TRANSFORMER PROTECTIONS
a) OVERALL DIFFERENTIAL PROTECTION (87 GT)
This protection which is used as the differential protection of the transformer, also covers
the generator and unit transformer. The differential transformer protection measuring
circuit is based on the well-known MERZ-PRICE circulating current principle.
Fig-1 shows the relay functional block diagram. The output from each bias resistance
transformer T3 to T5 proportional to the appropriate primary line currents, are rectified
and summed to produce a bias resistance voltage. Any resulting difference current is
circulated through the transformer T1 & T2. The output from T1 is rectified and combined
with the bias voltage to produce a signal, which is applied to the amplitude comparator.
The comparator output is in the form of pulses which vary in width depending on the
amplitude of the combined bias and difference voltages where the measurement of the
interval between these pulse indicate less than a preset time, an internal fault is indicated
and a trip signal initiated after a short time delay (1/f sec), level set by the bias.
An unrestrained high set circuit, which monitors the differential current, will over ride the
amplitude comparator circuit and operate the relay output element when the difference
current is above the high set settings.
Fig-2 shows the basic circuit diagram of the differential protection and fig-3 shows the
current direction of the restraint/differential transformers in the relay. The currents I1, I2,
& I3 are the output of generator CT, UT CT and GT CT respectively. These currents is
passing through the star connected restraint transformer, the algebraic sum of vector
(I1+I2+I3 = I4) is passing through the differential transformer, which will give the output
for operating the relay (87).
b) OVER FLUXING PROTECTION (59V/F)
This is designed to protect the transformer from damages caused by the flux density in the
core exceeds the designed value. The excessive flux can cause serious overheating of
metallic parts and in extreme case can cause localized rapid melting of generator and
transformer core laminations. Over fluxing can be caused by regulator failure, load
reduction or excessive excitation with generator off-line it can also result from decreasing
speed while the regulator or the operator attempts to maintain rated stator voltage. Its
main application is to protect the transformers where, unless considerable care is taken,
the flux density can become excessive during the running up or running down sequence.
The flux density in the core depends on the ratio of terminal voltage (V) divided by the
frequency (f). Normally the over fluxing withstand characteristics of the transformer are
120% over fluxing for 2 minutes
135% over fluxing for 1 minutes
140% over fluxing for 5 seconds.
Whenever the v/f ratio of the transformer exceeds the pre-set time, the relay will operate
and initiate
• Running down the AVR if the machine is off the bus bar.
• Tripping the GT breaker.
OPERATING PRINCIPLE:
The basic principle of the relay is to produce an alternating voltage, which is proportional
to the ratio of voltage & frequency, and to compare this with a fixed voltage. When the
peak of the alternating voltage exceeds the fixed dc reference, the first timer is started. At
the end of the fixed timer cycle the second adjustable timer is initiated.
To obtain the correct measuring quantity the applied voltage V is converted to a current
by means of a resistor R. This V/R is arranged to flow through a capacitor C to produce
an output voltage
V/2 π f RC.
Over fluxing relay which consists of Voltage/Frequency measuring circuit, which output
is given to a comparator, compares with dc reference and to give an output after a fixed
time delay of 0.5 to 1.0 seconds. After the end of fixed time delay, the 2nd variable timer
initiates. The fixed time auxiliary has one of its two pairs of contact wired out which is
normally arranged to operate a follower.
c) GENERATOR TRANSFORMER RESTRICTED EARTH FAULT PROTECTION (64)
In addition to overall differential protection, a restricted earth fault protection covering
the transformer HV winding only is provided. The zone of protection extends from CT
provided on the transformer neutral end to the CT provided on the transformer bushings.
The relay is high impedance type and high speed of operation. A non-linear resistance is
connected across the relay terminal to limit the voltage developed during serial internal
fault. This protection energizes Class-A trip of the turbo-generator.
REF relay
Transformer
SCHEME OF RESTRICTED E/F PROTECTION
d) GENERATOR TRANSFORMER BACK-UP OVER CURRENT PROTECTION FOR PHASE FAULT (51)
This protection consists of a 3 phase over current relay. The relay is 3-pole version of
very inverse time over current relay plus high set instantaneous over current relay. This
will act as the back up protection for the transformer fault due to the fault current flowing
from system side. This may also serve limited back up protection function for fault
external to the transformer. This will energize Class-A trip.
R
e) GENERATOR TRANSFORMER BACK UP EARTH FAULT PROTECTION (51N)
This is a simple inverse type over current relay connected to the neutral CT of
transformer. This relay provided back up function for fault both internal and external to
the transformer, This protection energizes Class-A trip.
f) OVER LOAD MONITORING (49)
Measuring oil temperature and winding temperature indirectly monitors the loading of the
generator transformer. The oil and winding temperature indicators are provided with
contacts for initiating alarms as a first stage and tripping as the second stage whenever the
oil and winding temperature limits are exceeded. The oil temperature /winding
temperature trips are routed through Class-C trip.
g) GAS PROTECTION (63)
A Buchholz relay is supplied along with the transformer. The relay has two contacts one
closes on slow gas formation and initiate alarm. The second contact closes of sudden
surge of oil flow in case of severe internal fault and this contact is wired for tripping the
unit in Class-A trip.
The relay consists of two float switches contained in a closed housing, which is located in
the pipe from transformer to conservator tank. Any internal fault in the transformer
comes, the oil decomposes and the generating gases which passes up the pipe towards the
conservator and is trapped in the relay. In this two float relay the top float responds the
slow accumulation of gas due to mild and incipient fault, the lower float being deflected
by the oil surges caused by a major fault. The float control contacts, in the first stage give
an alarm and second case to isolate the transformer.
• What are the set values of generator protection?
TYPE OF PROTECTION AND ITS SET VALUES
No. Type of Protection Set Values CT/PT Ratio Time Delay Class
1. Generator differential 0.5A(10%) 10000/5 Inst. Class-A1
2. Generator Inter turn 0.5A(10%) 5000/5 Inst. Class-A1
3 Generator reverse power 0.5% 10000/5 5 sec Class-A1
(stage 2 Tx trip)
4. 100% Stator Earth Fault ND = 5V(3r
harmonic 70
100%)
16.5 kV/110V 1.0 sec Class-A1
5. 2nd Rotor Earth Fault 1.0 mA --- --- Class-A1
6. Over Frequency 51.5 Hz 16.5 kV/110V 0.1 sec 86 BG
7. Over Voltage 120% 16.5 kV/110V 2.0 sec Class-A
8. Overall Differential 1.0A 10000/5A Inst. Class-A1
9. GT Restricted E/F 0.1A 800/1A Inst. Class-A1
10. GT Gas Protection --- ---- Inst. Class-A1
11. GT Fire --- --- Inst. