This document discusses generator protection techniques. It begins by explaining why protective systems are needed to protect expensive power system elements like generators. It then describes different types of generator faults and various protection schemes. These include stator protection using differential protection and its modifications. Rotor faults and their protections like rotor earth fault protection are also explained. The document provides details on other protections like overcurrent, overvoltage, vibration and overheating protections. It concludes by stating that protective devices help detect faults, notify maintenance, and disconnect faulty elements to ensure continuous and safe operation of power systems.

1. PROTECTION OF
GENERATOR
Submitted by:
Name: Subham Bhattacharjee
College: Techno India College of Technology
Stream: Electrical Engg. 6th sem
Roll no: 09187016021

2. INTRODUCTION :
 It is imperative need to install some protective system to protect the
expensive elements of modern power system such as generators,
transformers, station bus-bars, transmission lines etc. from different types
of faults which are likely to occur sooner & later.
 In generating station, as a continuous operation of generators is much
more necessary so the faulty part has to be cleared very quickly for
uninterruptable power supply. Unlike other apparatus, opening a breaker
to isolate the faulty generator is not sufficient to prevent further damage.
 The basic electrical quantities those are likely to change during abnormal
fault conditions are current, voltage, phase angle and frequency .
Protective elements utilizes one or more of these quantities to detect
abnormal conditions in a power system for taking further essential steps
to isolate the faulty equipment to keep the healthy part in normal working
condition.

3. GENERATOR FAULTS AND PROTECTION TECHNIQUES :
 Generator Faults Can be Considered Under The Following Heads :
i. Stator Winding Faults
ii. Field Winding or Rotor Circuit Faults
iii. Abnormal Operating Conditions
 Followings Are The Different Process of Protecting Generator :
i. Stator Protection
ii. Overload or Overcurrent Protection
iii. Overvoltage Protection
iv. Overspeed Protection
v. Protection Against Motoring
vi. Rotor Fault Protection
vii. Negative Sequence Protection Against Unbalanced Loads
viii. Protection Against Vibration
ix. Bearing Overheating Protection
x. External Fault Back-Up Protection

4. STATOR FAULTS AND ITS PROTECTION :
 Stator faults include the following-
i. Phase-to-earth faults
ii. Phase-to-phase faults
iii. Inter-turn faults
Phase-to-phase faults & Inter-turn faults are less common, these generally
develop into an earth-faults. Inter-turn faults are more difficult to be detected.
 The Different Protection Schemes for Stator Protection are :
i. Differential Protection
ii. Modified Differential Protection
iii. Biased Circulating Current Protection
iv. Self Balance Protection System
v. Balanced Earth-Fault Protection
vi. Stator Inter-Turn Protection
vii. Stator Overheating protection

5. DIFFERENTIAL PROTECTION :
 Two sets of identical CTs, each set is mounted on either side of
stator phase winding. The secondaries of these current
transformer sets are connected in star & their ends are connected
through pilot wires (shown in fig).
 At normal operating conditions, the currents at the two ends of
the protected section are same (I1=I2). So relay will not operate.
 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.
 The relays employed in this protection scheme are generally of
electro-magnetic type & are arranged for instantaneous
operation as faults are expected to be cleared as quick as possible

6. DIFFERENT KINDS OF DIFFERENTIAL PROTECTIONS :

7. MODIFIED DIFFERENTIAL PROTECTION :
 Generally protection is made for
80 to 85% of the winding in
Differential Protection scheme.
 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.

8. RESTRICTED OR BALANCED EARTH FAULT PROTECTION :
 In case of small size generators
the neutral end of 3 phase
winding is not available
because it is made inside the
generator and grounded through
some low resistance then
percentage differential relay for
ground fault is provided and is
known as restricted earth fault
protection.
 This scheme can be used only for
ground faults but not for phase
faults.

