Generators and their protection systems are designed to operate reliably over many years while withstanding some abnormal operating conditions. Monitoring devices supervise the machine and auxiliaries to minimize abnormal incidents. Despite monitoring, electrical and mechanical faults can still occur, so generators are equipped with protective relays to quickly disconnect the machine from the system if a fault is detected. Protection systems include devices that monitor temperature, vibration, pressure, and lightning arrestors, as well as relays that detect over voltage, over current, winding faults, mechanical faults, and other conditions. Relays provide protection for the stator, rotor, differential currents, negative phase sequences, loss of excitation, out of step conditions, and other faults. Protection schemes are classified based on

1. GENERATOR PROTECTION
22.07.2015

2. 2
GENERATOR PROTECTION
Generators are designed to run at high load factor for a large
number of years and permit certain incidents of abnormal
working conditions.
The machine and its auxiliaries are supervised by monitoring
device to keep the incidents of abnormal condition down to
minimum.
Despite the monitoring, electrical and mechanical faults may
occurred and generators must be provided with protective relays
which in case of a fault, quickly initiate and disconnect the
machine from the system.

3. 3
GENERATOR PROTECTION
Normal service conditions of generator is supervised by
 Temperature detector such as RTD, temp. gauge etc.
 Mechanical activated device such as vibration pickup,
pressure switches etc.
 Lightning arrestors to limit incoming surges.
Origin of disturbance and failure
 Over voltage.
 Over current.
 Winding Fault in stator and rotor
 Mechanical fault

4. 4
GENERATOR PROTECTION
1. Generator stator earth fault protection.
 95% stator earth fault protection.
 100% stator earth fault protection.
2. Generator rotor earth fault protection.
3. Generator differential protection.
4. Negative phase sequence current protection.
5 Generator loss of excitation protection.
6. Generator out of step protection.
7. Generator inter- turn fault protection

5. 5
GENERATOR PROTECTION
8. Generator under frequency protection
9. Generator over frequency protection
10. Generator back-up impedance protection.
11. Generator over load protection.
12. Generator reverse power protection
13. Generator over fluxing protection.
14. Generator over voltage protection.

6. 6
CLASS – A1 GENERATOR PROTECTION
 Inadvertent Energization
 GCB Breaker Failure
 Voltage Controlled Over Current
 GT Differential Protection
 UT Differential Protection
 WTI and OTI of UT Transformers
 WTI and OTI of GT Transformers
 Buchholz Trip of GT and UT
 PRV Trip of GT and UT
 SPR Trip of GT and UT
 OSR Trip of UT
 REF Protection of UT
 Over Current Protection of GT
 GT Over Hang Differential Protection

7. CLASS – A2 GENERATOR PROTECTION
 Generator Differential Relay
 Stator Earth fault Relay (100%+95%)
 Generator Reverse Power
 Generator Low Forward Power
 Generator Over Voltage
 Generator Under Frequency Stage 2
 Generator Over Fluxing
 Generator Loss of Excitation
 Generator Rotor Earth Fault
 Generator Excitation System Fault
 I & C Trip
7

8. CLASS - B GENERATOR PROTECTION
 Generator Under Frequency Stage 1
 Generator Over Frequency
 Generator Backup Impedance
 Negative Sequence Current
 Generator Pole Slip
 Cooling Water Loss
 Conductivity High High
8

9. 9
CLASSIFICATION OF TRIPPING
 Objective is to trip only the essential equipments so that Damage is
minimum, over speeding of set avoided. Impact of large set on the grid is
minimum and time to restart the unit is minimum.
 Class – AII Tripping
 This is adopted for those electrical faults for which tripping cannot be
delayed. This leads to simultaneous tripping of
 -Generator and Field circuit breaker.
 -Turbine tripping.

10. 10
CLASSIFICATION OF TRIPPING
 Class – B Tripping
 This is adopted for all turbine faults (Mechanical) and for some
electrical faults for which it is safe to trip the turbine. Subsequently the
generator is tripped through I & C trip or reverse power interlock.
 Class – A1 Tripping
 This is adopted for all faults beyond the Generator system, which can
be cleared by tripping of EHV circuit breaker alone.

