's instructions. Such setting up is done during acceptance trials of a new ship and at subsequent periodic surveys.

The alternator breaker has an over-current trip, but a major consideration is that the supply of power to the switchboard must be maintained if possible. The breaker is therefore designed to be tripped instantly only in the event of high over-current such as that associated with a short-circuit fault. When over current is not so high, a delay with an inverse time characteristic allows an interval before the breaker is opened. During this time the overload fault may be cleared.

Overload of an alternator may be due to an increased switchboard load or due to a serious fault, causing high current flow. Straightforward overload (apart from the brief overload due to the starting currents of motors) is reduced by the preference trips which are designed to shed non-essential switchboard load. Preference trips may be operated by relays set at about 110% of the normal full load. They open the breakers feeding ventilation fans, air conditioning equipment, etc. The non-essential systems are disconnected at timed intervals, hence reducing alternator load. A serious fault on the distribution side of the switchboard should cause the appropriate supply breaker to open, or fuse to operate due to over-current. Disconnection of faulty equipment will reduce alternator overload.

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