Marine Electrical System

Operational Consideration in Electrical Power Plant

Objectives • State common parameters of AC electrical supply onboard • Describe how the power is distributed to consumers using line diagram (incorporate shore supply and emergency source of power) • Describe the insulated neutral system and why it is preferred

Introduction • Auxiliary services ranging from ER pumps and fans, deck winches & windlasses to general lighting, catering & AC • Electrical power – used to drive most of these auxiliaries • Electrical power system - designed to provide secured supplies with adequate built-in protection for both equipment & operating personnel • General scheme - nearly common to all ships

Switchboard • To distribute generated electricity to where it is needed • Can be classified as one of following: –  Main switchboards  –  Emergency switchboards  –  Section boards - supplied directly/via transformers etc  –  Distribution boards

• Metal-clad, dead front switchboards are mandatory for AC systems

Distribution system • Main board - built in 2 sections which can operate independently in case one section damaged • One side carries port & fwd motors (group motor  starter) while other section carried stbd s tbd & aft motors • Central section used for control the main generators • Switchgear cubicles on generator panel sides used for  essential services, flanked by group motor starter   boards • Separate section will controls 3-phase 220V & lighting services

Distribution system (cont/…) • 440V/220V lighting transformers may mounted inside main swbd cubicle, or free-standing behind it • Main generator supply cables connected directly to their CB • Short copper bars, then connected to three bus bars which run through switchboard length • Busbars - may seen if rear door are opened, in special enclosed bus-bar duct • Swbd contain frequency meters, synchroscopes, wattmeters, wattmeters, voltage and current transformers, ammeter ammeter switches, voltage regulations & means for adjusting prime movers speed

Shore supply • Required during deadship - dry-docking for major overhaul • Log of supply kWh meter taken for costing purposes • Suitable connection box to accept shore supply cable accommodation entrance or emergency generator room • Connection box - suitable terminals including earthing terminal, dedicated CB, switch & fuses - protect cable linking to main switchboard • Plate giving details of ship’s electrical system (voltage and frequency) & method for connecting must provided • For AC supply, phase sequence indicator is fitted - indicate correct supply phase sequence - usually lamp

Shore supply (cont/…) • It is not normal nor mal practice to parallel shore supply with ship’s generators • Therefore, ship’s generators must disconnected dis connected  before shore supply resume connection – interlocked  provided • Shore supply may also connected directly to emergency board - ‘back feeds’ to main switchboard • When phase sequence indicator indicate reverse sequence, simply interchanging any two leads to remedy this fault • Incorrect phase sequence cause motors to run in reverse direction

Effect of higher voltage • Contribute to sparking condition • Current drawn proportional to terminal voltage • Cause excessive starting current • Motor overheat due to high current • Motor accelerates fast and may overload the drive

Effect of lower voltage • Motor draw more current to keep same s ame power output • Starting torque

V², thus

to 72.5%

• Take longer period to build up speed • High reactance motor will stalled • Overheating will occur  • Motor may stall & burn due to overheating – 49x full load heating • Star delta starter line voltage

58%

Effect of higher frequency • Motor run 20% faster, increase overall speed • Overload, overheated & overstress driven loads • Power produced

(speed)³

• Supply will reduce stator flux • Affect starting torque • Centrifugal load will rise by 73 %

Effect of lower frequency • Stator flux increases • Magnetising current will increase • Motor runs slower & hot • Speed reduced to 17% • Overheating will take place • Remedy is to slightly lower the voltage

Emergency power supply

Emergency power supply • Provided, in event of emergency (blackout etc), supply still available for  emergency lighting, alarms, communications, watertight doors & other  essential services - to maintain safety & safe evacuation • Source - generator, batteries or both • Self-contained & independent from other ER power supply • Emergency generator must have ICE as prime mover with own FO supply tank, starting equipment & switchboard • Must initiated following a total electrical power failure • Emergency batteries - ‘switch in’ immediately after power failure • Emergency generators - hand cranked, but automatically started by air /  battery possible - ensure immediate immediate run-up • Power rating - determined by size & ship role • Small vessels - few kW sufficient for emergency lighting

• • • • • • • • •

Larger & complicated vessels - may require hundreds of kW for  emergency lighting, chronological restarting & fire fighting supply Connected to own emergency swbd - located in compartment above water line  Normal operation - emergency board supplied from main board via ‘bus-tie’ Impossible to synchronise with main generators due to interlocks –  newer design permit short period of synchronising Starting automatically - initiated by relay which monitors normal main supply Falling mains frequency / voltage causes ‘start-up’ relay to operate generator starting equipment Arrangement for starting – electrical, pneumatic, hydraulic Regular tests - power loss simulation will triggers start sequence Detailed regulations - 1972 SOLAS Convention, IEE Regulations for  Electrical and Electronic Equipment of Ships, regulations from Classification Societies (LR, ABS, DNV etc) and etc

Insulated neutral system

Insulated system - totally electrically insulated from earth (ship’s hull)

Earthed neutral system

Earthed system has one pole or  neutral point connected to earth

General • Shipboard systems - insulated from earth (ship's hull) • Shore system - earthed to the ground • HV systems (>1000V) - earthed to ship's hull via neutral earthing resistor (NER) or high impedance transformer to limit earth fault current • Priority for shipboard - maintain electrical supply to essential equipment in event of single earth fault • Priority ashore - immediate immediate isolation earth-faulted equipment

3 basic circuit faults

An open-circuit fault is due to a break in the conductor, as at A, so that current cannot flow An earth fault is due to a break in the insulation, as at B, allowing the conductor to touch the hull or an earthed metal enclosure

A short-circuit fault is due to a double break in the insulation, insulation, as at C, allowing both conductors to be connected so that a very large current bypasses or "short-circuits" the load.

The preferred system?? • If earth fault occurs on insulated pole of ‘EARTHED DISTRIBUTION SYSTEM’ - equivalent to ‘short circuit’ fault • Large earth fault current would immediately ‘blow’ the fuse in line conductor  • Faulted electrical equipment immediately isolated from supply & rendered SAFE, but loss of equipment • Could create hazardous situation if i f equipment was classed ESSENTIAL

The preferred system??

The preferred system?? • If earth fault ‘A’ occurs on one line of ‘INSULATED DISTRIBUTOIN SYSTEM’ - not trip any protective gear & system resume function normally • Thus, equipment still operates • If earth fault ‘B’ developed on another line, 2 earth faults would equivalent to a short-circuit fault & initated protective gear  • An insulated distribution system requires TWO earth faults on TWO different lines to cause an earth fault current. • An earthed distribution system requires only ONE earth fault on the LINE conductor to create an earth fault current. • Therefore an insulated system is more effective than an earthed system - maintain supply continuity to equipment, thus  being adopted for most marine electrical systems

High voltage system • Shipboard HV systems - ‘earthed’ via resistor  connecting generator neutrals to earth • Earthing resistor with ohmic value - chosen to limit maximum earth fault current < generator full load current •  Neutral Earthing Resistor (NER) - assembled with metallic plates in air – due to single earth fault will cause circuit disconnected by its protection device

The preferred system??