Shipboard Electrical Distribution

References

Required• Introduction to Naval Engineering–Ch 17

Primary Power Characteristics

• US Residential– 115 VAC Power– 1 phase, 60 Hz frequency– Grounded system

• US Navy– 450 VAC Power– 3 phase, 60 Hz frequency– Ungrounded system

Primary Power Characteristics

• Why AC?– More power than DC for same size

• Why 3 Phase?– More power than single phase– Can operate with only 2 phases - redundancy

• Why ungrounded?– NOT Safety– Reliability: at least two faults are required to cause

system failure (grounded systems fail with one fault)

Basic Components

• Ship Service Generators– Primary source of power is rotating-field

generator– Powered by prime mover• SSTG (steam)• GTGS (gas)• EDG (diesel)

• Switchboard– Metal enclosure that serves as distribution center

(contains buswork, breakers, instruments, etc)

Basic Components• Bus Transfers– Allow transfer of load from one

bus to another (normal -> alternate)• ABT: senses voltage loss quickly

and auto transfers supply to alternate source to ensure continuity of power

• MBT: manual transfer, used for non-vital loads

• Disconnects– Used for infrequent

connections (shore power)

Basic Components

• Circuit Breaker– Used as switches to connect buswork– Trip open and break circuit if unsafe condition (over

current, under voltage, under frequency)

• Fuse– Simplest protective device– Fusible strip of wire melts to provide protection– Must be replaced once blown

Electrical Distribution

• Ship Service Distribution System– Provides normal and alternate power– Power from generators switchboards sent

to loads, power panels, etc.

• Emergency Power Distribution– Normally supplied by DG’s– Designed to provide power to vital systems during

a casualty

Electrical Distribution

• Casualty Power Distribution system– Use of portable cables for temporary

routing of power in severe casualty• Selective Tripping– Principle under which faults are

isolated as close to the associated load and furthest from supply as possible

– Done with load-specific fuses, switches, and breakers which trip “selectively” on various faults– i.e. Arc-fault protection

Other AC Generation Systems

• 115 VAC, single phase, 60 Hz used for smaller loads– Used for lighting, outlets, etc.– Used for vital loads (eg. reactor instrumentation)– Incorporates 450-115V step-down transformer

• 400 Hz Power– Most of electronics including fire control & sonar

use 400 Hz– 400 Hz MG used to convert 60Hz to 400 Hz

DC Generation Systems

• Battery: chemical potential due to dissimilar metals (Pb vs. PbO2) in presence of electron bridge (electrolyte)

• SSMG: converts AC <-> DC (large machine with an AC end & DC end)

• Static Inverters: solid state version of older SSMGs

• Rectifier: converts AC -> DC

Safety Considerations• #1 On-the-job KILLER of sailors• Working on energized gear requires CO’s permission• NUMEROUS electrical safety precautions– Electrical Tagout to de-energize where possible– Rubber gloves– Insulated tools– Diamond deck– CPR qualified personnel– Removal of jewelry/metal objects– Etc.

Take Aways• Explain why shipboard electrical systems are

3-phase and ungrounded.• List the primary and emergency sources of

shipboard electrical power• Describe the purpose of ABTs, circuit bkrs,

fuses and disconnects• Describe the principle of “selective tripping”.• Explain the safety precautions associated with

working on energized gear.

Any Questions?