Main equipment in the power plant is Generator. It's cost is much higher than any other equipment so we will have to protect the generator from all the possible faults and errors.

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 and to ensure 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.
 Protective system cost is 4-5%of the total cost

3. SWITCHGEAR
 Switchgear is a general term covering a wide range
of equipments concerned with switching and
protection.
Eg: Circuit breaker, Isolator, Earth switch etc.

4. DESIRABLE PROTECTION
ATTRIBUTES
Reliability
 Selectivity
Speed
Simplicity
 Economics

5. PROTECTION ZONES
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1. Generator or Generator-Transformer Units
2. Transformers
3. Buses
4. Lines (transmission and distribution)
5. Utilization equipment (motors, static loads, etc.)
6. Capacitor or reactor (when separately protected)
Unit Generator-Tx zone
Bus zone
Line zone
Bus zone
Transformer zone
Transformer zone
Bus zone
Generator
~
XFMR Bus Line Bus XFMR Bus Motor
Motor zone

6. MAIN EQUIPMENT FOR
SWITCHGEAR OPERATION
 Current transformer
 Potential transformer
 Relay
 Circuit breaker

7. 7
GEConsumer&Industrial
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VP
VS
Relay
• Voltage (potential) transformers are used to isolate and step down
and accurately reproduce the scaled voltage for the protective
device or relay
• VT ratios are typically expressed as primary to secondary;
14400:120, 7200:120
• A 4160:120 VT has a “VTR” of 34.66
Voltage Transformers

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GEConsumer&Industrial
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• Current transformers are used to step primary system currents to
values usable by relays, meters, SCADA, transducers, etc.
• CT ratios are expressed as primary to secondary; 2000:5, 1200:5,
600:5, 300:5
• A 2000:5 CT has a “CTR” of 400
Current Transformers

9.  Alarm
 Act at Abnormal condition.
 Disconnect.
 Fast operation.
 Use system supply.

10. Simple electromechanical relay

11. 1. Reed relay
2. Latching relay
3. Solid state relay
4. Solid state contact relay
5. Ratchet relay
6. Coaxial relay
7. Overload protection relay
8. Forced guided contact relay
9. Buchholz relay

12. •It like a fuse
•It is a switch
•Interrupt the faulty part
•Operation

13. 1.Voltage class
2.Current rating
3.Type of circuit breaker

14. 1.Air breaker circuit breaker
2.Miniature circuit breaker
3.Air blast circuit breaker
4.SF6 circuit breaker
5.Low oil circuit breaker
6.Vaccum circuit breaker

15. SF6 CIRCUIT BREAKER

16. PLANT LAYOUT

17. GE
N
UA
T
AV
R
220 kv bus
220 kv HVCB
6.6 KV CB
NGT
10.5 KV
220 KV
GT
EXT
TR
SER
TR
415 V AC
LA
Single line Diagram of generator connection

18. GENERATOR THEORY GENERAL OVERVIEW
AND TYPICAL SYSTEM

19. 500 MW TG ON TEST BED

20. NATURE OF FAULTS IN
GENERATOR
 Insulation failure.
 Tends to deteriate with rising temp.
 Insulation failure may cause inter-turn fault, ph to ph or
earth fault.
 Bring winding in to direct contact with core plates.
 Any failure to restrict earth fault may result into core
plate damage.
 Insulation of rotor winding is also important.

21. Fault Occur In Generator
• Stator Fault
• Rotor fault
• Abnormal Running Condition
1) Unbalanced Loading
2) Over loading
3) Over Speed
4) Over Voltage
5) Failure of Primer Mover
6) Loss Of Excitation
7) Excessive vibration
8) Difference in expansion between rotating
and stationary parts
9) Loss of synchronism

22. PROTECTION APPLIED TO
GENERATOR
 Relays to detect faults outside generator
 Relays to detect faults in side generator
 Over speed protections.
 Temp measuring device for bearings, stator
winding, Oil temp.

23. EQUIPMENT GROUNDING
 Prevents shock exposure of personnel
 Provides current carrying capability for the ground-fault current
 Grounding includes design and construction of substation
ground mat and CT and VT safety grounding

24. SYSTEM GROUNDING
 Limits overvoltages
 Limits difference in electric potential through local area conducting
objects
 Several methods
 Ungrounded
 Reactance Coil Grounded
 High Z Grounded
 Low Z Grounded
 Solidly Grounded

25. SYSTEM GROUNDING
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GEConsumer&Industrial
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1. Ungrounded: There is no intentional
ground applied to the system-however
it’s grounded through natural
capacitance. Found in 2.4-15kV
systems.
2. Reactance Grounded: Total system
capacitance is cancelled by equal
inductance. This decreases the current
at the fault and limits voltage across the
arc at the fault to decrease damage.
X0 <= 10 * X1

