A SEMINAR ON…….PROTECTIONOFALTERNATORPRESENTED BY :Basanta Ku. Jena,Electrical Engg.CET,BBSR
CONTENTS Mechanical Protections Failure of Prime-mover Failure of Field Over-current Over-speed Over-voltage Electrical Protections Unbalanced Loading Stator Winding Faults
INTRODUCTION The generating units,especially the larger ones,are relatively few in number and higher in individual cost than most other equipments,Therefore ,it is desirable and necessary to provide protection to the alternators.Followings protection are provided to alternator:-
FAILURE OF PRIME MOVERWhen input to the prime-mover fails,thealternator runs as a synchronus motor anddraws some current from supply system.This isknown as “inverted running”.In case of turbo alternator,failure of steamsupply may cause inverted running.If the steamsupply is gradually restored,the alternator willpick up load without disturbing the system.In case of hydro-generator ,protection againstinverted running is achieved by providingmechanical devices on the water wheel.Whenwater flows drops to an insufficient rate tomaintain electrical output,alternator isdisconnected from the system.
FAILURE OF FIELDThe chances of field failure of alternators areundoubtedly very rare.Even if it does occur,no immediate damage willbe caused by permitting the alternator to runwithout a field for a short period.In this case the alternator can be disconnetedmanually.So there is no need of automaticprotection.
OVER-CURRENT It occurs mainly due to partial breakdown of winding insulation or due to overload on the supply system. Overcurrent protection for alternator is considered unnecessary because of the following reasons: As the modern alternators have considerably high values of internal impedance,these will stand a complete short circuit at their terminals for suffient time without serious overheating. On the occurrence of an overload,the alternators can be disconnected manually.
OVER-SPEEDThe chief cause of overspeed is thesudden loss of all or the part of load on thealternator.Centrifugal devices mounted on theirdriving shafts trip the main valve of theprimemover when a dangerousoverspeed occurs.
OVER-VOLTAGEThe field excitation system of modern alternatorsis so designed that over voltage conditions atnormal running speed can’t occur.However,overvoltage in an alternator occurswhen speed of prime mover increases due tosudden loss of alternator load.Usually control governers are provided whichcontinuously checks the speed and prevents theover speed.
UNBALANED LOADINGUnbalanced loading arises from fault to earthor faults between phases on the circuitsexternal to the alternator.Unbalanced current may burn the mechanicalfixing of the rotor core or damage the fieldwinding.
OPERATIONUnder normal operating condition,algebraicsum of three currents flowing through the relayis zero and relay does not operate.When unbalancing occurs,resultant currentflows through the relay and relay trips thecircuit breaker to disconnet the alternator fromthe system.
STATOR WINDING FAULTSFault between phase andgroundFault between phasesInter-turn faults involvingturns of same phase winding
Differential ProtectionIt provides protection against phase to phaseand phase to groundCurrent at two ends of the protected sectionsare comparedUnder normal operating condions,thesecurrents are equal,hence no current flowsthrough the relay.When a fault occurs in the protectedzone,currents at two ends of the CT becomesunequal.Differential current flowing the the relayisolates the protected section from the system.
DIFFICULTY AND REMEDYIn most of the cases,alternator is locatedat a considerable distance from theswitchgear.As the relays are located close to thecircuit breaker,balancing resistances areinserted in shorter length of pilot wire tobalance the impedance on both side of therelay.
Differential Protection with Balancing Resistors
LIMITATIONIt is impossible to provide protection to thewhole winding when neutral earthingresistance is used.It protects only 85% of the winding.
Balanced earth fault protection Balanced earth fault protection is used forthose alternators in which neutral ends ofthe three phase are connected internally toa single terminal.It provides no protection against phase tophase fault.
Inter-turn fault protectionIt is used for multi-turn generators suchas hydro-electric generators which havedouble winding armatures.When a short circuit develops betweenadjacent turns in one of the armaturewindings,unbalanced current flows intwo winding.This unbalanced current flows throughthe relay to operate the circuit breaker.
CONCLUSIONElectricity is used in every step of ourlife which is generated by the alternator.The development of a country alsoindirectly depends on electricity.As electrical engineer,we all must bevery very much aware about theprotection of alternator.