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 V 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. PbO 2 ) 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?