This Presentation gives information on How Generator in Power Plants are protected with State of art technologies. Also provide information how latest Power System Protection technologies are more reliable operation.

1. GENERATOR PROTECTION
By – Parimal Bhagat
B.E Electrical, PG Diploma In Sub
T&D(NPTI,Bengaluru)

2. SYSTEM OVERVIEW
There are two Generator protection panel installed.
GRP Cubicle A & GRP Cubicle B
Both protection cubicles are having following IED’s
1) Micom P344 Generator Protection
2) Micom P342 Generator Protection
3) Micom P141 Excitation Transformer Protection

3. MICOM P344 RELAY INCORPORATED WITH
FOLLOWING PROTECTIONS
1) Generator Differential Protection (87G)
2) Inter turn fault Protection (50G1)
3) Negative sequence Over Current Protection (46G)
4) Stator Over Load(Current) Protection (50GS)
5) Voltage Restrained Over Current Protection (51V)
6) Dead Machine (61B)
7) Back Up Impedance (21G)
8) Over Fluxing (99G)

4. 9) 95% Stator Earth Fault Protection (64G1)
10) 100% Stator Earth Fault (64G2)
11) Breaker failure (50LBB)
12) Loss Of excitation with under voltage (27/40G)
13) Generator Rotor Earth fault (64F)
14) Generator Pole Slipping(Out of step) Protection (78G)

5. PROTECTION SCHEME TESTING
Generator Differential Protection (87G)
It operates on principle that current entering & leaving protection zone should be equal. Any
difference between that indicates fault current present in protected zone.

6. HEALTHY CIRCUIT :

7. By lenz law secondary current in healthy circuit will be just opposite to primary
current.
Since primary current is equal then circulating current also will be the same.
Id = i1-i2 =0Amp
where i1=i2

8. Heavy through current arising from external faults can cause one
CT to saturate more than other, resulting in difference between
secondary current produced by each CT’s, its essential to stabilize
the protection for these condition. A biasing technique where the
relay setting increased as the through current increased. The
through current is calculated as the average of the scalar sum of
current entering & leaving the zone of protection, this calculated
through current is then used to apply a percentage bias to increase
the differential setting.

10. RELAY SETTINGS :

11. INTER TURN FAULT PROTECTION (50G1)
In healthy 3 phase power system the addition of each phase to earth is normally
zero, as it is vector addition of three balanced vectors at 120° to another.
However when inter turn fault or earth fault occurs on primary system this balance
upset and residual voltage produced.
The neutral displacement voltage can be measured directly via the open delta PT .

12. RELAY SETTINGS:

13. STATOR OVER LOAD(50GS)
Protection reacts on current flowing on stator winding coil to protect winding from
overheat.
Overload can result in stator temp. rise which exceeds the thermal limits of windings.
Due to relatively large heat storage capacity of electric machine, infrequent
overload of short duration may not damage the machine, however sustained
overload of few percentage may result in premature aging and failure of insulation.

14. RELAY SETTINGS :
Over-load (50GS)
PHASE O/C
Menu text
Setting range
Setting
Min Max
I＞1 Function
Disabled, DT, IEC S Inverse, IEC V Inverse, IEC E Inverse, UK LT Inverse, UK
Rectifier, RI, IEEE M Inverse, IEEE V Inverse, IEEE E Inverse, US Inverse, US ST
Inverse
DT
I＞1 Direction Non-directional, Directional Fwd, Directional Rev Non-directional
I＞1 Current Set 0.08 In 4.0 In 4.50A
I＞1 Time Delay 0 100 5s (reduce load)
I＞2 Function
Disabled, DT, IEC S Inverse, IEC V Inverse, IEC E Inverse, UK LT Inverse, UK
Rectifier, RI, IEEE M Inverse, IEEE V Inverse, IEEE E Inverse, US Inverse, US ST
Inverse
IEC E inverse
I＞2 Direction Non-directional, Directional Fwd, Directional Rev Non-directional
I＞2 Current Set 0.08 In 4.0 In 4.75A
I＞2 Time Delay 0 100
I＞2 TMS 0.025 1.2 0.45

15. VOLTAGE RESTRAINED OVER CURRENT (51V)
This protection reacts on voltage, negative sequence voltage and current as a back up
protection to detect grounding and phase to phase short circuits.
The voltage restrained over current adjust the pickup setting and characteristic curve of the
overcurrent function based on the system voltage. The relay is set pickup current greater
than overload, and sensitivity increase as the voltage drops.
The voltage controlled overcurrent relays uses and under voltage element to the supervise
the operation of overcurrent element.
When voltage drops below the set value the under voltage element permits the operation
of the time over current units.

