AC GENERATOR OPERATIONAC GeneratorsSummaryThe important information covered in this chapter is summarized below.AC Generator Operation SummaryPower (kW) ratings of an AC generator are based on the ability of the primemover to overcome generation losses and the ability of the machine to dissipatethe heat generated internally. The current rating of an AC generator is based onthe insulation rating of the machine.There are three requirements that must be met to parallel AC generators:1) Their terminal voltages must be equal. A mismatch may cause highcurrents and generator or distribution system damage.2) Their frequencies must be equal. A mismatch in frequencies can causeone generator to "motor," causing an overload in the generators and thedistribution system.3) Their output voltages must be in phase. A mismatch in the phases willcause large opposing voltages to be developed, resulting in damage to thegenerators and distribution system due to high currents.The disadvantage of a stationary field, rotating armature is that the slip-ring andbrush assembly is in series with the load circuits and, because of worn or dirtycomponents, may interrupt the flow of current.A stationary armature, rotating field generator has several advantages: (1) a loadcan be connected to the armature without moving contacts in the circuit; (2) itis much easier to insulate stator fields than rotating fields; and (3) much highervoltages and currents can be generated.The advantage of the delta-connected AC generator is that if one phase becomesdamaged or open, the remaining two phases can still deliver three-phase powerat a reduced capacity of 57.7%.The advantage of a wye-connected AC generator is that each phase only has tocarry 57.7% of line voltage and, therefore, can be used for high voltagegeneration.ES-10 Page 14 Rev. 0
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