Generator rotor earth fault protection

EXPERIMENTAL PANEL OF GENERATOR ROTOR
EARTH Fault PROTECTION
A Project Report
Submitted in partial fulfilment of the requirements for the degree of
BACHELOR OF TECHNOLOGY (B. TECH)
In
ELECTRICAL ENGINEERING
M & V PATEL DEPARTMENT OF ELECTRICAL ENGINEERING
C. S. PATEL INSTITUTE OF TECHNOLOGY, CHANGA
ANAND, GUJARAT
Submitted By:
PARTH UKANI (12EE112)
KEYUR VAGHELA (12EE115)
ADITYA PARMAR (12EE141)
DHRUMIL SHIROYA (12EE142)
Guided By:
Mr Nilay Patel
Assistant Professor
Mr Kalpesh Patel
Lab. Tech.

M&V Patel Department of Electrical Engineering
C.S. Patel Institute of Technology
CHARUSAT, Changa
Certificate
This is to certify that the project report entitled “Experimental panel of generator
rotor earth fault protection” being submitted by Parth Ukani(12EE112), Keyur
Vaghela(12EE115), Aditya Parmar(12EE141), Dhrumil Shiroya(12EE142)to the
M&V Patel Department of Electrical Engineering for the award of the Degree of
Bachelor of Technology is a record of the work carried out by them under my guidance
and supervision.
Mr Nilay Patel
Assistant Professor
Mr Kalpesh Patel
Lab. Tech.
M& V Patel Department of Electrical
Engineering,
Charusat, Changa
Dr. Praghnesh Bhatt
Professor and Head
M& V Patel Department of Electrical
Engineering,
Charusat, Changa

Acknowledgement
Every project is an outcome of culmination of efforts of many. There are many teachers,
friends, work shop and electrical maintenance department and well-wishers who have
contributed to our work directly or indirectly. Although it is not permissible to name and
thank them all individually, we must make a sincere effort to name a few who have taken
personal interest and contributed to the successful completion of the project.
We pay our profound gratefulness to Mr. Nilay Patel for giving us an opportunity to carry
out the project work. We must thank him for sparing his valuable time from his busy
schedule; who was not only our source of inspiration but a guardian who looked after us in
all the frustration and disappointments.
We specially thank Mr. Kalpesh Patel for guiding and helping us in project work.
It gives us immense pleasure to express our deep sense of gratitude to Prof. Praghnesh
Bhatt (HOD, Department of Electrical Engineering, C.S. Patel Institute of Technology
Charusat, Changa) for providing us such an opportunity to work till late nights in the
campus and complete the project work by time.
PARTH UKANI (12EE112) Sign: ____________________
KEYUR VAGHELA (12EE115) Sign: ____________________
ADITYA PARMAR (12EE141) Sign: ____________________
DHRUMIL SHIROYA (12EE142) Sign: ____________________

Abstract
Generator is one of the most expensive and important equipment in power system network.
The generator must have good practice scheme because it demands 24 hours operation daily
otherwise any halt in operation of generator may overload rest of the system and cause power
oscillations and may cause loss of power system stability. On the other hand, failure to clear
a fault in very short time may cause damage to generator and whole system which may lead
to blackout. There are various faults occurring on different part of generator such as stator
earth fault, rotor earth fault, fault due to failure of excitation, prime mover, overload etc. The
various protective schemes are also employed to protect generator against the above faults.
The ’Rotor’ is one of the major parts of the generator and it requires good protective scheme
because faults in rotor increases the current in winding and may cause unbalance the air-gap
fluxes so that there will be serious vibrations, local heating which may lead to serious
damage.
The “GENERATOR ROTOR EARTH FAULT PROTECTION SCHEME” employed for
rotor earth fault. In this scheme during first rotor earth fault, there will be small fault current
and it does not require tripping of the generator so scheme gives only alarm. During the
second rotor earth fault, current will bypass the major part of the field winding so generator
must be tripped. To design and develop an actual protection scheme which will provide the
solution of the problems faced during the protection of Generator-rotor against earth fault.

