1. A SEMINAR ON…….
PROTECTION
OF
ALTERNATOR
PRESENTED BY :
Basanta Ku. Jena,
Electrical Engg.
CET,BBSR

2. CONTENTS
Mechanical Protections
 Failure of Prime-mover
 Failure of Field
 Over-current
 Over-speed
 Over-voltage
Electrical Protections
 Unbalanced Loading
 Stator Winding Faults

3. 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:-

4. FAILURE OF PRIME MOVER
When 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. Unbalanced Loading Protection

11. 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.

12. STATOR WINDING FAULTS
Fault between phase and
ground
Fault between phases
Inter-turn faults involving
turns of same phase winding

13. 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.

14. Differential Protection

15. 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.

16. Differential Protection with Balancing Resistors

17. LIMITATION
It is impossible to provide protection to the
whole winding when neutral earthing
resistance is used.
It protects only 85% of the winding.

18. 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.

19. Balanced Earth Fault Protection

20. 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.

21. Stator Inter-Turn Fault Protection

22. 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.

23. THANKS