This document provides an overview of generator protection. It discusses the working principles of turbo generators and their auxiliary systems. It then covers abnormal operating conditions that can cause faults and the necessity of protection. The document explains the working principles of protective devices and relays. It describes different types of generator protections like differential, interturn fault, reverse power, etc. It discusses tripping classification and provides details on specific protections like differential, interturn fault and reverse power relays.

1. GENERATOR PROTECTION
Presented by
1) MAURYA RAJKUMAR RAMASARE – 106151
2) RAHUL KUMAR VERMA - 106152
3) AMIT RANJAN - 106153
Under the guidance of,
Mr. SANJAY KUMAR –- Manager (operation)
DATE : 15/06/2022
NTPC kaniha

2. LIST OF CONTENT
 INTRODUCTION & WORKING PRINCIPLE OF TURBO GENERATOR
 AUXILIARIES OF GENERATOR FOR OPERATION
 ABNORMAL OPERATING CONDITIONS
 POSSIBLE FAULTS DUE TO ABNORMAL OPERATING CONDITIONS
 WORKING PRINCIPLE OF PROTECTION
 BASIC PROPERTIES OF PROTECTIVE DEVICES
 TYPES OF PROTECTIONS OF ALTERNATOR
 CONCLUSION
 REFERENCES

3. INTRODUCTION OF TURBO GENERATOR :
A electrical generator is a device that converts motive power from
turbine to electrical power by coupling to the same shaft.
The turbo Generator works on the principle of Electromagnetic induction.

4. AUXILIARIES REQUIRED FOR OPERATION OF GENERATOR
1) Stator water system :
 Stator water cooling is a closed loop system.
 The stator water cooler is shell and tube type heat
exchanger
 DM water is flows through shell.
 Thermal capacity of water as liquid is high. Pure water
has high electrical resistance and low conductivity.
 Polishing unit (mixed bed ion exchanger) is there to
maintain conductivity of stator water to desired level

5. 2) Seal oil system :-
 pressurized seal oil is provided on rotor shaft to prevent hydrogen escape at the shaft.
 Oil pressure is kept higher than the gas pressure.
 There are one AC seal oil pp. and one DC seal oil pp. which feed oil to the seal through
cooler and filter.
3) Gas system(H2) :-
 H2 has High heat transfer co-efficient capacity.
 Low density, so that low windage-friction loss.
 H2 is lightest gas with 0.09 gm/liter while air’s 1.3 gm/liter
 Thermal capacity of H2 as gas is high.
 Thermal conductivity of H2 is 5 times that of air.
 It Provides cooling to  ROTOR slots and winding, STATOR core , and both end parts.

6. GENERATOR CAPABILITY CURVE ANALYSIS
 “δ” = load angle
 “φ” = power factor angle
 Active Power (P) = VI COS∅ =
𝐸𝑉𝑠𝑖𝑛𝛿
𝑋𝑠
, Reactive Power (Q) = VI SIN∅ =
𝐸𝑉𝑐𝑜𝑠𝛿
𝑋𝑠
-
𝑉^2
𝑋𝑠
 Stable zone when dP/dδ is positive (i.e., up to δ=90°).
 Generator Excitation is controlled by A.V.R,
 Load angle limiter limits the negative reactive MVAR as
shown by the line EF.
 Rotor current limiter limits the rotor current by arc CG.
 Stator current limiter limits the stator current as arc FC.
 All these limiters ensure that the Generator, at any
point of time, operates within the framework EFCG for
electrical as well as thermal stability of the machine.

8. NECESSITY OF PROTECTION
The word 'protection' is generally used to describe the whole concept of protecting a power
system.
If the generator gets damage then the entire power generation, transmission, utilization part
will get disturb for longer period of time. Because there is no alternatives for generator.
Three main effects of fault, that state the necessity of protection:
1) It is likely to cause the individual generators in a power station,
or groups of generators in different stations, to lose synchronism
and fall out of step with consequent splitting of the system;
2) A risk of damage to the affected plant; and
3) A risk of damage to healthy plant.