Index
Chapter Title Page No.
1 Introduction 1
1.1 Types of Faults 1
1.1.1 Symmetrical Faults 1
1.1.2 Asymmetrical Faults 2
1.2 Causes of Faults 3
1.2.1 L-L-L-G Fault 3
1.2.2 L-L Fault 3
1.2.3 L-G Fault 3
1.3 Damage Cause by Fault 4
1.3.1 Thermal Damage 4
1.3.2 Electrodynamics Damage 4
1.4 The Functional Requirement of Protection System 5
2 Synchronous Generator 7
2.1 Introduction 7
2.2 Types of Synchronous Machine 8
2.3 Construction of Synchronous Machine 8
2.3.1 Stator Core 9
2.3.2 Rotor Core 9
3 Generator Protection 10
3.1 Introduction 10
3.2 Multi-CT Differential Protection for Generators 10
3.3 Stator Ground Fault Protection 11
3.4 High Impedance Grounding 11
3.5 Low Impedance Grounding 11

3.6 Loss of Excitation Protection 11
3.7 Protection against Unbalanced Operation 13
3.8 Overvoltage Protection 13
3.9 V/F Protection 13
3.10 Out-of-step Protection 14
3.11 A Note on Numerical Protection 14
3.12 Schemes of Generator Protection 15
4 Generator Rotor Earth Fault Protection 16
4.1 Introduction 16
4.2 Protective Schemes 16
5 Experimental Panel of Generator Rotor Earth Fault Protection 21
5.1 Introduction 21
5.2 Operation 21
5.3 Control Circuit 22
6 Equipment’s, Tools and Materials Used for Laboratory Simulation 23
6.1 Equipments 24
6.2 Description of Equipments Used for Laboratory Simulation 24
6.2.1 Semaphore Indicator 24
6.2.2 Contactor 24
6.2.3 Relay 24
6.2.4 Push Button 24
6.3 Tools 24
6.4 Materials Used In Laboratory Simulation 25
7 Conclusion
8 Bibliography

1
CHAPTER: - 1
INTRODUCTION
The modern electrical power systems to provide the demands that are spread over large areas
containing major component like generators, transmission and distribution lines, induction
motors etc. It is evident that in spite of all necessary precautions taken in the design and
installation of such systems; they do encounter abnormal conditions or faults. Some of them,
like short circuits may prove to be extremely damaging for not only the faulty components
but to the neighbouring components and to the overall power system network. It is of greatest
importance to limit the damage to a minimum by quickly isolating the faulty section without
disturbing the operation of the rest of the system. So power system has greatest importance
for better reliability of the system.
1.1. Types of Faults
Faults: The flow of current to the undesired path and abnormal stoppage of current are
termed as faults.
These Faults are classified as-
A) Symmetrical Fault
B) Asymmetrical fault
1.1.1. Symmetrical Fault
a) Triple line fault: -

2
b) Triple line to ground Fault: -
1.1.2. Asymmetrical Fault
a) Line to ground fault: -
b) Double line to ground fault: -

3
c) Double line fault: -
1.2. Cause of Fault
1.2.1 L-L-L-G Fault: -
Triple line to ground fault can occur in case of switching on of C.B. when the earth
switch is kept on.
1.2.2 L – L Fault: -
Two phases can be bridged together either in the machines on in transformer because
of failure of insulation between phases, particularly conductor of different phases are in the
same slot of stator of machine. In transmission lines, two phase wire may be get shortage
together by birds, kites strings or tree limbs.
Moreover, in monsoon, the two phase conductor may swing due to winds and storms.
Also dielectric strength of air reduces in monsoon when distance between these conductor
reduce due to swinging, a power arc may occur between them causing a line to line fault.
1.2.3 L-G Fault: -
A link to ground fault is the commonest fault and can occur because of flashover
across the line insulators or because of failure of line insulator, due to lighting or switching
over voltage or due to defective insulator.
Line to ground fault can occur in machine and transformer too. Abnormal stoppage of
current can occur due to open conductor or as result of voltage breakdown at equipment due
to the occurrence of fault the first kind some part of the system.

4
1.3 Damage Caused by Faults
The damage cause by faults are of two kinds in electrical equipment are as under,
(1) Thermal damage
(2) Electrodynamics damage
1.3.1 Thermal Damage
Fault current ranging from approximately two times about 8 to 10 times at rated full
load current of the equipment to be protected within conductor and insulation around it.
Equipment temp thus rich Exide the temp withstand value of the insulation used. When
insulation is thermally breakdown resulting In to another fault if remedial steps are not taken.
This is known as thermal breakdown of insulation.
1.3.2 Electrodynamic Damage
When fault current exceeds 8 to 10 times the full load rating of the equipment, the
repealing forces generated due to this large current would reshapes and destruct the whole
equipment structurally. The instantaneous tripping feature is required to be used avoid such
electrodynamics damage.
Element Percentage of fault
Overhead transmission lines 45-55%
Underground cables 8-12%
Switchgears 13-17%
Power transformers 10-14%
C.T.s & P.T.s 1-3%
Control circuits equipment 2-4%
Miscellaneous 7-9%
Table 1.1 Probability occurrence of fault in different element of a system.
Types of fault Percentage of occurrence
Line to ground fault 80-90%
Double line fault 6-10%
Double line to ground fault 3-7%
Triple line faults 2% or less
Table 1.2 Frequency of different type of fault