16. Voltage Restrained Overcurrent (51V)
I＞3 Status Disabled or Enabled Enabled
I＞3 Direction Non-directional, Directional Fwd, Directional Rev Non-directional
I＞3 Current Set 0.08 In 32 In 5.15A
I＞3 Time Delay 0 s 100 s 0.0s
I＞Function Link
Bit 0=VTS Blocks I＞1, Bit 1=VTS Blocks I＞2, Bit 2=VTS Blocks I
＞3, Bit 3=VTS Block I＞4, Bits 4~7 are not used.
Bit 3=VTS Block I>4

17. NEGATIVE SEQUENCE OVER CURRENT (46G)
NPS protection is provided to protect generator against the unbalanced load and as a back
up for uncleared unsymmetrical faults.
The Negative sequence current create magnetic flux in the air gap which rotates in
opposition to the rotor. This flux induce eddy current as twice the fundamental frequency in
the rotor which cause overheating of rotor body, wedged and retaining rings. The rotor body
gets heated instantaneously.

18. RELAY SETTINGS :

19. 95% STATOR EARTH FAULT PROTECTION (64G1)
There is a limit on the percentage of winding that can be protected by stator earth fault
element. For the earth fault close to generator neutral side, the driving voltage will very low
and hence fault current will be severely reduced. In practice approximately 95% of stator
winding will can be protected for the fault in last 5% of winding the fault current is very low
which can not be detected by this type of earth fault.
Single phase earth fault with in 85%-95% range from terminal of stator winding can be
protected by fundamental zero-sequence overvoltage protection.

20. SCHEME ARRANGEMENT FOR 95% & 100% STATOR
EARTH FAULT PROTECTION :

21. RELAY SETTING :

22. 100% STATOR EARTH FAULT PROTECTION (64G2)
In this protection 100% stator winding is covered in protection zone. Low probability
fault occurring in 5% of the stator winding near to star point is due to mechanical
damage such as creepage of conductors, loosening of boult etc.
Most of the generator produce third harmonic voltage due to non linearities in
magnetic circuit of generator design, under normal operating condition distribution of
third harmonic voltage along the stator winding. The value increases with the load.
To detect the fault in last 5% of winding the relay is provided with under & over
voltage element.

23. The third harmonic under voltage element is applicable when the neutral voltage measured
is available at the neutral end of the generator.
To prevent false tripping operation when the machine is dead or under no load condition
power interlock is used to detect load.

24. SCHEME ARRANGEMENT FOR 95% & 100% STATOR
EARTH FAULT PROTECTION :

25. RELAY SETTING :
V<Inhibit Set 80V
P<Inhibit set Disabled, Enabled Enabled
P<Inhibit set 20.00W
Q<Inhibit set Disabled, Enabled Disabled
S<Inhibit set Disabled, Enabled Disabled

26. POLE SLIPPING PROTECTION (78G)
Sudden changes or shock in electrical system such as line switching operation, transient
system fault, generator governor failure, excitation system failure, reconnection of islanding
system without synchronization, large jumps in load or fault may lead to power system
oscillations which appears as regular variations of current and angular separation between
systems.
At times, this power swing becomes so sever that it can lead lost of synchronism between
generator and system, this condition recognized as out of step or pole slipping.

27. OVER FLUXING PROTECTION (99G)
Over fluxing or over excitation of generator can increase if the ratio of voltage to
frequency (V/HZ) increased certain limits. High voltage or low frequency can rise V/Hz
ratio, it will produce high flux densities in magnetic core of machines, this can cause core of
generator to be saturated & stary flux to be induced in un laminated components that have
not been designed to carry this flux, the resulting eddy current in solid components (e.g. core,
bolts & supporting structure) can cause rapid overheating & damage.
Over fluxing likely to occur during machine start up & shutdown while the generator is not
connected to the system.
Failure in automatic control of excitation system or errors in manual control of machine field
current, could allow excessive voltage to be generated.
Sudden loss of load could cause an overvoltage condition, if generator excitation system
does not respond properly.

28. BREAKER FAILURE PROTECTION (50LBB)
The LBB protection is used as back up protection of main protection, during this
protection, relay sense the breaker position, flow of current & trip command
generated by itself.

29. RELAY SETTINGS

30. BACK UP IMPEDANCE PROTECTION (27G)
Generator is source of power & it will supply fault current until they are cleared by
the protection system.
Back up protection applied at the generator so that faults are cleared in the event of
downstream protection/breaker fails to operate.
Under impedance element is set to monitor the system impedance at machine
terminal, if the measured impedance falls below set value then relay will operate.

31. DEAD MACHINE (61B)
Accidental energization of breaker when machine is not running can cause severe
damage to machine.
If the breaker is closed when machine is not running then generator will began act as
induction motor with the surface of rotor core & rotor winding slot wedges acting as
rotor conductor ,this abnormal current will result in overheating and damaged.
The relay element is enabled when the machine is not running ,there for element can
have very low current setting, the undervoltage threshold typically set to 85% of
nominal voltage to ensure that machine is not running.

