17253038 marine electrical system

8/6/2019 17253038 Marine Electrical System

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Objectives

State common parameters of AC electrical

supply onboard

Describe how the power is distributed toconsumers using line diagram (incorporate

shore supply and emergency source of power)

Describe the insulated neutral system and why

it is preferred


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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 bothequipment & operating personnel

General scheme - nearly common to all ships


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Distribution system

Main board - built in 2 sections which can operateindependently in case one section damaged

One side carries port & fwd motors (group motorstarter) while other section carried stbd & aft motors

Central section used for control the main generators

Switchgear cubicles on generator panel sides used foressential services, flanked by group motor starter

boards Separate section will controls 3-phase 220V &

lighting services


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Distribution system (cont/)

440V/220V lighting transformers may mounted inside mainswbd cubicle, or free-standing behind it

Main generator supply cables connected directly to their CB

Short copper bars, then connected to three bus bars whichrun through switchboard length

Busbars - may seen if rear door are opened, in specialenclosed bus-bar duct

Swbd contain frequency meters, synchroscopes, wattmeters,voltage and current transformers, ammeter switches, voltageregulations & means for adjusting prime movers speed


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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 earthingterminal, dedicated CB, switch & fuses - protect cable linkingto main switchboard

Plate giving details of ships electrical system (voltage andfrequency) & method for connecting must provided

For AC supply, phase sequence indicator is fitted - indicatecorrect supply phase sequence - usually lamp


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Shore supply (cont/)

It is not normal practice to parallel shore supply withships generators

Therefore, ships generators must disconnectedbefore shore supply resume connection interlockedprovided

Shore supply may also connected directly toemergency board - back feeds to main switchboard

When phase sequence indicator indicate reverse

sequence, simply interchanging any two leads toremedy this fault

Incorrect phase sequence cause motors to run inreverse direction


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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


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Effect of lower voltage

Motor draw more current to keep same power output

Starting torque V, 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%


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Effect of higher frequency

Motor run 20% faster, increase overall speed

Overload, overheated & overstress driven

loads

Power produced (speed)

Supply will reduce stator flux

Affect starting torque Centrifugal load will rise by 73 %


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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


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Emergency power supply


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Emergency power supply

Provided, in event of emergency (blackout etc), supply still available foremergency lighting, alarms, communications, watertight doors & otheressential 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 FOsupply 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 run-up

Power rating - determined by size & ship role

Small vessels - few kW sufficient for emergency lighting


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Larger & complicated vessels - may require hundreds of kW foremergency 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 mainsupply

Falling mains frequency / voltage causes start-up relay to operategenerator starting equipment

Arrangement for starting electrical, pneumatic, hydraulic

Regular tests - power loss simulation will triggers start sequence

Detailed regulations - 1972 SOLAS Convention, IEE Regulations forElectrical and Electronic Equipment of Ships, regulations fromClassification Societies (LR, ABS, DNV etc) and etc


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Insulated neutral system

Insulated system - totally electrically

insulated from earth (ships hull)


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Earthed neutral system

Earthed system has one pole or

neutral point connected to earth


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General

Shipboard systems - insulated from earth (ship's hull) Shore system - earthed to the ground

HV systems (>1000V) - earthed to ship's hull vianeutral earthing resistor (NER) or high impedancetransformer to limit earth fault current

Priority for shipboard - maintain electrical supply toessential equipment in event of single earth fault

Priority ashore - immediate isolation earth-faultedequipment


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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, as at C,

allowing both conductors

to be connected so that a

very large current by-

passes or "short-circuits"

the load.


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The preferred system??

If earth fault occurs on insulated pole of EARTHEDDISTRIBUTION SYSTEM - equivalent to shortcircuit 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 equipment was

classed ESSENTIAL


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If earth fault A occurs on one line of INSULATEDDISTRIBUTOIN 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 protectivegear

An insulated distribution system requires TWO earth faults onTWO different lines to cause an earth fault current.

An earthed distribution system requires only ONE earth faulton the LINE conductor to create an earth fault current.

Therefore an insulated system is more effective than anearthed system - maintain supply continuity to equipment, thus

being adopted for most marine electrical systems



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High voltage system

Shipboard HV systems - earthed via resistorconnecting generator neutrals to earth

Earthing resistor with ohmic value - chosen to limit

maximum earth fault current < generator full loadcurrent

Neutral Earthing Resistor (NER) - assembled withmetallic plates in air due to single earth fault will

cause circuit disconnected by its protection device


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17253038 marine electrical system

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