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1.4 The Functional Requirement of a Protection System
1) Reliability:
Reliability is a product of two factors; dependability and security. For relay
system protection, dependability is defined as the ability to trip for a fault within its
protective zone while security is the ability to refrain tripping when there is no fault in the
protective zone.
While not practical to use, it could be of interest to illustrate the concepts by looking at the
two extremes; 100% dependability and security. 100% dependability would be achieved by a
protection system that is in constantly tripped state, hence there is no possibility that there
would be a fault that would not to be detected. 100% security would be achieved by disabling
the protection system entirely so that it could not trip. From this we can see that while high
dependability and high security are desirable, they will both have to be less than 100%.
Generally, an increase in dependability will decrease security, and vice versa. However,
measures to increase dependability may not penalize security to an equal degree and the aim
of a protection system design is to be find the optimum combination of the two factors in
order to provide adequate reliability of the protection system.

6
2) Selectivity:
Security is the ability not trip when not called for. To put a number on security
is not as easy as for dependability. A simple method would be to compare the number of false
trips for faults external to the protected zone as compared to the number of external faults.
However, this does not consider other phenomena; false trips due to relay failure, trips on
stable power swings, inrush currents or other phenomena that are not necessarily classified as
power system faults. Even an ‘external fault’ is not readily defined as it depends on what
extent of the adjacent power system is included in the fault count.
A practical approach to determined security was made by the ‘Transmission
protective relay System Performance Measuring Methodology IEEE/PSRC Working Group
I3. The Working group suggested to measure security as the number of false trips relative to
the total number of events recorded during a time period. While this does not provide a
security estimate according to the strict definition of security it certainly is a useful
measurement for proactive relay system performance comparisons.
3) Sensitivity:
The relaying equipment must be sufficiently sensitive so that it operates
reliably when required under the actual conditions that produces least operating tendency.
4) Speed:
The relay must operate at the required speed. It should be too slow which may result
in damage to the equipment not should it be too fast which may result in undesired operation.
5) Discrimination:
A protective system should be able to discriminate between fault and load
conditions even when the minimum fault current is less than the maximum load current. A
fault and an overload, sometimes, look similar to the protective scheme. There must be some
methods and means to discriminate between such similar-looking conditions. There are other
situations also, where the relay has to be sharp enough to distinguish between the two. The
magnetising inrush current at the instant of switching shall not be misinterpreted as an actual
internal fault. In modern interconnected system, conditions like power swings can mislead a
distance relay to mal-operate while protecting the transmission lines.
6) Stability:
The term stability is often used to describe the quality of a protective system by
virtue of which it remains inoperative under specified conditions usually associated with high
values of fault currents. It is a quality that only unit systems can possess because they are
required to remain inoperative under all conditions associated with faults outside their own
zones.

7
CHAPTER: - 2
SYNCHRONOUS GENERATOR
2.1 Introduction
Synchronous machines are AC machines that have a field circuit supplied by an external DC
Source. Synchronous machines are having two major parts namely stationary part stator and a
rotating field system called rotor.
In a Synchronous generator, a DC current is applied to the rotor winding producing a rotor
magnetic field. The rotor is then driven by external means producing a rotating magnetic
field, which induce a 3-phase voltage within the stator winding.
Field winding are the winding producing the main magnetic field (rotor winding for
synchronous machines); armature windings are the windings were the main voltage is
induced (stator windings for synchronous machines).

8
2.2 Types of Synchronous Machines
1. Hydro-generators: The generators which are driven by hydraulic turbines are
called. These are run at lower speeds less than 1000 rpm.
2. Turbo-generators: These are the generator driven by steam turbines. These
Generators run at very high speed of 1500 or above.
3. Engine driven Generators: These are driven by IC engines. These are run at as
speed less than 1500 rpm.
Hence the prime movers for the synchronous generators are Hydraulic turbines, Steam
turbines or IC engines.
Hydraulic turbines: Pelton wheel Turbines: Water head 400 m and above
Francis turbines: Water head up to 380 m
Kaplan turbines: Water head up to 50 m
Steam turbines: The synchronous generators run by steam turbines are called turbo
generators or turbo alternators. Steam turbines are to be run at very high speed to get higher
efficiency and hence these types of generators are run at higher speed.
Diesel Engines: IC engines are used as prime movers for very small rated generators.
2.3 Construction of synchronous machines
1. Salient pole Machines: These type of machines has salient or projecting pole with
concentrated field windings. This type of construction is for the machines which are driven
by hydraulic turbines or Diesel engines.
2. Non-salient pole or Cylindrical rotor or Round Rotor Machines: These machines are
having cylindrical smooth rotor construction with distributed field winding in slots. This type
of rotor construction is employed for the machine driven by steam turbines.
2.3.1 Stator core:
The stator is the outer stationary part of the machine, which consist of
 The outer cylindrical frame called yoke, which is made either of welded sheet
steel, cast iron.
 The magnetic path, which comprises a set of slotted laminations called stator core
pressed into the cylindrical space inside the outer frame. The magnetic path is