26. SYSTEM GROUNDING
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GEConsumer&Industrial
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3. High Resistance Grounded: Limits
ground fault current to 10A-20A. Used
to limit transient overvoltages due to
arcing ground faults.
R0 <= X0C/3, X0C is capacitive zero
sequence reactance
4. Low Resistance Grounded: To limit
current to 25-400A
R0 >= 2X0

27. SYSTEM GROUNDING
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GEConsumer&Industrial
Multilin
5. Solidly Grounded: There is a
connection of transformer or generator
neutral directly to station ground.
Effectively Grounded: R0 <= X1, X0 <=
3X1, where R is the system fault
resistance

28. generator
NGT NGR RELAY
GENERATOR EARTHING

29. Stator protection:
Stator faults include the following-
i. Phase-to-earth faults
ii. Phase-to-phase faults
iii. Inter-turn faults
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

30. INTER-TURN FAULT
PROTECTION

31. 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 high
voltage CTs.
• For protection against inter-turn faults the
following protection schemes are used.
(1)Cross differential protection.
(2)Residual voltage protection.

32. mmmmmm
mmmmmm
mmmmmm
mmmmmm
mmmmmm
Loading
resistor
Over voltage relay
With time delay
STATOR EARTHFAULT RELAY

33. exciter
P.B
Field wdg
Voltage relay
ROTOR E/F RELAY

34. Rotor earth fault protection:
• DC injection method or AC injection method.
• The dc or ac voltage is impressed between the field
circuit and ground through a sensitive overvoltage
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.

35. Rotor earth fault protection
AC Injection method

36. GENERATOR PROTECTION
1 ST ROTOR E/F PROTECTION
(64R1)
D.C. INJECTION METHOD.

37. Rotor temperature alarm
• It is provided in large
generators.
• It indicates the level of
temperature but not the
actual hot spot
temperature.
• The relay measures the
temperature by
measuring the resistance
.(as shown in fig)

38. GENERATOR PROTECTION
• Abnormal Operating Conditions
The
"Wild"
Power
System
G
Exciter
Loss of Field
Loss of Field
Overexcitation
Overexcitation
Overexcitation
Open
Circuits
Loss of
Synchronism
Inadvertent
Energizing,
Pole Flashover
Abnormal
Frequency
Abnormal
Frequency
Breaker
Failure
Reverse
Power
Over
Power

39. 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.
Which may cause-
 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.

40. Differential protection of generator:

41. Differential protection using balancing resistor:

42. Modified differential protection

43. 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 does not operate.
• 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.

44. External fault back-up protection

45. External fault back up
protection:
• Over-current and earth-fault protection is
provided for back-up protection of large sized
generators protected by differential
protection.
• Induction type IDMT relay is used for this
purpose.

46. STEAM
VALVE
C.B
TRIP
Protective relay
Reverse power relay
Reverse power relay scheme

47. REVERSE POWER PROTECTION
 Failure of the prime mover of a generator set ,will
keep the set running as a synchronous
compensator, taking the necessary active power
from the net work and could be detrimental to to the
safety of the set, if maintained for any length of time.
The amount of power taken will depend on the type
of prime mover involved. It ranges from 5% to 25%.

48. m
m
46
mm
Zc ZA
A
B
C
Ia
Ib
Ic
VZC
VZA
POSITIVE SEQ
Ia
IbIc
VZC
VZA
VZA+VZC
X Y
NEGATIVE SEQUENCE
Negative phase sequence protection

49. 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 overheating
and damage to the rotor.
• This can be protected by negative sequence
current filter with over current relay.

50. Negative phase sequence protection:

51. Excite
r
FUSE T1
T2
FUS
E
TRIP
SHUNT
FILED WDG
Field failure protection

52. FIELD FAILURE PROTECTION
 Loss of generator field excitation under normal running
conditions may arise due to any of the following condition.
1. Failure of brush gear.
2. unintentional opening of the field circuit breaker.
3. Failure of AVR.
When generator on load loses it’s excitation , it starts to
operate as an induction generator, running above synchronous
speed. cylindrical rotor generators are not suited to such
operation , because they do not have damper windings able to
carry the induced currents, consequently this type of rotor will
overheat rather quickly.

53. 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 line due to
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

54. Overcurrent protection:
• Overloading of the machine causes overheating in the
stator winding.
• This can be prevented by using over-current relay with
time delay adjustment.
• 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.

55. GENERATOR PROTECTION
Name Input Protecting to
Differential protection Differential Current Stator core and winding
Stator earth fault Voltage Stator core and winding
Over current Current Stator core and winding
Over voltage Voltage Stator core and winding
Interturn short circuit Current Stator core and winding
Rotor Earth Fault Current Rotor winding
Over and under
frequency
Frequency Turbine protection
Reverse power flow Voltage and current Turbine protection
Loss of excitation Voltage and current Power System Protection
Back up protection for
lines
Voltage and current Generator protection