Marine Electrical System v4y47

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 & aft motors • Central section used for control the main generators • Switchgear cubicles on generator 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, voltage and current transformers, 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 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

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 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 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 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, as at C, allowing both conductors to be connected so that a very large current byes 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 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??