9
laminated to reduced eddy currents, reducing losses and heating. CRGO
laminations of 0.5 mm thickness is used to reduce the iron losses.
 A set of insulated electrical windings are placed inside the slots of the laminated
stator. The cross-sectional areas of these windings must be large for the power
rating of the machine.
 For a 3-phase generator, 3 sets of windings are required, one for each phase
connected in star. Fig. 1 shows one stator lamination of a synchronous generator.
In case of generator where the diameter is too large stator lamination cannot be punched in on
circular piece. In such cases the laminations are punched in segments. A number of segments
are assembled together to from one circular laminations. All the laminations are insulated
each other layer of varnish.
2.3.2 Rotor Core:
An alternator rotor is made up of a wire coil enveloped around an iron core.[11]
The
magnetic component of the rotor is made from steel laminations to aid stamping conductor
slots to specific shapes and sizes. As currents travel through the wire coil a magnetic field is
created around the core, which is referred to as field current.[1]
The field current strength
controls the power level of the magnetic field. Direct current (DC) drives the field current in
one direction, and is delivered to the wire coil by a set of brushes and slip rings. Like any
magnet, the magnetic field produced has a north and a south pole. The normal clockwise
direction of the motor that the rotor is powering can be manipulated by using the magnets and
magnetic fields installed in the design of the rotor, allowing the motor to run in reverse or
counter clockwise.

10
CHAPER: - 3
GENERATOR PROTECTION
3.1 Introduction
Generator protection and control are interdependent problems. A generator has to be
protected not only from electrical faults (stator and rotor faults) and mechanical problems
(e.g. Related to turbine, boilers etc.), but it also has to be protected from adverse system
interaction arising if generator going on out of step with the rest of system, loss of field
winding etc. Under certain situations like internal faults, the generator has to be quickly
isolated (shut down), while problems like loss of field problems required an ‘alarm’ to alert
the operator. Following Is a descriptive list of internal faults and abnormal operating
conditions.
1. Internal Faults
a. Phase and / or ground faults in the stator and associated protection zone
b. Ground faults in the rotor (field winding)
2. Abnormal operating condition
a. Loss of field.
b. Overload.
c. Overvoltage.
d. Under and over Frequency.
e. Unbalanced operating e.g. single phasing.
f. Loss motoring e.g. Loss of prime mover.
g. Loss of synchronous (out of step).
h. Sub synchronous oscillation.
3.2 Multi – CT Different Protection for Generators
Typical interconnection for different protection of generators is shown in Fig 1. With
numerical relay, the circulatory as shown in Fig is not be hard wired. Instead, equivalent
computations can be done in microprocessor. For differential protection, it is important to
choose CTs from same manufacturer with identical turns ratio to minimize CT mismatch. To
improve security, percentage differential protections is preferred. The accuracy of the
differential protection for generators is expected to be better than that of differential
protection for transformers, as issues like fluxing, magnetizing inrush, no load current and
different voltage rating of primary and secondary are non – existent.

11
3.3 Stator Ground Fault Protection
Most faults in a generator are a consequence of insulation failure. They may lead to turn – to
– turn faults and ground faults. Hence ground faults protection is very essential for
generators.
Three types of grounding schemes are used in practice
1) Faults Protection with high impedance grounding
2) Fault protection with low impedance grounding
3) Hybrid grounding
3.4 High Impedance grounding
It is used to limit the maximum fault current due to fault in winding near generator terminals
to 1-10 A primary. This reduces iron burning in the A primary. This reduces iron burning in
the generator and it help in avoiding costly repairs. Fig 2 below shows a typical circuit
connection. There is an inverse time overvoltage unit connected across the resistor to trip the
breaker on overvoltage, which is a consequences of large zero sequence currents flowing in R
due to the fault. High impedance grounding reduces sensitivity for both feeders ground
protection and differential protection in the stators of the generators. Alternative to high
impedance grounding is low impedance grounding.
3.5 Low Impedance Grounding
The advantage pf low impedance grounding is improved sensitivity of the protection.
However, if the fault is not cleared quickly, the damage to equipment can be much higher. It
is possible to engineer ground (zero sequence) differential protecting using directional ground
overcurrent relaying as shown in Fig 3. The basic idea is to compare the sum of terminal
Ia+Ib+Ic
current with neutral current. If the two are identical, there is no internal ground fault.
Conversely, a differential in the two quantities an internal ground fault on the generator.
3.6 Reduce or Loss of Excitation Protection
What is it?
Reduction or loss of excitation to the field winding is an abnormality rather than a fault. If the
field winding is completely lost, then in principle, synchronous generators will try to mimic
an induction generator. This mode of operation is possible provided that power system to
which generator is connected is strong enough to provide necessary reactive power support.
Recall that an induction generator has no field winding and hence it cannot generate reactive
power. If adequate reactive power support is not available (a strong possibility!), then the
generator will have to be shut down. It is likely that field winding will be accidentally shut