11. 11
STATOR E/F RELAY
 Ground faults, particularly single phase to ground, are the most likely to
occur, particularly on generators equipped with isolated-phase bus
connecting the stator winding terminals to the step-up transformer. Modern
generators are usually STAR -connected with the neutral grounded through a
high resistance to limit the magnitude of the ground fault current.
 A solidly grounded synchronous generator will supply a single phase to
ground short circuit current of about 120% of the three-phase short circuit
current. Since the single phase to ground short circuit is the most likely, an
impedance is usually introduced in the neutral of generators to limit the
ground fault current.

12. 12
STATOR E/F RELAY
 a) 95% Stator Earth Fault Protection
 Ground faults particularly phase to ground are most likely to occur.
generators are grounded through a reactance to limit the current typically
about 10 A.
 The relay is normally set to operate at 5% of maximum neutral voltage
with a time delay of 0.5 sec. with this voltage setting approximately 95% of
stator winding are covered by this protection.

13. 13
STATOR E/F RELAY
 b) 100% Stator E/F Protection:
 As some voltage is required to pickup the 95% E/F relay, approximately
5% of winding remains unprotected. Some time ground faults may occur very
near to the neutral due to mechanical damage and thus 100% protection to
stator winding is required.
 The basic principle is that an external low frequency is injected into the
generator star point. Maximum of 1% of rated generator voltage is injected.
 The voltage/current check unit is used to prevent mal-operation of the
relay at the generator standstill condition.

14. 14
STATOR E/F RELAY
Basic Principle of Voltage Injection into the
Generator Starpoint

15. 15
ROTOR E/F RELAY
 The field or rotor winding of generator is unearthed. Thus single earth fault of
field is not much harmful. But a second earth fault acts as short circuit causing severe
damage to the field winding and mechanically endangers the rotor due to magnetic
unbalance.
 Rotor E/F relay injects a DC voltage to the rotor field winding and measures the
insulation resistance. The protection has two stages, gradual deterioration of the
insulation initiates alarm, a solid earth fault initiates trip.
 Alarm value 5 k Ohm.
 Trip value 1 k Ohm.

16. 16
ROTOR E/F RELAY

17. 17
GEN. DIFFERENTIAL PROTECTION
.
The main protection for generator stator winding is differential
protection. In order to protect generator stator winding from
severe damage during internal faults like phase to phase and
earth faults, a differential current principal is used to detect the
faults.
CTs with identical ratio are used for both neutral and line side
of the generator. Hence under normal service conditions and
external faults with unsaturated CTs the relay operating current
remains almost zero.

18. 18
GEN. DIFFERENTIAL PROTECTION
.

19. 19
NEGATIVE PHASE SEQ. PROTECTION
Unbalanced load protection detects unbalanced loads of three-
phase induction motors. Unbalanced loads create a counter-
rotating field which acts on the rotor at double frequency. Eddy
currents are induced at the rotor surface leading to local
overheating in rotor end zones and slot wedges. Another effect of
unbalanced loads is overheating of the rotor.
Negative sequence current is Produced by due to Load
unbalances i.e.
-Single phase load.
-Transmission line open conductor
-Breaker pole discrepancy etc.

20. INTERTURN FAULT PROTECTION
20
The interturn fault protection detects faults between turns within a
generator winding (phase). This situation may involve relatively high
circulating currents that flow in the short-circuited turns and damage the
winding and the stator. The protective function is characterized by a high
sensitivity.
Modern medium size and large size turbo generators have the
stator winding designed with only one turn per phase per slot. For these
machines inter-turn faults can only occur in case of double ground faults
or as a result of severe mechanical damage on the stator end winding.
For this type of generator voltage transformers with open delta
can be used to detect interturn fault.

21. 21
.
INTERTURN FAULT PROTECTION
R
Y
B
Vsec Δ=Vpri Δ
R
110/1.732
110/1.732
Setting = 3 V of T= 0.5 Sec

22. 22
.The inadvertent energizing protection serves to limit damage by
accidental connection of the stationary or already started, but not
yet synchronized generator, by fast actuation of the mains
breaker. A connection to a stationary machine is equivalent to
connection to a low-ohmic resistor. Due to the nominal voltage
impressed by the power system, the generator starts up with a
high slip as an asynchronous machine. In this context,
unpermissibly high currents are induced inside the rotor which
may finally destroy it.
INADVERTENT ENERGISATION

23. 23
.
UNDER FREQUENCY PROTECTION
Due to prolong operation of units in case of under freq.
particularly last stage of LP turbine gets damage. Thus an
under frequency Class – B trip is generally provided at 47.5 Hz
after 15 Sec. Under frequency Class AII trip is provided at 47
Hz after 4 Sec. It should also trip the generator circuit breaker;
this is to prevent under frequency operation of loads
connected to the system supplied by the generator.

