The document discusses generator protection systems. It introduces the basic electrical quantities used for protection like current, voltage, phase angle and frequency. Protective relays use one or more of these quantities to detect faults. The document then discusses different types of relays and circuit breakers used for protection. It describes various protection zones like generator, transformer, bus, line and utilization equipment zones. The rest of the document elaborates on different protection schemes for generators including stator protection, rotor protection, loss of excitation protection and reverse power protection.

1. GENERATOR
PROTECTION
Prepared By
Priten Vasa

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.
•Desirable Protection Attributes
• Reliability
• Selectivity
• Speed
• Simplicity

4. RELAYS & CIRCUIT BREAKERS
• TYPES OF RELAYS
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
• CIRCUIT BREAKERS TYPES BASED
ON VOLTAGE CLASS AND CURRENT
RATING
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

5. Advantages Of Digital Relay
Multifunctional
Compatibility with
digital integrated
systems
Low maintenance
(self-supervision)
Highly sensitive,
secure, and
selective
Adaptive
Highly reliable
(self-supervision)
Reduced burden
on
CTs and VTs
Programmable
Versatile
Low Cost

6. GEN
UA
T
AVR
220 kv HVCB
6.6 KV CB
NGT
10.5 KV
220 KV
GT
EXT
TR
SER TR
440 V AC
LA
Single line Diagram of generator connection

7. Protection Zones
7
GE Consumer & Industrial
Multilin
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

8. SCHEME OF GENERATOR PROTECTION
• CLASS A TRIPPING
 This is adopted for those electrical faults of Generator and Generator transformer unit
auxiliary transformer for which tripping can not be delayed.
· This leads to simultaneous tripping of
- Generator Transformer HV side CB
- Field Circuit Breaker
- LV side incomer breakers of UAT
- Auto changeover from unit to station for unit auxiliaries and tripping of turbine
• CLASS B TRIPPING
 This is adopted for all turbine faults (Mechanical) and for some Electrical faults of Generator,
Generator transformer and unit Auxiliary transformer for which it is safe to trip the turbine.
 ·Subsequently the Generator is tripped through low forward powerInterlock
 Ensures that unit does not over speed due to trapped steam in the Turbine during the
shutdown and also the loss of power to the grid From the generator is not sudden.

9. CLASS C TRIPPING
• This is adopted for all faults beyond the Generator system which can be cleared
by tripping of Generator transformer HV side CB alone.
• In this case the TG set runs with HP-LP bypass system in operation and the
Generator continues to feed the unit auxiliary load through unit auxiliary
transformers.

11. 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
Abnormal Operating Conditions

13. FAULT OCURRENCE & FAULT CLASSIFICATION
• 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
• 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

16. • Limits overvoltages
• Limits difference in electric potential through local area
conducting objects
• Several methods
i. Ungrounded
ii. Reactance Coil Grounded
iii. High Z Grounded
iv. Low Z Grounded
v. Solidly Grounded
System Grounding

17. 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
4.High Resistance Grounded: Limits ground
fault current to 10A-20A. Used to limit transient
overvoltages due to arcing ground faults.
5.Low Resistance Grounded: To limit current to
25-400A
3.Solidly Grounded: There is a connection of
transformer or generator neutral directly to
station ground

18. Protection Relay – ANSI Standards
1.Current protection functions
 ANSI 50/51 – Phase overcurrent
 ANSI 50N/51N or 50G/51G – Earth fault or sensitive earth fault
 ANSI 50BF – Breaker failure ANSI 46 -Negative sequence / unbalance ANSI 49RMS – Thermal overload
2.Directional power protection functions
• ANSI 32P – Directional active overpower
• ANSI 32Q/40 – Directional reactive overpower
3.Voltage protection functions
 ANSI 79 – Reclose the circuit breaker after tripping
4.Machine protection functions
 ANSI 37 – Phase undercurrent
 ANSI 48/51LR/14 – Locked rotor / excessive starting time
 ANSI 66 – Starts per hour
 ANSI 50V/51V – Voltage-restrained overcurrent
 ANSI 26/63 – Thermostat, Buchholz, gas, pressure, temperature detection
 ANSI 38/49T – Temperature monitoring by RTD