12
off and usually loss of synchronous will required appreciable time to effect. Hence, it is
preferable to first raise an alarm for operator to restore field which, generator has to be shut
down.
Consequences
Prima- facie, consequence of reduced excitation may not appear to be dramatic, but it can
lead to end –region over-heating in turbo – alternators. Hence, this abnormality has to be
detected and an alarm has to be raise for the operator. The ultimate measure would be to shut
down the generator. Fig XY shows the reactive power capability curve of a generator. It can
be seen that in the lagging power factor – operating region, limits are determined either by
rotor field heating limit or by armature heating limit. During the leading power factor-
operating region, it is the iron end region-heating limit due to eddy currents that is
detrimental to the machine. Turbo-alternators may not have adequate reactive power
absorption capability. Hence, they are seldom operated with leading power factor. Typically
leading power factor operation of generators results when the field excitation is reduced.
Hence, limitations on the reactive power absorption set a lower limit on the reduction on field
excitation system.
How?
Protection system for synchronous – generators should detect reduced or loss of excitation
condition, raise an alarm and if the abnormality persists, trip the generator. This can be
achieved by use of distance relays that are installed at generator terminals. Directionally, they
look into the generator. For this purpose, we need to interpret capability curve on the R-X
plane. If the complex power generated is given by P+JQ then apparent seen by the distance
relay installed on the ground terminals is given by
Zapp =
|𝑉|2
𝑃−𝑗𝑄
=
|𝑉|2
𝑃2+𝑄2
(𝑃 + 𝑗𝑄) ------------------(1)
For simplicity, we have referred impedance has been referred to the primary side. Fig yy
shows the capability curve transferred to R-X plane using eqn. (1).
To protect the generator two distance relays and directional units are used. To protect
generator against complete loss of field, inner circle is used. The relay operates when the
impedance vector moves into this circle. Operating time of about 0.2 to 0.3 seconds are used
with a complete shutdown of the generators. The diameter of this circle is of the order of
𝑋 𝑑with the upper part of the circle 50-75% of 𝑋 𝑑below the origin.
The larger circle is used to detected reduced or partial loss of excitation system. Directional
blinder may be used to limit pickup on normal operating condition.

13
3.7 Protection against unbalanced operation
Quite often, a generator is connected to grid using a transformer. The ∆/𝑌 winding on the
generator side, traps the zero sequences current from flowing through the phase winding.
However, positive and negative sequence currents will find their way into stator winding.
Positive sequences currents cannot discriminate between balanced and unbalanced operating
conditions. On, the other hand, negative sequence currents clearly indicate the abnormality.
Hence, it can be used as an effective discriminator for unbalanced system operation. Negative
sequence currents create an mmf wave in opposite direction to the direction of rotation of
rotor. Hence, it sweeps across the rotor induces second harmonic currents in rotor, which can
cause severe overheating and ultimately, the melting of the wedge in the air gap. ANSI
standard have established that the limits can be expressed as∫ 𝑖2
2
𝑑𝑡 = 𝑘, where 𝑖2is the
negative sequence currents flowing. The machine designer establishes constant k. It can be in
the range of 5-50. An inverse – time overcurrent relay excited by negative sequence current
can be used for this protection.
Generator Motoring: why and how?
If the mechanical input to the prime mover is removed while the generator is in service, then
rotors mf wave will tend to drive the rotor, just like an induction motor. This is dangerous to
both steam and hydro turbine. In steam turbines, it may lead to overheating while in hydro
turbine it would cause cavitation of the turbine blades. The motoring of generator can be
detected by reserves power flow relays having sensitivity of 0.5% of rated power output with
time delay of approximate 2 seconds.
3.8 Over Voltage protection
How?
On its face value, over voltage protection should be more or less straightforward. First, one
should raise an alarm if the over voltage is above 110% of rated value. There would a
subsequent trip if it persists for 1 or more. Very larger over voltage of the order of 120% of
rated value or above, will lead to trip within approximately 6 seconds.
Why?
Terminal voltage of a generator is controlled by an automatic voltage regulator (AVR). If the
load current (I)on the generator reduces, the AVR would automatically reduce the field
current so as to reduce open circuit emf E to maintain constant terminal voltage V. However,
loss of a VT fuse, incorrect operation or setting of AVR etc. can lead to over voltage which is
detrimental to the generator. Steady state over voltage will lead to saturation of iron, both for
generator and the unit transformer connected to it. This will lead to large magnetizing

