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 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,

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 by-passes 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??

• 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??

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??