9. STATOR INTER-TURN PROTECTION :

10. OVERCURRENT OVERVOLTAGE
PROTECTION PROTECTION
 Overloading of the generators  Overvoltage protection is required
may be caused either due to in case of hydro-electric or gas
turbine generators but not in case
i. Partial breakdown of of turbo generators.
winding insulation  Over voltage may be caused due to-
ii. Excessive load on the
power supply system i. Transient over voltage in
the transmission line due
 In occurrence of such to lightening.
faults, the generator can be ii. Defective operation of
disconnected from the system the voltage regulator.
manually. iii. Sudden loss of load due
 In modern alternators it has to line tripping.
not so much importance though  The protection is provided with an
unnecessary overloading over voltage relay.
interfere with the continuity of  It is usually of induction pattern
the supply with an IDMT Characteristic

11. ROTOR FAULTS AND ITS PROTECTION :
 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 its operation.
 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.
 The Different Protection Schemes for Rotor Protection are
i. Rotor Earth Fault Protection
ii. Loss of Excitation Protection
iii. Protection Against Rotor Overheating because of Over excitation
iv. Rotor Temperature Alarm
v. Automatic Field Suppression & Use of Natural Circuit Breaker
vi. Alternative Arrangement of Field Suppression

12. ROTOR EARTH FAULT PROTECTION :
 A high resistance is connected across
the rotor circuit and its mid point is
grounded through a sensitive relay.
 The another method of rotor earth
fault protection is done by
i. DC injection method
ii. 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).
 A single earth fault in rotor circuit
will complete the path and the fault
is sensed by the relay.
 DC injection method is simple & has
no problems of leakage current.

13. LOSS OF EXCITATION PROTECTION :
 It is caused by accidental tripping of
field breaker, short-circuit in the field
circuits, poor brush contact or
operating errors.
 In absence of field current alternators
runs as an induction generator &
currents are inducted in the rotor teeth
and wedges & also draws reactive
power from the system.
 Under-current moving coil relay is
connected in series with field winding
for loss of excitation protection.
 The relay mostly used for this type of
protection is Directional distance type
relay.

14. ROTOR OVERHEATING PROTECTION BY TEMPERATURE ALARM :
 This type of protection
scheme is installed generally
in large alternators.
 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).
 The relay measures the ratio
of voltage & current.

15. 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 generator
itself.
 Hence all protection system not
only trip the generator circuit
breaker but also trip the
“automatic field discharge
switch “

16. 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.

17. 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 against external
phase-to-phase faults & earth faults.
 Induction type IDMT relay is used for this purpose.
 High se, definite minimum time, induction type, inverse over-current, earth
fault relays are required for back-up protection of generators.

18. PROTECTION AGAINST VIBRATION AND BEARING
OVERHEATING PROTECTION :
 Rotor earth fault protection & Negative sequence protection of
generator against unbalanced loads prevent vibration under those
circumstances.
 A vibration detector mounted on one of the bearing pedestals & on
one of the upper guide bearing in case of horizontal & vertical shaft
generating set respectively.
 Bearing overheating can be detected by a relay accounted by
i. A thermometer-type bulb inserted in the hole of the bearing
ii. A resistance-temperature-detector relay
 If lubricating oil is circulated through the bearing under pressure, the
oil temperature should be taken into account for an alarm if oil
circulation is stopped

19. CONCLUSION :
 Switchgears & other protective devices are used in the
electrical machines & in the power system elements
i. To detect the abnormal faulty condition
ii. To notify this information to the maintenance officer
of that factory or generation station
iii. To disconnect the faulty elements from the healthy
system or shutdown the faulted apparatus for
continuous operation of the machines, for safety of
this expensive electrical devises as well as for the
safety of the workers
 The protective device does not prevent the cause of fault or
does not prevent ta occurrence of fault , it is used after the
occurrence of fault for quick action to remove those faulty
section for further normal operation .

20. REFERENCES :
A Course In Power Systems
by J.B.Gupta
 Modern power System Analysis
by D.P.Kothari & I.J.Nagrath
 www.wikipedia.com
 www.google.co.in

21. THANK YOU