Sepam - HV/MV Protection and control units (ENG) - Trinet

AC generator protectionintroductionAC generator operation can be altered byboth faults within the machine and bydisturbances occurring in the electricalsystem to which it is connected.An AC generator protection systemtherefore has a dual objective: protectingthe machine and protecting the system.types of faultsFaults such as overloads, unbalance andinternal phase-to-phase faults are the sametype for AC generators as for motors.However, there are other types of faults thatare characteristic of AC generators.When a short circuit occurs in a anelectrical system close to an AC generator,the fault current looks like that shown infigure 1.The maximum short-circuit current valueshould be calculated taking into account themachine's substransient impedance X"d .The value of the current detected by aprotection device, which is very slightly timedelayed(by about 100 milliseconds), shouldbe calculated taking into account themachine's transient impedance X’d.The value of steady state short-circuitcurrent should be calculated taking intoaccount the synchronous impedance X.This current is weak, generally less than theAC generator's rated current.Internal phase-to-frame faultThis is the same type of fault as for motorsand its effects depend on the groundingsystems adopted. A particularity in relationto motors, however, lies in the fact that ACgenerators can operate decoupled from theelectrical system during the start-up andshutdown periods, and also when operatingfor testing or on stand-by. The groundingsystem may differ depending on whether theAC generator is coupled or decoupled andthe protection devices should be suitable forboth cases.Field lossWhen an AC generator previously coupledwith a system loses its field, it becomesdesynchronized from the system. It thenoperates asynchronously, overspeedingslightly, and it absorbs reactive power.Motor-like operationWhen an AC generator is driven like a motorby the electrical power system to which it isconnected and it applies mechanical energyto the shaft, this can cause wear anddamage to the driving machine.Voltage and frequency variationsVoltage and frequency variations duringsteady state operating are due to themalfunction of the related regulators. Thesevariations create the following problems:too high a frequency causes abnormalmotor overheating,too low a frequency causes motor powerloss,variations in frequency cause variations inmotor speed which can bring aboutmechanical damage,too high a voltage puts stress on all partsof the network,too low a voltage causes torque loss andan increase in current and motoroverheating.current subtransient phenomena transientcontinuoustfig.1Merlin Gerin protection guide 41