9. Abnormal operating conditions :-
 Unsafe act and unsafe conditions
 Overcurrent or overloading
 unbalanced load (negative sequence)
 overtemperature
 Over voltage and under voltage
 Over excitation and under excitation
 Over frequency and Under frequency (Asynchronous running )
 Over-fluxing (V/F ratio not maintained)
 generator motoring (failure of steam supply)
 failures in the machine control system (i.e. AVR or governor failure)
 failures in the machine cooling system
 failures in the primary equipment (i.e. breaker flashover)
 open phase (sudden load gets disconnected)

10. Possible faults in generator due abnormal conditions
 Stator Earth Faults
 Stator winding interturn fault.
 Stator winding short circuited with other phase.
 Rotor Earth Faults
 Rotor Interturn faults
 Unit transformer faults
 External faults (i.e. Mechanical disturbance, thermal limit, overspeed)

11. WORKING PRINCIPLE OF PROTECTION
 A protective relay is a SWITCHGEAR device which
sense the electrical or relevant parameter, if the fault
level is greater than the set value, then the relay coil will
initiates the operation of the circuit breaker to isolate the
defective element from the rest of the system.
 They are compact and self-contained devices which can
detect abnormal conditions also controlled from control
room.
 The protective relaying scheme mainly includes as,
current transformers, potential transformers, relays,
trip circuits, battery, circuit breaker, auxiliary switch
content and in modern relay electronic switches, which
can be used for controlling and providing time delay.

12. BASIC PROPERTIES AND REQUIRMENTS OF PROTECTIVE
DEVICES :-
 Speed of fault detection to trip the circuit
 Selectivity of fault
 Sensitivity should be high
 Higher Reliability
 Simplicity
 Economy

13. TYPES OF PROTECTIONS
BASICALLY THERE ARE TWO MAIN CATOGORIES OF PROTECTION ARE USED,
1) Main Protection
2) Back-up Protection
 The main protection is the first line protection which provides quick-action and selective clearing of
a fault within the boundary of the circuit section. The main protection is provided in each section of
an electrical installation.
 Some times, Failure of the primary protection occurs because of the failure of DC supply circuit,
CT/PT m/m error, stuck in circuit breaker.
 The backup protection provides the back up to the main protection whenever it fails in operation or
its cut out for repairs. The backup protection is essential for the proper working of the electrical
system

14. DIFFERENT PROTECTION SCHEMES
The Generator Protection at (NTPC) mainly include following protections as IEEE standard
1) Differential protection (87G)
2) Stator Earth fault protection(64G)
3) Stator inter turn protection (95)
4) Loss of excitation (40G)
5) Under & over Frequency protection (81G)
6) Reverse Power Relay (32R)
7) Under & over voltage (59G)
8) Volt per hertz (24G)
9) Negative sequence overcurrent (46)
10) Loss of synchonism (78G)
11) Generator Overload protection(49G1)  alarm stage-105%
12) MAIN OPERATING TRIP RELAY (186G/286G/386G/486G)

15. TRIPPING CLASSIFICATION OF GENERATOR
 Tripping classification of generator is based on the need of isolation of generator on the basis of type of
fault.
 Generator electrical Trips are mainly classified as Class A, Class B, Class C
Class A trip (Simultaneous tripping):- it Provides fastest isolation of generator from the system. This tripping
mode is used for all internal generator faults and severe abnormalities in the generator protection zone.
Isolation is achieved by simultaneously tripping the generator breakers and field breaker, shutting down the prime mover
by closing the turbine valves and transferring unit auxiliary loads to standby power.
Class-A1 trip:-The protections for the faults in the generator
which need immediate isolation are grouped under this class-A1.
1. Generator differential protection
2. Stator earth fault protection
3. Generator over voltage protection
4. Starting over current protection
5. Generator inter-turn protection
6. Generator negative phase sequence
Class-A2 trip:- The protections for the faults in GT and
unit transformer which need immediate
isolation are grouped under this class-A2.
1. Back up impedance protection of generator
2. Differential protection of GT/UT
3. Over fluxing protection of generator
4. Trip of GT
5. Fire protection of GT
6. Restricted earth fault

