Protections of the transformer, generator, and motor.

1. Equipment Protection
Protection of Transformer, Generator
and Motor

2. Types of Faults
• Faults in auxiliary equipment which is a part of transformer
such as bushings, tap changer etc
– Transformer oil; low level exposes live parts such as the leads to
bushing. Oil level indicator with alarm give indication to this condition
– Oil temperature; rise in oil temperature may be an indication of
overload or failure of cooling system. A thermometer with alarms will
indicate rise of oil temperature.
• Faults in transformer winding and connections
– Winding Insulation failure; high impulse voltage , mechanical forces,
overload
– Core insulation failure;
• Overloads and external short circuits

3. Buchholz
relay

4. Transformer : Earth fault
protection
Magnitude of fault current
depends on
•Source impedance
•Neutral earthing impedance
•Transformer leakage reactance
•Fault voltage
•Winding connection

5. Fault current-DY
transformer- neutral
solidly grounded
Fault on secondary Y side.
Current magnitude depends on
leakage reactance of transformer
winding which is minimum near
neutral end and maximum at the
middle of winding
Primary fault current varies because of
change in transformation ratio

6. GENERATOR PROTECTION

7. • Protection of turbo-generators is the most
complex and elaborate.
– A Generator is a large machine and is connected
to bus-bars. It is accompanied by unit
transformers, auxiliary transformer and bus
system.

8. Generator connection
Typical small generators connection to
system
Large Generator connection to system
There are two methods by which generators are connected to Bus:
Generators connected directly to bus Generators connected via
delta-star unit transformer to HV bus i.e., to transmission Line

10. Possible Generator Faults

11. Usual practices of Generator
Protection

12. overloading
• Continued over loading may increase the winding
temperature to such an extent that the insulation
will be damaged and its useful life reduced.
• Temperature rise can also be caused by failure of
cooling system. In large machines thermal
elements (thermo-couples or resistance
thermometers) are embedded in the stator slots
and cooling system.
• Electrical overcurrent protection can not sense
the failure of cooling system

13. Unbalanced loading
Un-balanced loading on generator can be due to:
 1. Unsymmetrical faults in the system near the generating
station.
 2. Mal-operation of a circuit-breaker near generating station, the
three phases not being cleared.
Negative sequence protection senses unbalanced loading of
generators.
Continued unbalanced loads, equal to or more than 10% of the rated
current cause dangerous heating of the cylindrical rotor in turbo-
generators. Salient pole rotors in hydro-generators often include
damper windings and are, therefore, much less affected by
unbalance loading (negative phase sequence currents).

14. Stator faults
The stator faults include:
1. Phase-to-earth faults
2. Phase-to-phase faults.
3. Stator inter-turn faults.
Phase to phase faults and phase inter-turn faults
are less common.
Inter turn faults are more difficult to detect.

15. stator winding faults
• Stator winding faults involve armature winding and must therefore be
cleared quickly by complete shutdown of the generator. Only opening the
stator circuit does not help since the e.m.f. is induced in the stator winding
itself. The field is, therefore, opened and de-energized by "Field
Suppression".
• With resistance earthing, protection to full (100%) winding is not
possible.
• Leakage reactance depends on square of the number of turns of stator
winding.
• Fault at some x% of the winding, will therefore reduce leakage reactance
by square of x% turns, making leakage reactance negligible . Current in
fault loop then solely depends on earthing resistor

16. Phase to earth fault
• These faults normally occur in the armature
slots. The damage at the point of fault is
directly related to the selected neutral
earthing resistor. With fault currents less than
20 A, negligible burning of the iron core
will result if the machine is tripped within few
seconds. The repair work then amounts to
changing the damaged coil without re-
stacking the core laminations.

17. Methods of connecting alternator
neutral to ground

18. • Resistor earthing is used when generator is
directly connected to station bus. In this
case the IDMT relay used in neutral is
coordinated with other over current relays
in the system
• When generator is connected via delta-Y
transformer, no coordination is required
with other relays since earth loop is
restricted to generator and transformer
primary(delta)
• For combined generator-transformer
protection system, neutral is grounded
through a voltage transformer.

19. • With Resistance earthing it is impossible to
provide protection to the whole winding, the
percentage of stator winding protected being
dependent on the value of earthing
resistance.
• Another problem with resistance earthing-
resonance may occur between generator
stator winding distribution capacitance and
Earth resistance. The limiting value is given by
fC
Rn


6
106

20. If, however, the earthing resistor is selected to pass a
much larger earth-fault current (> 200 A) severe
burning of the stator core, will take place, necessitating
re-stacking of laminations. Even when a high speed
earth-fault differential protection is used, severe
damage may be caused owing to the large time
constant of the field-circuit and the relatively long time
required to completely suppress the field flux. In the
case of high earth-fault currents it is therefore normal
practice to install a circuit breaker in the neutral of the
generator in order to break the path of earth fault
current.

21. Field suppression
• As the source of fault current is itself the stator
winding in which voltage is induced due to field
flux, the exciter must be disconnected and stored
field energy of both alternator and exciter be
dissipated in the resistance . For this purpose,
– one trip coil opens the connection of generator field
winding from exciter and simultaneously shunts it to a
resistance,
– second trip coil opens the neutral circuit breaker to
break the earth fault current path
– And third trip coil opens the exciter armature circuit
and shunts the exciter field winding across a resistance
for flux collapse

22. Differential
Protection
Star winding
Delta winding

23. Rotor
fault

24. MOTOR PROTECTION

25. Possible Faults and abnormal
conditions-Induction motor
• Supply side problems
– Faults at motor’s Terminal
– Unbalanced supply
– Single phasing
– Reduced voltage
– Phase reversal
• Motor’s inside faults
– Phase fault
– Ground fault
– Inter turn fault
• Load side Faults
– Bearing Failure
– Prolonged over loads
– Jam Rotor

26. Coordination of Over current relay
with motor starting current
• Induction motor
– Istart.= 6 to 8 IFL
• To coordinate, OC relay characteristics must lie above
starting characteristics and below thermal capability
curve of the motor

27. Phase Faults
At motor terminals, fault current is
much greater than Full load current.
Hence, an instantaneous OC relay with
high pick up can be used
Faults inside motor:
Small size motor- protected by HRC
fuse
Big size motor- OC relay is used. Relay
characteristics must lie below motor’s
thermal characteristics
High impedance ground fault:
Fault current less than IFL
Use of current balance type
protection( core balance CT) is used

28. Unbalance supply
Negative sequence current flow at
almost double frequency
Rotor offers very high resistance due
to skin effect and becomes excessively
heat up and so is the stator
Motors have certain I2t thermal
capability ;usually around a value of 40

29. Overload Protection
Overload device thermal
characteristics must lie below motor
thermal curve (I2t)
Thermal overload relays provide good
protection against short, medium and
long duration overloads but
insufficient for heavy overloads
Induction type overcurrent relay
provides good protection against
heavy overloads.