32. RELAY SETTINGS :

33. LOSS OF EXCITATION WITH UNDER
VOLTAGE(27/40)
Complete loss of excitation may arise as a result of accidental tripping of excitation
system, failure of field breaker , an open circuit or short circuit happened in DC
circuit, flashover of any slip ring or failure of excitation source.
When excitation of synchronous generator fails its internal emf will decay and output
of active power will reduce and it will draw large reactive power grid, hence
machine will act as a induction generator, after filed failure circulating current will
induce in rotor core and rotor heat, also stator winding may get damaged severely.

34. RELAY SETTING

35. ROTOR GROUND FAULT PROTECTION (64R)
The Generator rotor earth fault protection is provided to protect the generator rotor
and exciter rotor winding against earth fault. The rotor earth fault relay sense the
resistance between rotor winding and ground through carbon brushes mounted on slip
rings.

36. MICOM P342 RELAY INCORPORATED WITH FOLLOWING
PROTECTIONS :
1. Low Forward Power Protection (37G)
2. Reverse Power Protection (32G)
3. Under frequency Protection (81U)
4. Over frequency Protection (81O)
5. Over Voltage Protection (59G)
6. Under Voltage Protection (27G)
7. Generator Stator Water flow minimum
8. Excitation system failure
9. Excitation transformer temp. high

37. LOW FORWARD POWER PROTECTION (37G)
When machine is generating and CB connecting the generator to the system is
tripped, the electrical load on the generator is cut, this could lead to generator over
speed if the machine input power is not reduced quickly.
Trapped system in the turbine, risk of over speed damage to such sets, interlock is
used for non urgent tripping of the generator breaker and excitation system with a
low forward power check.

38. Relay Settings

39. REVERSE POWER PROTECTION (32G)
It is back up protection to low forward protection. If the generator prime mover falls,
generator is connected in parallel with another source of electrical supply will begin as
“Motor” . The generator will operator as a synchronous motor and drive the turbine. Here in
this condition draw the real power (MW) from the bus & and export reactive power (MVAR)
to grid.
If the filed breaker closed but field current is zero, the generator will act as an induction
motor. Here in this condition generator will draw real power as well as reactive power from
the grid

40. In this condition generator will not be harmed by synchronous motoring but steam
turbine can be harmed due to winding loss of causing overheating of turbine blade if
continued long enough.
The sensitivity and setting of the relay is depending on the type of prime movers.
Causes of Reverse Power Protection :
Two or more machines are running in parallel and prime mover of machine fails.

41. RELAY SETTINGS

42. OVER VOLTAGE (59G)
The over voltage protection will act as a back up protection of the over fluxing
protection. The over voltage protection is required to protect the stator conductor
insulation of the generator. This is directly connected to Gen. phase side PT secondary
terminals.
Causes of over voltage :
Prime mover over speed due to sudden loss of load.
Voltage regulator (AVR) is defective.

43. RELAY SETTING

44. OVER FREQUENCY PROTECTION (81O)
The over frequency protection is provided to protect the generator against over
speed. Sever high speed operation of generator set could result in damaged.
Causes of over frequency :
Demand of active power suddenly decreased in power grid
Full or partial load rejection (Load thrown off)
Over shedding of load during major disturbance. Generator CB oprn during load.

45. RELAY SETTING :

46. UNDER FREQUENCY PROTECTION (81U)
The under frequency protection is used to protect the generator against operation at
low frequency.
If the frequency goes down, the speed of turbine get reduced, hence the steam may
get condensed on turbine blades and damage them.
Causes of under frequency :
Demand of active power suddenly increased in grid.
Islanded system two or more system running in parallel on more than 70% load &
outage of nay machine due to fault increased the load to another machine will result
in under frequency operation.

47. RELAY SETTING :

48. UNDER VOLTAGE (27G)
Generator are usually designed to operate continuously at a minimum voltage of 95% of its
rated voltage, while generate power at rated frequency.
Generator terminal voltage goes lower than 85% of its rated voltage may result in
undesirable effects such as reduction in stability limits, import of excessive reactive power
from grid, malfunctioning of voltage sensing devices and equipment.
The under voltage relay works in secondary value if PT which is connected with generator
output terminals.

49. RELAY SETTING:

50. NON DIRECT PROTECTION
❑Generator stator water flow minimum
Generator CW flow minimum link to be closed, protection simulated from C&I cabinet ( if
stator water pump not running then signal will come high)
❑Excitation system fault
Protection simulated from AVR end at micom P342 relay assigned o/p becomes high & EXT
trip LED becomes high.
❑Excitation transformer temp. high
Protection simulated from excitation transformer end for low setting at P342 relay o/p
assigned becomes high.
For high setting at P342 assigned relay becomes high.

51. Thank You