16. Class-B Trip:-
The protections for the faults in the
generator which do not need immediate isolation are
grouped under this class-B. This results in tripping of
turbine first. (if the fault is in steam cycle, under that
condition turbine will be tripped first while generator
will continue to run utilizing trapped steam till reverse
power relay operates)
GCB is tripped when the reverse power flows.
1. Stator water flow very low
2. Turbine protection
3. Turbine hand trip
4. Loss of excitation
5. Generator reverse power
Class-C Trip:-
when disturbance arises on grid side,
generator is isolated from the grid by opening the
generator transformer HV side breaker. in this case
only generator is isolated from the grid but generator
continues to feed station loads,
1. Unbalance or negative sequence protection
2. Back up impedance protection
3. Under frequency
4. Over frequency

17. GENERATOR DIFFERENTIAL
PROTECTION(87G1)
 For the differential protection one
group of CT are located at neutral
side and another group of CT located
at generator phase side.
 Under normal operating condition
torque produced by restraining coil
(Tr) is greater than operating coil
torque(To) so relay remains
inoperative (Tr > To).
 When internal fault occur operating
torque exceeds restraining torque
causes trip circuit contacts close to
open circuit breaker (Tr < To).
Set value:- 125volt

18. STATOR INTERTURN FAULT
PROTECTION(95G1)
 The primaries of CT’s are inserted in this
parallel path and the secondary’s are cross
connected as shown in fig.
 Under the normal operating condition current
flows through both path is same, so no
current will flow through the relay operating
coil.
 During the inter turn fault the phase winding
currents flowing through two parallel paths
will be different and operating coil current is
proportional to the difference of current
which will close the trip circuit and isolate
the machine from the power system.
Set value  2volt (5sec),
10volt (1sec)

19. REVERSE POWER PROTECTION
(32G1)
 Whenever the Turbine output is lesser
than the generator no load(core loss) and
copper losses, then generator starts
consuming power from grid and work as
electrical motor to drive rotor and coupled
turbine.
 In this protection direction of overcurrent
relay is kept as reverse direction and rest
of principle of relay and circuit breaker
remains same.
Set value: - 0.5% (10 sec)

20. LOCAL BREAKER BACKUP PROTECTION (LBB)
 Due to some external fault on the circuit breaker
such as trip circuit loose contact, delayed in CB
operation, internal mechanism issue, and CT/PT
measurement error etc.
 Under such condition CB unable to isolate
generator under fault condition. and Failure of the
breaker leads to loss of protection, single
phasing, negative phase sequencing etc.
 LBB protection Monitors both trip and closing
position of the generator’s breaker. The LBB
protection operates when the breaker trip is
initiated after a suitable time interval if
confirmation of the breaker tripping from three
poles is not received. LBB energize the grid
circuit breaker associated with the generator
breaker. Once the LBB protection opted, then the
entire station is called as blackout.

21. STATOR EARTH FAULT
PROTECTION(64G1)
 Short circuits between the stator winding in the slots or core
are the most common electrical fault in Generators.
 Interturn faults, which is normally difficult to detect, will quickly
convert into a ground fault and it will tripped by the stator
Earth fault protection.
 it is a common practice to ground the generator neutral
through a Grounding Transformer called Neutral Grounding
Transformer NGT having a loading resistor across its
secondary.
 This method of Earthing is called High Impedance earthing
where the Earth fault current is limited to 5-10 Amps.
GENERATOR 95%STATOR E/F PROTECTION (64G2)
Set value 7.5volt(1.5sec)
GENERATOR 100%STATOR E/F PROTECTION (64G1) 20mA
Alarm-20ohm (10sec) trip-10ohm (1sec)

22. OVER SPEED PROTECTION
 As the rotor speed increases, the force from the
plunger increases on the spring. Once the
centrifugal force increases from the speed of 110%
rpm, the plunger overcomes the spring force
causing the plunger to came outward.
 A stationary lever, set with a relatively tight
clearance, is situated such that when the plunger
moves out, the lever is struck.
 The lever is integral with the emergency
mechanical trip device. When the mechanical trip
is actuated, the hydraulic oil is dumped to the
drain, which results in the immediate closing of the
valve rack and trip valve.
ALARM SPEED : 107% (RPM)
TRIP SPEED : 110% (RPM)

