A SEMINAR ON…….

PROTECTION OF ALTERNATORPRESENTED 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,the alternator runs as a synchronus motor and draws some current from supply system.This is known as “inverted running”.In case of turbo alternator,failure of steam supply may cause inverted running.If the steam supply is gradually restored,the alternator will pick up load without disturbing the system.In case of hydro-generator ,protection against inverted running is achieved by providing mechanical devices on the water wheel.When water flows drops to an insufficient rate to maintain electrical output,alternator is disconnected from the system.

FAILURE OF FIELD

The chances of field failure of alternators are undoubtedly very rare.

Even if it does occur,no immediate damage will be caused by permitting the alternator to run without a field for a short period.

In this case the alternator can be disconneted manually.So there is no need of automatic protection.

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-SPEED

The chief cause of overspeed is the sudden loss of all or the part of load on the alternator.

Centrifugal devices mounted on their driving shafts trip the main valve of the primemover when a dangerous overspeed occurs.

OVER-VOLTAGE

The field excitation system of modern alternators is so designed that over voltage conditions at normal running speed can’t occur.

However,overvoltage in an alternator occurs when speed of prime mover increases due to sudden loss of alternator load.

Usually control governers are provided which continuously checks the speed and prevents the over speed.

UNBALANED LOADING

Unbalanced loading arises from fault to earth or faults between phases on the circuits external to the alternator.

Unbalanced current may burn the mechanical fixing of the rotor core or damage the field winding.

Unbalanced Loading Protection

OPERATION

Under normal operating condition,algebraic sum of three currents flowing through the relay is zero and relay does not operate.

When unbalancing occurs,resultant current flows through the relay and relay trips the circuit breaker to disconnet the alternator from the system.

STATOR WINDING FAULTSFault between phase and ground

Fault between phases

Inter-turn faults involving turns of same phase winding

Differential Protection

It provides protection against phase to phase and phase to ground

Current at two ends of the protected sections are compared

Under normal operating condions,these currents are equal,hence no current flows through the relay.

When a fault occurs in the protected zone,currents at two ends of the CT becomes unequal.Differential current flowing the the relay isolates the protected section from the system.

Differential Protection

DIFFICULTY AND REMEDY

In most of the cases,alternator is located at a considerable distance from the switchgear.

As the relays are located close to the circuit breaker,balancing resistances are inserted in shorter length of pilot wire to balance the impedance on both side of the relay.

Differential Protection with Balancing Resistors

LIMITATION

It is impossible to provide protection to the whole winding when neutral earthing resistance is used.

It protects only 85% of the winding.

Balanced earth fault protection

Balanced earth fault protection is used for those alternators in which neutral ends of the three phase are connected internally to a single terminal.

It provides no protection against phase to phase fault.

Balanced Earth Fault Protection

Inter-turn fault protection

It is used for multi-turn generators such as hydro-electric generators which have double winding armatures.

When a short circuit develops between adjacent turns in one of the armature windings,unbalanced current flows in two winding.

This unbalanced current flows through the relay to operate the circuit breaker.

Stator Inter-Turn Fault Protection

CONCLUSION

Electricity is used in every step of our life which is generated by the alternator.

The development of a country also indirectly depends on electricity.

As electrical engineer,we all must be very very much aware about the protection of alternator.

THANKS