14
currents, unacceptable flux patterns, over-heating, which can damage the power apparatus.
Hence generators have to be protected against overvoltage.
3.9 V/F Protection
During start-up or shut shown, the speed of the generator will deviate significantly from the
nominal speed. As per emf equation (E= 4.44f∅N), over fluxing of the core is not simply a
consequence of over voltages with to nominal voltage. Rather over fluxing occurs when V/F
ratio exceeds nominal value, over voltage protection is implemented after normalizing
voltage by the frequency of the generator.
3.10 Out-of-Step Protection
With modern generators having large 𝑋 𝑑 and EHV transmission having low reactance, it is
likely that the electrical Centre, a consequence of out of step condition would be within the
generator step-up transformer unit. To detect this condition, distance relay looking into the
generator (or into the transformer – generator unit) should be installed. Even a distance relay
used for loss-of-field protection will pick –up on such power swing. If the swing moves out
of the relay characteristic, before the timer runs down, then, no trip action will be initiated.
However, if the swing persists for sufficient time, the loss-of- excitation distance relay will
operate on power swing.
3.11 A Note on Numerical Protection
At the point of time, there are no new principles to be introduced to be from the numerical
relaying perspective. The differential protection scheme can be implemented by either using
sample comparison (Time domain approach) or by using phasor comparison (frequency
domain approach). Time domain approach can be faster, than phasor comparison approach.
The DFT approach with 1-cycle window will required one cycle to latch on to the phasor.
Usually, the time constant associated with DC offset currents for generator faults will be
large. Hence, decaying do offset can be approximated by DC signal, which implies the full
cycle DFT will be able to reject it. However, will half-cycle estimation, mimic impedance
should be used. Sample comparison approach is immune to dc-offset problem but building
reliability with such an approach requires a polling scheme.
In other words, reliability is obtained at the cost of time by ascertaining that successive
samples return the trip decision. One can even implement a hybrid approach where in one
switches from time domain to frequency domain approach. The decision to switch will
depend upon the speed of rotation

15
3.12 Schemes of Generator Protection
DEVICE
NUMBER
FUNCTIONS
21 Distance – Distance protection
24 Volts/Hz – protection for generator over-excitation
25 Synchronism check – synchronism check when paralleling
27 Under Voltage – Under voltage protection
27-3N Under Voltage – Third harmonic under voltage protection
32 Reverse Power – Anti-motoring protection
40Q Loss of Field – Protection of failure of excitation system(Reactance based)
40Z Loss of Field – Protection of failure of excitation system(Impedance based)
46 Current negative Sequence – Unbalance current protection
47 Voltage negative Sequence – Unbalance voltage protection
49 Temperature – Stator thermal protection
51 Time Over current – Phase over current protection
81O Over frequency – Protection for over frequency
81U Under frequency – Protection for under frequency
86 Lockout Relay – A latching trip relay or device that requires an operator to reset
87G Differential – Generator current differential protection
87N Differential – Ground differential protection
87T Differential – Transformer current differential protection

16
CHAPTER: - 4
GENERATOR ROTOR EARTH FAULT PROTECTION
4.1 Introduction
If the rotor winding is ungrounded, a is the usual practice, a fault to earth has no effect, but a
second fault to earth will increase current in part of the winding and may also unbalance the
air-gap fluxes so that there will be serious vibration which may lead to serious damage. A
rotor second earth-fault may also cause local heating which slowly distort the rotor causing
dangerous eccentricity this can also cause vibrations and serious damage.
4.2 Protective Scheme:
Figure 4.1 shows one of the methods of detecting rotor earth-faults. The field circuit, as
shown in figure is biased by a dc voltage. If a ground fault occurs, current will pass through a
very sensitive dc relay which can initiate alarm or class A trip as required. The relay is
sensitive polarized moving iron relay. The dc voltage is impressed because in case of ac, the
relay cannot be made very sensitive. This is because the relay may pick-up due to the current
that normally flows through the capacitance of the rotor winding to its core and thereafter
through between this capacitance and the relay inductance. Also, even if this is small, it will
pit the bearing unless a special collector brush is fitted to rotor shaft.
In unattended station, a protective relaying equipment must be arranged to trip the main and
field breakers of the generators when the first ground fault occurs. In attended stations, the
usual practice is to sound an alarm at the occurrence of rotor first earth fault. Should the
second earth fault occur, the main and field breakers of the generator must be instantaneously
tripped. However, this practice involves a little risk because the vibration caused by second
earth fault cannot be stopped instantly and also the two ground faults may occur together or
quick succession.
The detailed scheme of first and second earth fault protection of rotor is shown in fig 4.1. The
circuit of figure is for the selector switch on ‘rotor first earth-fault’ position. Figure gives the
control circuit for alarm and annunciation. Figure 4.1 is the simplified circuit for selector
switch on ‘Balance’ position.
Figure 4.2 is control circuit for effecting tripping of the main and field breaker of generator.
The protective scheme of figure uses a selector switch having four position as shown P1 and
P2 are coarse and fine potentiometer respectively and L is a choke. The selector switch is at
rotor 1st
E/F position initially.

