Shipboard Electrical Distribution

Objectives Principles of operation of a three-phase AC shipboard power distribution system Functions of the various elements of a ship's electrical distribution system Ship's service vs emergency power distribution systems. - The student will comprehend the principles of operation of a three-phase AC shipboard power distribution system. - The student will comprehend the functions of the various elements of a ship's electrical distribution system. - The student will comprehend the difference between ship's service and the emergency power distribution systems.

Objectives Requirements for paralleling generators Vital and non-vital systems Casualty power system - The student will comprehend the function of the casualty power system. - The student will identify vital and non-vital systems. - The student will comprehend measures which may be employed to counter the ship's magnetic field. - The student will comprehend the requirements for paralleling generators prior to placing an electrical load on them.

Reference Introduction to Naval Engineering, Chapter 17, pp. 319-332 Principles of Naval Engineering, Chapter 20, pp. 20-21 - 20-35

ELECTRICAL DISTRIBUTION COMPONENTS

Switchboard Enclosure that houses various electrical instruments, indicating devices, protective devices, and regulating apparatus required for the control of the generators and for the distribution of electricity Contains buswork and distributes electrical power from generator to various circuits

Fuse An electrical protective device consisting of a fusible (easily melted) metal-alloy strip of wire encased in a cartridge If current exceeds a predetermined value, sufficient heat is generated to melt the fuse causing an open circuit Protects the circuit/equipment from possible damage due to excessive current

Circuit Breaker An electrical device that opens due to various trip setpoints (over-current, under-voltage, under-frequency) to protect generators and loads Circuit breakers trip open, and may be reset when the operators close the circuit breaker Circuit breakers may be operated manually or electrically, and can be used as switches in some circuits

Automatic Bus Transfer Also known as “ABT” A device that senses a loss of power from a normal (N) source and will automatically disconnect the load from the normal source and connect it to an alternate (A) source ABTs can be designed to transfer loads very quickly (within 50 ms)

Manual Bus Transfer Also known as “MBT” Can connect a load to either a normal (N) or alternate (A) source, but unlike ABTs (which shift automatically) must be shifted manually

Generators Powered by gas turbine, diesel or steam US Navy typically uses: 450 Volt, 3 phase, 60 Hz Power approximately 750 - 3000 kW

Shipboard Electrical Distribution Systems How we get electrical power from generators to loads

2S 1S BUS TIE N A SSTG #3 FIRE CONTROL SSTG #1 A N SSTG #4 SSTG #2 N ABT SSTG #3 FIRE CONTROL SSTG #1 A N SSTG #4 MBT SSTG #2 2S 1S N ABT STEERING PANEL E N N EMERGENCY GENERATOR SWITCHBOARD #2 EMERGENCY GENERATOR SWITCHBOARD #1 E ABT ABT E TO VITAL LOADS TO VITAL LOADS EDG #2 EDG #1

Ship’s Service Distribution System (SSDS) Supplied by ship’s service generators Provides both normal and alternate power supplies to all ship’s loads Electrical power is distributed as follows: Generators Distribution switchboards Load centers Distribution panels Loads Electrical power is normally distributed through circuit breakers

Emergency Power Distribution System Emergency (E) power is normally supplied by two or more emergency diesel generators (EDGs) which will start automatically upon a loss power to the SSDS Located in forward and after ends of the ship and low in the ship Physical separation between SSDS and the emergency power distribution system minimizes the possibility battle damage will cause a total loss of electrical power

Casualty Power Distribution System (CPDS) Allows for temporarily connecting vital systems to any available power source Permits maintaining electrical power to equipment necessary to keep the ship afloat, allow the ship to move out of the danger area, or operate armament such as antiaircraft guns, missiles, or radar detectors Includes suitable lengths of portable electrical cable, permanently installed casualty power bulkhead terminals and risers, and terminal boxes for the equipment concerned

Vital v. Nonvital Vital loads have multiple power sources and are usually connected via an ABT Fire fighting pumps Steering Weapons systems Nonvital loads are not required for safe operation of the ship Power receptacles in berthing Laundry equipment Galley equipment

Grounded vs. Ungrounded Shipboard electrical power is ungrounded (all cabling is insulated from the ship’s hull) Ungrounded systems optimize continuity of power (increase equipment reliability), but do not increase personnel safety

Grounded Distribution Source Load Power

Ungrounded Distribution Source Load Power

Generators Operating In Parallel Phase: must be “in phase”, bring one generator in parallel with another at the instant both machines are “in phase” Voltage: must have the same output voltages Frequency: the oncoming generator frequency must be slightly higher than the running generator