Marine Electrical System
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Transcript of Marine Electrical System
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Marine Electrical System
By
DR. Oladokun Sulaiman Olanrewaju
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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
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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 both equipment & operating personnel
• General scheme - nearly common to all ships
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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
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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 & 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
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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, voltage and current transformers, ammeter switches, voltage regulations & 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 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
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Shore supply (cont/…)
• It is not normal practice to parallel shore supply with ship’s generators
• Therefore, ship’s generators must disconnected 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
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Emergency power supply
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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 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 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
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Insulated neutral system
Insulated system - totally electrically
insulated from earth (ship’s 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 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 isolation earth-faulted equipment
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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 ‘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 equipment was classed ESSENTIAL
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The preferred system??
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• 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
The preferred system??
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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
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The preferred system??
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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 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%
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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 %
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Effect of lower frequency
• Stator flux increases
• Magnetizing current will increase
• Motor runs slower & hot
• Speed reduced to 17%
• Overheating will take place
• Remedy is to slightly lower the voltage