17
Should the earth-fault occur on the rotor winding as shown in the figure, rotor first earth-fault
relay 64F1 will operate. The operation of 64F1 will energies the timer relay 2/6F1. After the
pre-set time, an alarm will be sounded in the control room of the power station.
Once the balancing is done, the selector switch is to be changed to “test” position. The only
change in the circuit of the “test” positon is that the galvanometer is replaced by the rotor
second earth fault relay 64F2. As the bridge is already balanced, no current passes through
the coil of earth fault relay 64F2 and it does not operate. The operation of 64F2 can be tested
here by disturbing the balance. It is to be noted that no tripping will be effected during testing
as the selector switch is on ‘test’ position.
After testing the balancing again, the selector switch is shifted to ‘rotor 2nd
E/F position.
Should the second earth fault occur, the balance will be disturbed and relay 64F2 will trip the
generator. 64F1 and 64F2 are moving coil type DC relays and are made sensitive about 1mA.
It is worthwhile to note that the rotor first earth fault will initiate an alarm while the second
earth fault protection is Class A protection. The second earth fault protection is instantaneous
in operation.
Fig: 4.1 Rotor 1st and 2nd Earth Fault Protection Scheme
Fig:4.2 Basic Control Circuit

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Fig:4.3 Selector switch on rotor 1st earth fault position
Fig:4.4 Selector switch on Balance position

19
Fig:4.5 Selector switch on Test position
Fig:4.6 Selector switch on Test position
Should be earth –fault occur on the rotor winding as shown in the figure, rotor first earth-fault
relay 64F1 will operate. The operation of 64F1 will energies the timer relay 2/64F1. After the
pre-set time, an alarm will be sounded in the control room of the power station.

20
Once the alarm is heard, the operator will change the selector switch to “Balance” position. In
the balance position, the circuit reduces to a whetstone bridge with parts of the field winding
F1 and F2, resistive part y of potentiometer P1 and parallel combination of X and R plus P2
as four arms. This bridge can now be balanced using coarse, medium and fine controls
respectively of a selector switch and using the coarse potP1 and potP2. Choke L protects the
galvanometer and relay 64F2 against switching transients.
Once the balancing is done, the selector switch is to be changed to “test” position. The only
change in the circuit of the “test” position is that galvanometer is replaced by the rotor second
earth fault relay 64F2. As the bridge is already balanced, no current passes through the coil of
earth fault relay 64F2 and it does not operate. The operation of 64F2 can be tested here by
disturbing the balance. It is to be noted that no tripping will be effect during testing as the
selector switch is on ‘test’ position.
After testing the balancing again, the selector switch is shifted to ‘rotor 2nd
E/F’ position.
Should the second earth fault occur, the balance will be disturbed and relay 64F2 will trip the
generator. 64F1 and 64F2 are moving coil type DC relays and are made sensitive about 1
mA.
It is worthwhile to note that the rotor first earth fault will initiate an alarm while the second
earth fault protection is class A protection. The earth fault protection is instantaneous in
operation.

21
CHAPTER: -5
EXPERIMENTAL PANEL OF GENERATOR ROTOR EARTH FAULT
PROTECTION
5.1 Introduction
Generator rotor winding and its associated dc supply electric circuit is typically fully
insulated from the earth (i.e. ground). Therefore, single connection of this circuit to earth will
not cause flow of any substantial current. However, if second earth fault appears in this
circuit circumstances can be quite serious. Depending on the location of these two faults such
operating may cause:
 Partial or total generator loss of field
 Large dc current flow through rotor magnetic circuit
 Rotor vibration
 Rotor displacement sufficient to cause stator mechanical damage
Therefore, practically all bigger generators have some dedicated protection which is capable
to detect the first earth fault in the rotor circuit and then, depending on the fault resistance,
either just to give an alarm to the operating personnel or actually to give stop command to the
machine.
5.2 Operation
 Switch on the AC supply.
 By pressing the ON push button field winding gets the DC supply through rectifier
(230V AC – 24V DC).
 Semaphore indicator gives the indication that field winding is getting continuous DC
supply. On that state Yellow indication lamp shows that ‘Waiting for first earth fault’.
 Now connect terminal A and 1 to detect the 1st
earth fault by relay 64F1. By turning
on the toggle switch F1, we make the first earth fault on field winding. On that state
Red indication lamp shows that ‘First earth fault occurred’.
 Now remove the connection of terminal A and 1. Connect terminal B, C, D and 2,3,4
respectively. This state is for balancing the bridge by rheostat. It can be confirmed by
the zero indication in digital meter. Yellow indication lamp shows that ‘Balance’
mode.
 Now keep the connection of B, C and 2,3 respectively as it is and replace the
connection of D and 4 with E and 5 respectively. The test mode is indicated by Green
indicator lamp. In Test mode digital meter is replaced by Relay(64F2). By using the
rheostat make the circuit to lose its balanced condition, so the fault current will flow
thorough the relay and we can check that relay is in working condition. This
indication is given by the Yellow lamp that shows ‘Relay is in working condition’.
 Again go to the BALANCED mode by making connection as stated in previous steps.
 Now connect terminal F and 6. By turning on the toggle switch F2, we make the 2nd
earth fault on field winding. On that state Red indicator lamp shows that ‘2nd
earth

