This document discusses generator protection systems. It describes how differential protection uses CTs to detect faults by measuring differences in current. Modified differential protection is discussed as a way to protect the full winding. Other protections mentioned include restricted earth fault protection, stator protection against phase and interturn faults, rotor earth fault protection using dc injection, loss of excitation detection, overload protection using temperature sensors, and negative sequence protection to prevent rotor overheating. The conclusion emphasizes that protective relays act after a fault occurs to ensure safety and equipment protection.

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EE DEPARMENT SEMINAR April 23, 2012

2. Introduction:
In a generating station the generator and transformer
are the most expensive equipments and hence it is
desirable to employ a protective system to isolate the
faulty equipment as quickly as possible to keep the
healthy section in normal operation aanndd ttoo eennssuurree
uninterruptable power supply.
The basic electrical quantities those are likely to
change during abnormal fault conditions are current,
voltage, phase angle and frequency . Protective relays
utilizes one or more of these quantities to detect
abnormal conditions in a power system.

3. Generator protection:
Stator protection
Differential
Restricted eeaarrtthh ffaauulltt
Negative sequence current
Loss of load

4. Cont..
Rotor earth fault
Over speed
OOvveerr vvoollttaaggee
Loss of field
Back –up over current

5. Differential protection:
CTs are provided at each end of the generator winding
which is to be protected.(as shown in fig.)
When there is no fault the differential current (I1-I2)
through the relay is zero. So the relay wwiillll nnoott ooppeerraattee..
When the fault occurs the balance is disturbed and
differential current (I1-I2) flows through the operating
coil of the relay causing relay operation and the trip
circuit of the circuit breaker is closed.

6. Differential protection of generator:

7. Differential protection using balancing resistor:

8. Biased differential protection

9. Modified differential protection:
Generally protection is made for 80 to 85% of the
winding.
If any fault occurs near the neutral point then the fault
current is very small and relay ddooeess nnoott ooppeerraattee..
Modified differential protection scheme is used to over
come this.
Two phase elements (PC and PA) and balancing
resistor(BR) is connected in star and the earth
relay(ER) is connected between the star point and
neutral pilot wire.

10. Modified differential protection

11. Restricted or balanced earth fault
protection:
In case of small size generators star point is not
available because it is made inside the generator and
grounded through some low resistance then
percentage differential relay for ggrroouunndd ffaauulltt iiss
provided and is known as restricted earth fault
protection.
This scheme can be used only for ground faults but not
for phase faults.

12. Restricted or Balanced earth fault protection

13. Stator protection:
Stator faults include the following-i.
Phase-to-earth faults
ii. Phase-to-phase faults
iii. IInntteerr-ttuurrnn ffaauullttss
From these phase faults and inter turn faults are less
common ,these usually develop into an earth faults.
This causes-
• Arcing to core
• Damage of conductor and insulation

14. Stator inter-turn fault protection:
Inter-turn fault on the same phase of the stator
winding cannot be detected by transverse differential
protection as it does not disturb the balance between
the currents in neutral and hhiigghh vvoollttaaggee CCTTss..
For protection against inter-turn faults the following
protection schemes are used.
(1)Cross differential protection.
(2)Residual voltage protection.

15. Cross differential protection:
Used in case of hydro-electric
generator having
double winding armature.
As shown iinn ffiigguurree rreellaayy
Rc provides protection
against phase to ground
and phase to phase fault.
The relay R1 provides
protection against inter-turn
faults.

16. Residual voltage protection:
These method is used in
case of alternators those
don’t have parallel stator
windings.
During normal operation
VRES=VRN+VBN+VYN=0.
In case of fault VRES is not
zero and this residual
voltage operates the relay.

17. Rotor faults:
Faults in the rotor circuit may be either earth faults or
between the turns of the field winding .
Field circuits are normally operated un-earthed. So a
single earth fault will not affect iittss ooppeerraattiioonn..
But when a second fault arises then field winding is
short circuited and produce unsymmetrical field
system which leads to unbalanced forces on rotor and
results in excess pressure and bearing and shaft
distortion.

18. Rotor earth fault protection:
The rotor earth fault protection is done by “dc
injection method or ac injection method”.
The dc or ac voltage is impressed between the field
circuit and ground through a sensitive oovveerrvvoollttaaggee
relay and current limiting resistor or capacitor(in case
of ac).
But dc source is generally used as over-current relay in
case of dc is more sensitive than ac.
A single earth fault in rotor circuit will complete the
path and the fault is sensed by the relay.

19. Rotor earth fault protection

20. Rotor temperature alarm
It is provided in large
generators.
It indicates the level of
temperature bbuutt nnoott tthhee
actual hot spot
temperature.
The relay measures the
temperature by
measuring the resistance
.(as shown in fig)

21. Loss of excitation protection:
When the excitation of generator is lost it operate as a
Induction generator. It derives excitation from the
system and supply power at leading power factor.
WWhhiicchh mmaayy ccaauussee-
A fall in voltage so loss of synchronism system
instability.
Over heating of rotor due to induction current on it.
A protection having MHO characteristic
is used to detect loss of field.

22. Over voltage protection:
Overvoltage protection is required in case of hydro-electric
or gas turbine generators but not in case of
turbo generators.
Over voltage may be caused due to-
Transient over voltage in the transmission lliinnee dduuee ttoo
lightening.
Defective operation of the voltage regulator.
Sudden loss of load due to line tripping.
The protection is provided with an over voltage relay.
It is usually of induction pattern with an IDMT
Characteristic

23. Overload protection:
Overloading of the machine causes overheating in the
stator winding.
This can be prevented by using over-current relay with
time ddeellaayy aaddjjuussttmmeenntt..
But overheating not only depends on over-current but
also the failure of the cooling system in the generator.
So temperature detector coils such as thermistors or
thermocouples are used at various points in stator
winding for indication of the temperature.

24. Reverse power protection:
When prime-mover fails machine starts motoring and
draws electrical power from the system and this is
known as inverted operation .
The generator can be protected ffrroomm iinnvveerrtteedd
operation by using single-element directional power
relay(reverse power relay) which senses the direction
of power flow.

25. Negative phase sequence
protection:
Unbalance may cause due to single phase fault or
unbalanced loading and it gives rise to negative
sequence current .
This current in rotor causes rotor oovveerrhheeaattiinngg aanndd
damage to the rotor.
This can be protected by negative sequence current
filter with over current relay.

26. Negative phase sequence protection:

27. Automatic field suppression and
use of neutral circuit breaker:
In case of a fault in the generator and though the
circuit breaker is tripped ,the fault continues to fed as
long as excitation will exist because emf is induced in
the ggeenneerraattoorr iittsseellff..
Hence all protection system not only trip the generator
circuit breaker but also trip the “automatic field
discharge switch “.

28. Automatic field suppression and use of neutral
circuit breaker

29. External fault back up protection:
Over-current and earth-fault protection is
provided for back-up protection of large
sized generators protected bbyy ddiiffffeerreennttiiaall
protection.
Induction type IDMT relay is used for this
purpose.

30. External fault back-up protection

31. Conclusion:
Protective relays are used to detect electrical faults and
to alarm, disconnects or shutdown the faulted
apparatus to provide personnel safety and equipment
pprrootteeccttiioonn..
A protective relay does not prevent the appearance of
faults rather takes action only after a fault has occurred
in the system.

32. References:
Electrical power system
by C.L.Wadhwa
Electrical power
bbyy JJ..BB..GGuuppttaa
www.wikipedia.com

33. Thank
yyoouu

34. Any Queries
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