Electrical Installation on Ship's

AC Generators - how they work

Vast majority of ships use alternating current (ac) generators (often called alternators). The basic principle of an ac generator is very simple. Pairs of magnetic poles are driven (by the prime mover) past fixed coils of wire.

 An alternating electromotive force (emf) which has a sinusoidal waveform, is induced into each coil:

622 V

440V   440 V

 

EMF (voltage)  generated depends on the strength of magnetic flux ((D) and the rate at which this flux cuts the coils –

EMF µ Strength of magnetic field x Speed of rotation EMF µ Strength of magnetic field x Speed of rotation

The FREQUENCY (measured in Hertz) of the emf generated is the number of waveform cycles per second. Above figure shows 1 cycle.

These two basic relationships for emf size and frequency dictate how to control the voltage and frequency output of a generator.

In practice the speed is maintained practically constant by the generator's prime mover (governor) which fixes the output frequency  normally 60 Hz. The constant speed then allows the size of generated emf   (440V RMS) to be controlled by the adjusting the strength of magnetic field (excitation).

 

Generator Rating

The generators form the heart of the electrical design and their correct sizing is the key to a safe, workable and economical system. Main generator electrical power ratings range from, typically, 100kW to 2MW at 440V, 60Hz. Capacity of generators depends up on the nature of the load. In addition to satisfying load requirements under different conditions e.g.

• Cargo operation
• Anchorage
• Drydock
• Navigation at sea
• Manoeuvring

The generator often works on its own and is subjected to large system load swings during starting of equipment like air compressors, winches, ballast pumps, cargo machinery, and air-conditioning.

International maritime regulations (e.g. SOLAS Chapter II-1,), require at least two generators for a ship's main electrical power system. The generators are normally driven from their own dedicated diesel engine but this can be expensive, taking up additional space that could be used for other purposes. For ships engaged on long sea voyages, it can be economical to drive the generators from the main propulsion plant  (shaft generator) or from waste heat (Turbo alternator).

International maritime regulations also require at least one electrical generator to be independent of the speed and rotation of the main propellers and associated shafting and accordingly at least one generator must have its own prime mover.
If a minimum of two generators is provided, one of which is driven from the propeller shaft, failure of one of the generators could make the ship non-compliant with the International regulations.

For this reason many owners opt to provide three generators. One is used for the normal sea load (e.g. the shaft generator), leaving two available to meet any unusually high loads or to provide security when manoeuvring. Alternately, the third is retained as a standby set able to provide power should one set fail in service or require specific maintenance work. 

Main Switchboard
The main elements of a marine distribution system are the main and emergency switchboards, power panel boards, motor controllers, lighting and small power panel boards. The system is generally designed such that under all normal conditions of operation, power is distributed from the main switchboard.

The main switchboard is generally located  in the main engine room or machinery control room. These locations are normally below the ship's waterline or below the uppermost continuous deck of the ship i.e. the bulkhead or main deck. Consequently, in the event of a fire or flooding it is likely that the main generators and switchboard would be disabled.

To ensure that electrical supplies are available to emergency and safety systems, an emergency generator and associated emergency switchboard will be located above the main deck in a separate space, completely isolated from the main machinery spaces.

Motor Controls

It is often convenient to group motor driven auxiliaries according to their function, e.g. fuel and lubrication oil services, accommodation ventilation systems, machinery ventilation systems, and domestic service systems. This can often simplify the machinery control functions and required protection systems.

Protective systems are provided to shut down all ventilation systems and all fuel oil systems in the event of fire from a remote location.

Describe the use of circuit-breakers and fuses

A circuit breaker safely makes and breaks the power supply to an electrical circuit. In case of short circuit when current flow exceeds the safe limit, circuit breaker trips and interrupts the power supply. Circuit breaker can be reset and used again. The fuse on other hand burns out and needs to be replaced. Before carrying out maintenance, the power supply to the circuit is cut off by switching off and locking the main circuit breaker.

The purpose of a fuse isto protect a circuit from damage when excessive current flows. Fuses are designed to blow rapidly before excessive current can damage the circuit they protect. When a fuse blows it must be replaced by a fuse of same type and same rating.

One can remove the fuse and be sure that equipment is dead, if there is need to work on it.

Emergency Power

WHY  EMERGENCY SOURCE OF ELECTRICAL POWER IS PROVIDED?