22
fault is occurred and also field breaker will cut off DC supply and semaphore
indicator will show that the DC supply is off. By pressing Reset push button, Red
indication will show that AC supply is off and experiment can be repeated again.
Fig: Control Circuit

23
CHAPTER: 6
EQUIPMENTS, TOOLS AND MATERIALS USED FOR LABORATORY
SIMULATION
NO. EQUIPMENT TOTAL NO.
USED
RATINGS
1
Semaphore indicator 1 230V AC
2
Resistors 3 9Ω/6A, 9Ω/6A, 18Ω/6A
3
Rheostat 4 9Ω/6A, 9Ω/6A,9Ω/6A,180Ω/6A
4
Contactor 6 Coil Voltage – 230V
Current Rating – 10 A
5
Push Button 3 440V/10A
6
Indicator Lamp 8 230V/20mA
7
Digital Volt Meter 2 660V
8
Relay 4 6V/6A, 12V/6A

24
6.2 Description of Equipment Used for Laboratory Simulation
6.2.1 Semaphore indicator
Semaphore indicator is design to perform on instrumentation, control & relay panel
mimic diagram. Semaphore Indicators have provision for indicating ‘open’ or ‘close’ position
of the circuit breaker. Armed with Dial Needle, which tilts towards right, when the testing
equipment is in ‘open’ position. Whereas, for ‘closed’ position, the needle position itself and
bridge a gap between the lines. In this manner, continuity in the mimic diagram is
maintained.
6.2.2 Contactor
‘E’ type and ‘I’ type cores are used. One coil is wound on centre limb of ‘E’ core.
Arm is joint with ‘I’ type core. When supply is given to coil, it gets energised and magnetic
field is created. ‘I’ core is gets attracted towards ‘E’ core and hence each contacts gets closed.
6.2.3 Relay
On this unit, one pair of contact is provided to seal- in the operating coil, thereby
eliminating any ill – effects of bounce on the main contacts.
A second pair of reinforcing contacts is connected in the trip circuits and a third pair is wired
out for alarm purposes.
6.2.4 Push button
A push-button or simply button is a simple switch mechanism for controlling some
aspect of a machine or a process. Buttons are typically made out of hard material,
usually plastic or metal. The surface is usually flat or shaped to accommodate the human
finger or hand, so as to be easily depressed or pushed. Buttons are most often biased
switches, though even many un-biased buttons (due to their physical nature) require
a spring to return to their un-pushed state
6.3 Tools
The following tools were used during construction of panel.
 Insulated combination player 250mm.
 Insulated long nose plier 150mm
 Drill Machine
 Insulated screwdriver 250mm
 Tester.
 Connector
 Wire Tray
 PCB
 Round file

25
 Hack – saw
 Soldering iron
 Cutter Machine
6.4 Material used in Laboratory for Simulation
The following materials were used during construction of panel.
 Wire: 2.5 mm2 wires were used. We have used different coloured wires i.e.
Red, Black and Blue.
 Casing capping was used for wiring purpose.
 Different lugs were used for connection purpose, i.e. round type, pin type and
U-type.
CONCLUSION
By experimental panel of Generator rotor earth fault protection, we can study the operation of
rotor earth fault protection scheme in actual field and Conclude that there is very little fault
current during first earth fault so that there is no need of protection during first earth fault, but
in second earth fault serious consequences occurs and generator needs a protection against
vibration and overheating.
BIBILIOGRAPHY
Power System Protection and Switchgear
By Nirmal Kumar Nair, Vijay Makwana, Bhuvanesh Oza, Rashesh Mehta

Generator rotor earth fault protection

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