Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGThe ship's service generators and distributionswitchboards are interconnected by bus tiesso that any switchboard can be connected to feedpower from its generator to one or more of theother switchboards. The bus ties also connecttwo or more switchboards so that the generatorplants can be operated in parallel (or the switchboardscan be isolated for split-plant operation).In large installations, power distribution toloads is from the generator and distributionswitchboards or switchgear groups to load centers,to distribution panels, and to the loads,or directly from the load centers to some loads.On some ships, such as large aircraft carriers,a system of zone control of the ship'sservice and emergency power distribution isprovided. Essentially, the system establishes anumber of vertical zones, each of which containsone or more load center switchboardssupplied through bus feeders from the ship'sservice switchgear group. A load center switchboardsupplies power to the electrical loadswithin the electrical zone in which it is located.Thus, zone control is provided for all powerwithin the electrical zone. The emergencyswitchboards may supply more than one zone,depending on the number of emergency generatorsinstalled. Figure 20-23 shows the ship'sservice and emergency power distribution systemin a large aircraft carrier.In smaller installations (fig. 20-24) the distributionpanels are fed directly from thegenerator and distribution switchboards. Thedistribution panels and load centers (if any) arelocated centrally with respect to the loads theyfeed to simplify installation. This arrangementalso requires less weight, space, and equipmentthan if each load were connected to a switchboard.At least two independent sources of powerare provided for selected vital loads throughautomatic bus transfer equipment. The normaland alternate feeders to a common load run fromdifferent ship's service switchboards and arelocated below the waterline on opposite sides ofthe ship to minimize the possibQity that bothwill be damaged by a single hit.The lighting circuits are supplied from thesecondaries of 450/115-volt transformer banksconnected to the ship's service power system.In large ships, the transformer banks are installedin the vicinity of the lighting distributionpanels, at some distance from the generatorand distribution switchboards. In small ships,the transformer banks are located near thegenerator and distribution switchboards andenergize the switchboard buses that supply thelighting circuits.EMERGENCY POWERThe emergency power distribution system isprovided to supply an immediate and automaticsource of electric power to a limited number ofselected vital loads in the event of failure of theship's service power distribution system. Theemergency power system, which is separate anddistinct from the ship's service power distributionsystem, includes one or more emergencydistribution switchboards. Each emergencyswitchboard, supplied by its associated emergencygenerator, has feeders which run to the bustransfer equipment at the distribution panels orloads for which emergency power is provided.The emergency generators and switchboardsare located in separate spaces from those containingthe ship's service generators and distributionswitchboards. As previously noted, thenormal and alternate ship's service feedersare located below the waterline on oppositesides of the ship. The emergency feeders arelocated near the centerline and higher in theship (above the waterline). This arrangementprovides for horizontal separation between thenormal and alternate ship's service feedersand vertical separation between these feedersand the emergency feeders, thereby minimizingthe possibility of damaging all tluree types offeeders simultaneously.The emergency switchboard is connected byfeeders to at least one and usually to two differentship's service switchboards. One of theseswitchboards is the preferred source of ship'sservice power for the emergency switchboardand the other is the alternate source. Theemergency switchboard and distribution systemare normally energized from the preferredsource of ship's service power. If both the preferredand the alternate sources of ship's servicepower fail, the diesel-driven emergencygenerator starts automatically and the emergencyswitchboard is automatically transferredto the emergency generator.When the voltage is restored on either thepreferred or the alternate source of the ship'sservice power, the emergency switchboard isautomatically retransferred to the source thatis available (or to the preferred source, if voltageis restored on both the preferred and thealternate sources). The emergency generator510