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1. SUB: SWITCH GEAR AND
PROTECTION
PROTECTION OF ALTERNATOR &
BUSBAR
PREPARED BY:- S. N.KHALAS,
LECTURER IN ELECTRICAL ENGINEERING
DEPARTMENT,
V.P.M.P POLYTECHNIC

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. Need for Busbar Protection
Need For Bus Protection
 In its absence fault clearance takes place in zone II of
distance relay by remote end tripping.
 This means slow and unselective tripping and wide
spread black out.
Effect of delayed clearance
 Greater damage at fault point
 Indirect shock to connected equipments like shaft of
generator and windings of transformer

24. Requirements of Busbar Protection
 Must have short tripping time as possible.
 Must be able to detect internal faults (sensitivity).
 Must be absolutely stable to external faults (stability).
 Must be able to detect and trip only faulty part of
busbar system (selectivity)
 Must be secure against maloperation due to auxilliary
contact failure.

25. Types of Busbar Protection
Differential type of busbar protection is divided into
two groups.
 Low impedance scheme :
Low impedance scheme uses biased differential relay.
 High impedance Scheme:
High impedance scheme uses a stabilizing resistor in series
with the differential relay.

26. Differential Relay Principle
Busbar protection relays works on the differential principle
i.e. comparing the currents entering and leaving a protected
object.
If those currents matches the protected object is assumed
to be in healthy condition and relay remains stable (non
operating) . If there is a difference in magnitude of currents,
it is assumed that there is some internal fault and the
differential relay operates.
Differential
C.T C.T
Protected object

27. Single busbar Protection (Healthy condition)
BUSBAR
ZONE
Busbar
87
Current
entering
the bus
Current
leaving
the bus
Under healthy condition the current entering
the busbar and leaving the busbar will be same
and the CT secondary current circulates thru
the secondaries. No current flows thru the
relay. Hence the relay will remain restrained
(non operating).
P1
P1
P2
P2
S2
S2
S1
S1

28. Single busbar Protection (fault within bus)
BUSBAR
ZONE
P1
P1
P2
P2
S
2
S
2
S
1
S
1
Busbar
87
Current
entering
to the bus
Current
enters
from the
remote
bus
Under faulty condition all remote
busbars feed into fault and the
direction CT secondary currents
becomes additive and flows
though the rlay.

29. Single busbar Protection scheme
• Here, in the figure above we assume that at normal condition feed, A, B, C& D,
carries current IA, IB, IC, and ID . Now, according to Kirchhoff's current law, at node
K, IA + IB+ IC+ID = 0
• So, it is clear that under normal condition there is no current flows through the
busbar protection tripping relay.
• Now, say fault is occurred at any of the feeders, outside the protected zone. In
that case, the faulty current will pass through primary of the CT of that feeder.
This fault current is contributed by all other feeders connected to the bus. So,
contributed part of fault current flows through the corresponding CT of
respective feeder. Hence at that faulty condition, if we apply KCL at node K, we
will still get, iR = 0.
BUSBAR
ZONE
87
A D
B C
Rela
y (R)
K

30. When fault is occurred on the bus itself. The fault current is contributed
by all feeders connected to the bus. Hence, at this condition, sum of all
contributed fault current is equal to total fault current. The sum of all
secondary currents is no longer zero. It is equal to secondary equivalent
of fault current.
So at this condition current starts flowing through 87 relay and it makes
trip the circuit breaker corresponding to all the feeders connected to this
section of the busbar. As all the incoming and outgoing feeders,
connected to this section of bus are tripped, the bus becomes dead.
This differential busbar protection scheme is also referred as current
differential protection of busbar.

31. THANKS