19. 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-
I. Arcing to core
II. Damage of conductor and insulation
Generator Stator Ground Fault Protection
87N, 51N, 59N & 27-3N
Generator Stator Phase Fault Protection
87G

20. •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.
Stator inter-turn fault protection

21. Stator Earth Fault Relay

22. Typical Example Stator Earth Fault

23. Rotor Earth Fault E/F 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.

24. LOSS OF EXCITATION
CAUSES
• Field open circuit
• Field short circuit
• AVR control Failure
• Accidental tripping of field Breaker
• Loss of supply to main Exciter
• Poor Brush contact in Exciter
• Field Current Breaker Latch Failure
• Slip ring Flash Over
Consequences
• With mechanical power remaining
intact and field excitation is loss it
would attempt to remain in
synchronization by acting as a
INDUCTION GENERATOR
• This phenomenon can be explained
by following energy flows

27. Exciter
FUSE T1
T2
FUSE
TRIP
SHUNT
FILED WDG
Field failure protection

28. Types of Differential Protection.
Differential Protection protection using Balancing Resistors

29. Biased Differential Protection

30. 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(Inverse Definite Minimum
Time) Here current is Inversely proportional to time
More the rise off current or voltage faster the relay will be operated

31. 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.

32. Over /Under Frequency Protection 81 O/U
• Causes:
Significant load addition
Sudden reduction in mechanical input
power
Loss of generation
Loss of load
•Underfrequency can cause:
Higher generator load currents
Overexcitation
Turbine blade fatigue
•Overfrequency can cause:
 Overvoltage on hydro turbines
 Abnormal frequency presents hazards
to other parts of the plant such as:
 Steam turbine vibrations and
increased stresses on blades
Reduced capacity of auxiliary
equipment
 High temperatures caused by
increased excitation current
Overexcitation of transformers

33. Out of step Protection(78)
• High peak currents and off-frequency operation can occur
when a generator losses synchronism
• Causes winding stress, high rotor iron currents, pulsating
torques and mechanical resonances
• Conventional relaying approach – analyzing variations in
apparent impedance as viewed at generator terminals
• Variation in impedance can be detected by impedance
relaying and generator separated before the completion of one
slip cycle

34. Impedance Variation Detection

35. 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

36. Inadvertent Energization Protection
(27, 50, 60, 81U, 62 and 86)
 Protects against closing of the generator breaker while
machine is not spinning / on turning gear
 Caused by operator error, breaker flash-over, control circuit
malfunction.
 Two schemes illustrated:
 Frequency supervised overcurrent
 Voltage supervised overcurrent

37. Inadvertent Energization Protection
Frequency Supervised Overcurrent

38. Frequency Supervised Overcurrent &
• Uses an underfrequency relay (81U) to enable
a sensitive instantaneous overcurrent relay
(50)
• Overcurrent relay picks up at 50% or less of
expected inadvertent energizing current.
• Frequency relay contacts must remain closed
if sensing voltage goes to zero
• Voltage balance relay (60) protects against
loss of sensing
• Time delay relay (62) protects against sudden
application of nominal voltage during
inadvertent energization,
allowing overcurrent to trip lockout relay (86)
• Lockout relay must be manually reset
 Same illustration as frequency supervised
overcurrent except 81U replaced by 27
• Undervoltage setpoint of 85% of the
lowest expected emergency operating
level
Voltage Supervised Overcurrent

39. Loss of Voltage Transformer Protection (60)
Common practice on large systems to use two
or more VTs
• One used for relays and metering
• The other used for AVR
• VTs normally fused
• Most common cause of failure is fuse failure
• Loss of VT protection blocks voltage based
protective
functions (21, 32, 40 …etc)
• Loss of VT protection measure voltage
unbalance, typical setting is 15%

40.  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 from inverted operation by using
single-element directional power relay(reverse power relay) which
senses the direction of power flow.
 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%.
Reverse Power Protection

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

42. ThankYou