A ship’s electrical distribution scheme generally follows shore practice.
This allows normal industrial equipment to be used on board ship after being “marinised”, where necessary, to withstand the rigours of a sea-life (e.g. it must withstand the vibration, humidity, high temperature / ozone, sea-water, etc. encountered in various parts of the ship).
The majority of ships have a 3-phase d. c., 3-wire, 440 V insulated-neutral system.
This means that the neutral point of star-connected generators is not earthed to the ship’s hull.
For continental European vessels, a 380 V, 3-phase system is common.
Ships with very large electrical loads have generators operating at high voltages (HV) of 3.3 kV, 6.6 kV and 11 kV.
Such high voltages are economically necessary in high power systems to reduce the size of current, and hence reduce the size of conductors and equipment required.
Operating at such high voltages is becoming more common as ship size and complexity increase, e. g. for large cruise liners and big ships.
Offshore oil and gas production platforms operate at up to 13.8kV, where equipment weight saving is important.
Distribution systems at these high voltages usually have their neutral points earthed through a resistor or earthing transformer to the ship’s hull.
Frequency of ship electrical system
The frequency of an a. c. power system can be 50 Hz or 60 Hz.
In Europe and most of the world the national frequency is 50 Hz but is 60 Hz in North America and in a few other countries.
The most common power frequency adopted for use on board ships and offshore platforms is 60 Hz.
This higher frequency means that motors and generators run at higher speeds with a consequent reduction in size for a given power rating.
Lighting and low power single-phase supplies usually operate at the lower voltage of 220 V a. c. although 110 V a. c. is also used.
These voltages are derived from step-down transformers connected to the 440 V system.
Electrical distribution on ship
The distribution system is the means by which the electrical power produced by the generators is delivered to the various ship’s motors, lighting, galley services, navigation aids, etc. which comprise the ship’s electrical load.
The electrical energy is routed through the main switchboard, then distributed via cables to section and distribution boards then ultimately to the final load consumers.
The circuit-breakers and switches are the means of interrupting the flow of electric current, and the fuses and relays protect the distribution system from the damaging effects of large fault currents.
The system is called a radial or branching system. This distribution system has a simple and logical structure.
Each item of load is supplied at its rated voltage via the correct size of cable and is protected by the correctly rated protection device.
The main electrical load is divided into essential and non-essential services.
Essential services are those required for the safety of personnel and for the safe navigation and propulsion of the ship. They include certain supplies to navigational aids, communications, machinery spaces, control stations and steering gear.
Essential services on ship
The essential services may be supplied directly from the main switchboard or via section boards or distribution boards.
Emergency supplies are necessary for loads which are required to handle a potentially dangerous situation.
Ship generator overload protection
To maintain generator operation during an overload, a preferential load shedding arrangement is employed. This is achieved by a special overload relay, called a preference trip relay.
If a generator overload develops, the preference trip relay sets an alarm and acts to trip selected non-essential loads.
This reduces the generator load so that it may continue to supply essential circuits.
Each generator has its own normal overcurrent relay to trip its own circuit breaker which is typically high set at 150% with a 20 seconds delay.
In addition, each generator has its own preference overload trip, this being low set generally at 110 % current, instantaneous operation.
If a generator overload condition develops, its preference overload trip will operate to energise the timing relay.
The timing relay then operates to disconnect non-essential services in a definite order at set time intervals, e.g.
1st trip – air conditioning and ventilation – 5 seconds
2nd trip – refrigerated cargo plant – 10 seconds
3rd trip – deck equipment – 15 seconds
This order of tripping obviously varies with the ship type.
When sufficient non-essential load has been disconnected, the preference overload trip resets and no further load is disconnected.
The generator preference trip system can also be initiated by low generator frequency or by low speed at the generator prime-mover.
In many cases the preference trip protection is incorporated in a combined electronic relay which also monitors generator overcurrent and reverse power.
To maintain the preference relay trip settings as originally specified they must be periodically tested by calibrated current injection.
Preferential load shedding, generator scheduling and load sharing is usually part of an overall power management system (PMS) under computer control.
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