23. NEGATIVE SEQUENCE OR CURRENT UNBALANCE
PROTECTION(46G)
 Negative-sequence quantities has a rotation
opposite to the sequence of power system. it will
reverse the rotating stator current and induces
double frequency currents in rotor structures. The
resulting heating can damage the rotor very quickly.
 With the advancement of solid-state switches and
microprocessor technology, relaying is now
available to provide generator protection over a full
range of unbalance conditions.
Set value:- 7.5% of Im (alarm=6sec)
high setting -60% of Im (3sec)

24. ROTOR EARTH FAULT PROTECTION
 The rotor earth fault may be caused due to insulation
failure of winding or inter-turn fault may leads to local
heat and gets contacted with rotor body.
 Here, The one terminal of the sensitive over voltage
relay is connected to the exciter, and the other
terminal is connected to the negative terminal of the
DC source. The positive terminal of the DC source is
grounded.
 When the earth fault occurs, the fault current will
complete the circuit path, and the fault is sensed by
the over-voltage relay.
 This relay will send the tripping command to the
breaker.

25. LOSS OF FIELD OR LOSS OF EXCITATION
PROTECTION(40G1)
Generator failure due to LOE occurs nearly 70% of all
generator failures.
Loss of field or loss of excitation results in loss of
synchronism between rotor flux & stator flux but the
generator is still connected with the grid. The generator
works as an induction generator at higher speed and draws
reactive power from the grid. This will result in the flow of
slip frequency currents in the rotor body as well as severe
torque oscillations in the rotor shaft.
As the rotor is not designed to sustain such currents or to
withstand the high alternating torques which results in rotor
overheating, coupling slippage and even rotor failure.
Loss excitation protection operates under many
circumstances such as diode failure, rotor winding short
circuit, AVR failure, etc.
Negative Offset MHO relay is used to protect under such
condition.

26. OPERATING REGION IN R-X PLAN
 in order to detect the LOE, the protection scheme applies
the generator terminal voltage, current, active power or
reactive power output as the input value and calculates the
generator characteristic values to determine the LOE fault.
 In normal operation condition, the generator generates
active and reactive power to the system which means both
R and X are positive. and the terminal impedance is located
in the first quadrant in R-X plane.
 When the excitation is lost, the generator starts drawing
reactive power from the system and X becomes negative
from the LOE relay point of view. As a result, the terminal
impedance loci in R-X plane moves to the forth quadrant
and the endpoint of terminal impedance ranges between the
sub-transient reactance and synchronous d-axis reactance.
The endpoints depend on the initial load.
Char 1/2 ->10sec without under volt
Char 1/2 ->1.5sec with excitation under volt

27. POLE SLIP PROTECTION or OUT OF STEP
PROTECTION (98G)
 Under some extreme faulty condition such as heavy
line fault, lightning impulse, switching transient
generator may loss its synchronism and Pole
slipping occurs.
 Pole slipping is nothing but a generator’s rotor
produces weak magnetic field or low excitation for
the given input or the physical slip occurs in the
rotor’s shaft.
 It can cause severe mechanical stresses within the
rotor and experiences a sudden physical
disturbance and electrical shift in position relative to
the stator generator rotor angles, large swings in
power flows, large fluctuations of voltages and
currents, and eventually lead to a loss of
synchronism. this will putt winding at risk, causing
shaft damage.
 When the loss of synchronism occurs, then
generator impedance starts increasing. The relay
set by the generator impedance. When the
impedance increases to threshold value then the
relay starts operating.