Emergency switchboard is smaller than the main switchboard. It is provided so that in the event of an emergency involving a total power failure, a supply will still be available for emergency lighting, alarms, communications, watertight doors and other services necessary to maintain safety and to permit safe evacuation of the ship by lifeboats.

Emergency switchboard is generally arranged to be above the bulkhead deck and readily accessible from open deck.

POWER SUPPLY FOR THE EMERGENCY SWITCH BOARD

• Emergency switchboard is normally supplied power from main switchboard through BUS TIE CIRCUIT BREAKER.

• When the mains power fails, the emergency switchboard is supplied by its own independent power source (emergency generator).The emergency source of power must be self contained and not dependent upon any other engine room power supply. It should be capable of functioning when the ship is inclined by 22.5⁰ and/or trim of 10⁰

Starting arrangement for emergency generator

• Emergency generating sets shall be capable of being readily started in their cold condition at a temperature of 0⁰C.

• Each emergency generating set arranged to be automatically started with compressed air, hydraulic or electric means. The stored energy for starting should be sufficient for 3 consecutive starts.
• Additional energy for 3 consecutive starts with in 30 mins is available.

• The energy storage device (electric battery or compressed air) is always kept charged.

• Some generators may have manual starting by hydraulic accumulator or hand cranking.

Sources of Emergency Power Supply

 

A.        ACCUMULATOR BATTERY capable of carrying load, with not more than

12% volt drop during discharge period. Automatically supplying power to emergency switchboard in case of mains power failure. or

B. GENERATOR, capable of starting automatically from cold within

45 seconds, and using fuel of flash point above 43⁰C

TRANSITIONAL SOURCE (BATTERIES) Power for half an hour is provided along with emergency diesel generator

(a) In Passenger ships - To supply power during momentary blackout before emergency generator comes on load within 45 seconds, so that passengers don’t panic.

(b)In Cargo ships - To supply selected power, if the emergency generator is hand starting type.

INTERLOCKS FOR PROTECTION

Interlocks are provided such that power is supplied from only one source at a time to the emergency switchboard i.e. mains supply or emergency generator, so that the circuit breakers of the two supplies cannot be closed together.

Closing of two circuit breakers in a.c. system, without synchronising the phase, will cause short circuit and damage to the equipment.

List the essential services to be supplied from the emergency generator

 

Emergency switchboard supplies power to the following essential services.

TIME PERIOD - FOR PASSENGER SHIPS - 36 HOURS (SOLAS).

FOR CARGO SHIPS            18        HOURS. (SOLAS)–

(Transitional batteries supply power to service  NO.1 & 2 for half an hour)

SERVICE No 1.EMERGENCY LIGHTS

(a) Embarkation station on deck and overside.

(b) Lighting in alleyways, staircase and exits.

(c) Machinery space,   generators and their control position.

Emergency switchboard, steering compartment, emergency fire pump, storage for fireman's outfit, emergency bilge pump.

(d)       Navigation lights, not under command lights.

 SERVICE NO 2. COMMUNICATION AND ALARMS

(a)        Internal communications as required in an emergency, radio,

            telephones.

(b)        Fire detection and alarm system.

(c)        Intermittant operation of day light signalling lamp, ship's

            whistle, manual fire alarms.

3.NAVIGATIONAL AIDS-Radar,ARPA,Radio direction finder,Gyrocompass, Echo sounder,Radio equipment,Rudder angle indicator,speed log etc.

4. EMERGENCY FIRE PUMP - if electrically driven.

5. STEERING GEAR - for 30 minutes for a vessel above 10,000 tons.

Supplementary emergency lighting for ro-ro  passenger ships

Consists of fixed or portable lighting.

Fixed Lighting

All passenger public spaces and alleyways are provided with supplementary electric lighting that can operate for at least 3 h when all other sources of electrical power have failed and under any condition of heel.

The illunimation provided is such that the approach to the means of escape can be readily seen.

The source of power for the supplementary lighting consists of accumulator batteries located within the lighting units that are continuously charged, where practicable, from the emergency switchboard. These batteries are replaced at regular interval as per maintenance requirements.

Or

Portable lighting

a portable rechargeable battery operated lamp is  provided in every crew space alleyway, recreational space and every working space which is normally occupied unless supplementary emergency lighting.

NOTE - It must be noted that there is no fixed arrangement, because all ship's differ in some respect. This gives the basic concept. Some ships may have a diesel driven emergency fire pump. Others may have only batteries as an emergency power source, which supply limited power to few essential services. This depends upon the type of ship and safety regulations applicable to it.