SHIPBOARD ELECTRICAL DISTRIBUTION SYSTEM

 The function of the ship's electrical distribution system is to convey the generated electrical power to the electrical consumers onboard. let's discuss in detail about this system.

ELECTRICALCTRIAL DISTRIBUTION SYSTEM

The distribution system is the means by which the electrical power produced by the generators is delivered to the various motors, lighting, galley services, navigation aids, etc. which comprise the ship's electrical load.

It can be broadly classified as main supply and emergency supply.

BLOCK DIAGRAM OF SHIP'S ELECTRICALDISTRIBUTION SYSTEM

MAIN SUPPLY

The main supply 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 or the equipment necessary for

loads that are required to handle a potentially dangerous situation. They include certain supplies to navigational aids, communications, machinery spaces, control stations, steering gear, fire services, etc. The essential services may be supplied directly from the main switchboard or via section boards or distribution boards.

The majority of ships have a 3-phase a.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, the 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 even 11 kV. Such high voltages are economically necessary for high power systems to reduce the size of the current, and hence reduce the size of conductors and equipment required.

Transformers interconnect the high voltage and low voltage distribution sections of the system. 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.

 The produced electrical energy is routed through the main switchboard, The main board supplies bulk power to motor group starter boards(often part of the main board), .

section boards, and distribution boards

SHIP'S ELECTRICAL DISTRIBUTION SYSTEM

 PROTECTIVE DEVICES

Protection equipment like circuit-breakers and fuses, strategically placed throughout the system automatically disconnects a faulty circuit within the network. The operational state of a distribution system is indicated by the monitors for power, voltage, and current and by protection relays for over currents and earth faults at each main control center.

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.

EMERGENCY SUPPLY

An emergency electrical power service must be provided on board in the event of a main power failure. Such a supply is required for emergency lighting, alarms, communications, watertight doors, and other services necessary to maintain safety and permit safe evacuation of the ship.

It is powered by an emergency generator or batteries which come into load during main supply failure.

The emergency switchboard has two sections, one operating at 440v and other at 220 v. The 440v supply, under normal circumstances is taken from the main switch board through bus tie breaker , loss of main power causes bus tie breaker to be tripped and the supply is taken over directly by the emergency generator. 

The bus tie breaker prevents supply to main bus during main supply breakdown.

SHORE SUPPLY

A shore supply is required so that the ship's generators and their prime movers can be shut down for a major overhaul during a dry-docking period(click here to know more about shore supply). If the shore supply frequency differs from the ship's normal frequency then, ideally, the shore supply voltage should differ in the same proportion(click here to know more).

BATTERY CHARGING CIRCUIT

Battery charging circuits are provided which are stepped down and rectified to a 24V DC supply to keep the battery pack fully charged which are used to power emergency lighting and other essential supplies during main supply failure.