28. PROTECTION AGAINST VIBRATIONS:-
Vibration detector may be mounted on one of the bearing pedestals in the case of
a horizontal shaft generating set, It may be set to trip the machine or initiate an
alarm when the radial deflections of certain duration exceed a per-determined
value.
OVER VOLTAGE PROTECTION(59G1):-
Generally, overvoltage occurs due to sudden load through off, AVR malfunctioning,
power transformer taps changer failure, lightning strike on the transmission line,
turbine over speed etc.
The power system must be isolated when the system voltage greater than set
limit. Severe overvoltage causes the winding or electrical insulation failure, over
fluxing (v/f), transformer’s core saturation etc.
The over voltage protection is provided with an over voltage relay which has
two units one instantaneous relay set for pick up at about 130-150% of rated
voltage and the another IDMT relay set for pick up at about 110% of rated voltage.
The over voltage protection can be considered as a backup to the
Volts-per-Hertz protection (Over fluxing).
Trip command: 116v(2sec) , 126v(1.5sec)

29. OVER FLUX or OVER EXCITATION PROTECTION
(99G1):
Over fluxing (105-110%) increases core loss because of increase in
core temperatures due to hysterics & eddy currents loss. If they
continue for Long term, which decrease the life of the stator
insulation. Over fluxing normally can be caused by over speed of the
turbine or over excitation during Off-line condition(no load), and load
rejection or AVR mal-functioning during On-line condition.
The generator EMF is depending on the Excitation of field winding
and the frequency is depending on the speed of the alternator.
The relay is designed to monitor the ratio of EMF(V) to frequency.
Increasing the ratio of v/f is called over fluxing.
Set Alarm at 1.1 times of flux (20sec)
High set value at 1.4 times of flux (3sec)

30. BEARING OVER HEATING PROTECTION:-
Bearing over heating can be detected by a relay which is actuated by a thermometer-type bulb
inserted in a hole of bearing, or by a resistance-temperature-detector relay, such as used for
stator over heating protection, with the detector embedded in the bearing.
STATOR FRAME OVER HEATING PROTECTION:-
For the protection of the turbo generator against any possible fire accident multiple fire detector
relays are provided on either side of the stator winding The set of contacts will close when the
temperature surrounding the fire relay exceeds 80° C.
When the relay temperature exceeds 100° C, These contacts are connected to CO2 fire
extinguishing system.
ABNORMAL FREQUENCY PROTECTION(81G1):-
The abnormal over frequency on the machine may be due to improper speed control adjustment,
severe grid disturbance or sudden load thrown off.
If the unit trips due to abnormal frequency protection then change the governor speed until
machine reaches full speed. Even after 2 to 3 attempts the machine is running at lower speed,
means the governor of the turbine is faulty.
Under frequency trip 48Hz (1sec), 48.5Hz (2sec) Over frequency trip 52Hz (1sec), 51.5 Hz (2sec)

31. CONCLUSION
A generator is the most important and costly equipment in a power
system or plant. It is subjected to more number of troubles than any other equipment.
The basic function of protection applied to generators is therefore to reduce the outage
period to a minimum by rapid discriminative clearance of faults. To recover generator
from this kind of faults generator protections schemes of plant is designed in such a
way that grouping of generator protection are made respect to plants need, the
protection of complete unit and the stability of the system due to disturbance, in a
generator should be considered in addition to protection of the generator itself. and by
considering convenience of operation of every protection with ease by all employees
and engineers at NTPC.

32. REFERENCES
1) NTPC staff PPTs of generator operation and maintenance and its auxiliaries.
2) Operation engineer’s Hand Book Talcher kaniha, 3000MW units TSTPS/NTPC Ltd
3) Turbine overspeed trip protection by CHARLES R. RUTAN
Senior Engineering Advisor Lyondell/Equistar Chemicals, LP Alvin, Texas
4) The art & science of protective relaying by C. RUSSELL MASON
5) operation & maintenance manual for 500 mw turbo-generator with cooling system
by bharat heavy electricals limited(BHEL) haridwar.
6) Investigation on Generator Loss of Excitation Protection in Generator Protection Coordination
Master Thesis Examiner in KTH: Mehrdad Ghandhari Supervisor in KTH: Yuwa Chompoobutrgool
. supervisor in ABB: Jianpeng Wang
7) IEEE Standard C37.102, IEEE Guide for AC Generator Protection.
8) Case Study on Generator Protection in N.T.P.S. (Eklahare) by Diksha Amrutkar, Rajashree Chavan, .
. Sushama Avhad
9) google images.