24. 24
.
OVER FREQUENCY PROTECTION
Prolonged operation of units at higher frequency than rated is
also dangerous to turbine and associated electrical system.
Thus a alarm is generally provided at 51 Hz and trip is
provided at 51.5 Hz.

25. 25
.
LOW IMPEDANCE PROTECTION
Machine impedance protection is used as a selective
time graded protection to provide shortest possible tripping times
for short-circuits in the synchronous machine, on the terminal
leads as well as in the unit transformer. It thus also provides
backup protection functions to the main protection of a power
plant or protection equipment connected in series like generator,
transformer differential and system protection devices.

26. 26
.
OVER LOAD PROTECTION
The power that can be generated is limited by the steam
production and hence cannot rise unnoticed for any
appreciable period above the present value. Over load setting
is done considering the capability of a particular unit and
temperature rise of stator winding.

27. 27
.
REVERSE POWER PROTECTION
The purpose of the reverse power relay is basically to prevent
damage on the prime mover i.e. turbine.
If the driving torque becomes less than the total losses in the
generator and the prime movers, the generator start to work
as a synchronous motor. In case of steam turbine reduction of
the steam flow reduces the cooling effect on the turbine blades
and overheating may occurs.

28. 28
.
OVER VOLTAGE PROTECTION
During start-up of generator, voltage is obtained by automatic
voltage regulator. After synchronization the voltage level of the
generator will dictate voltage of the system grid.
If the generator circuit breaker tripped while machine is running
at full load, increase in the terminal voltage will occur if AVR is
switched to manual control

29. 29
.
OVERFLUXING PROTECTION
The magnetising flux in the core is directly proportional to v/f.
The losses are proportional to the level of excitation and
hence the temperature rise increases.
NORMAL V/F= 110/50=2.2;
FOR 24-1: 2.2*1.15=2.53; V/F=2.53; V=2.53*50; V=126.5(L-L);
V=73.04V (L-E)
FOR 24-1: 2.2*1.2=2.64; V/F=2.64; V=2.64*50; V=132.0(L-L);
V=76.2V (L-E)

30. 30
.
LOSS OF EXCITATION PROTECTION
Under-excitation or a total loss of excitation can result from a short circuit or
open circuit in the excitation circuit, a mal-operation of the automatic
voltage regulator, incorrect control of generators and transformers, or in the
event of a generator connected to a system with capacitive load. In this
context under-excitation means that the excitation of the synchronous
machine is less than required for stable operation at a particular power
level. This excitation limit determines the steady state stability characteristic
of the generator. If the excitation is not sufficient to provide the power
demanded of the generator, then this stability limit is exceeded. The
machine will slip and thereby obtain the required excitation from the
connected three phase system. Depending on the construction of the
generator

31. 31

32. 32

33. 33

34. 34
.
OUT OF STEP PROTECTION
Depending on power network conditions and feeding
generators, dynamic occurrences such as load jumps, short-
circuits not disconnected quickly enough, auto-reclosure
or switching actions, may cause system swings. Such power
swings endanger power network stability. Stability problems often
result from active power swings which can lead to pole-slipping
and generator overloading.

35. 35

36. 36

37. 37
Za = MAX. POWER SWIG FREQUENCY
Zb = TRANSIENT DIRECT AXIS REACTANCE OF THE GENERATOR i.e =
Xd’
Zd-Zc= REACTANCE OF THE NETWORK + REST OF THE
TRANSFORMER
Zc = TRANSFORMER REACTANCE
Zb + Zc = POWER SWING ANGLE BETWEEN GENERATOR AND
TRANSFORMER
Zb + Zd = POWER SWING ANGLE BETWEEN GENERATOR AND
NETWORK

38. THANKYOU
22.07.2015