Shipboard Electrical Distribution …“Shifting the Electric Plant to a Normal Full Power Lineup.”
Topics Primary Power Electrical Distribution System Components of systems Selective tripping Other AC power generation systems DC Power generation Safety Considerations
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) Arc Fault System 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. Parallel requirements for closing circuit breakers Emergency Power Distribution Normally supplied by DG’s Designed to provide power to vital systems during a casualty
The “Electric Plant”
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 fault as possible and furthest from power supply Done via fuses, switches, and breakers (most current system is arc-fault) Vital vs. Non-vital loads
Other AC Generation Systems 115 VAC, single phase, 60 Hz used for smaller loads Used for lighting, outlets, etc. 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 to replace SSMG’s Rectifier: converts AC -> DC
Degaussing Circulation of DC current around the hull to reduce the magnetic signature. Degaussing stations Magnetic exploders
Safety Considerations #1 On-the-job KILLER of sailors Working in vicinity of 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.
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