hazard zone
TRANSCRIPT
-
8/14/2019 Hazard Zone
1/26
Operational Consideration in
Electrical Power Plant
Dr. Oladokun Sulaiman
-
8/14/2019 Hazard Zone
2/26
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
-
8/14/2019 Hazard Zone
3/26
-
8/14/2019 Hazard Zone
4/26
-
8/14/2019 Hazard Zone
5/26
-
8/14/2019 Hazard Zone
6/26
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
-
8/14/2019 Hazard Zone
7/26
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 foressential services, flanked by group motor starter
boards
Separate section will controls 3-phase 220V &lighting services
-
8/14/2019 Hazard Zone
8/26
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 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, voltageregulations & means for adjusting prime movers speed
-
8/14/2019 Hazard Zone
9/26
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 linking
to main switchboard Plate giving details of ships electrical system (voltage and
frequency) & method for connecting must provided
For AC supply, phase sequence indicator is fitted - indicatecorrect supply phase sequence - usually lamp
-
8/14/2019 Hazard Zone
10/26
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 reversesequence, simply interchanging any two leads toremedy this fault
Incorrect phase sequence cause motors to run inreverse direction
-
8/14/2019 Hazard Zone
11/26
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
-
8/14/2019 Hazard Zone
12/26
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%
-
8/14/2019 Hazard Zone
13/26
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 %
-
8/14/2019 Hazard Zone
14/26
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
-
8/14/2019 Hazard Zone
15/26
Emergency power supply
-
8/14/2019 Hazard Zone
16/26
Emergency power supply
Provided, in event of emergency (blackout etc), supply still available for
emergency 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 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
-
8/14/2019 Hazard Zone
17/26
Larger & complicated vessels - may require hundreds of kW foremergency lighting, chronological restarting & fire fighting supply
Connected to own emergency swbd - located in compartment abovewater line
Normal operation - emergency board supplied from main board viabus-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 for
Electrical and Electronic Equipment of Ships, regulations fromClassification Societies (LR, ABS, DNV etc) and etc
-
8/14/2019 Hazard Zone
18/26
Insulated neutral system
Insulated system - totally electrically
insulated from earth (ships hull)
-
8/14/2019 Hazard Zone
19/26
Earthed neutral system
Earthed system has one pole or
neutral point connected to earth
-
8/14/2019 Hazard Zone
20/26
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 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
-
8/14/2019 Hazard Zone
21/26
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.
-
8/14/2019 Hazard Zone
22/26
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
-
8/14/2019 Hazard Zone
23/26
-
8/14/2019 Hazard Zone
24/26
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 faultswould 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
-
8/14/2019 Hazard Zone
25/26
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
-
8/14/2019 Hazard Zone
26/26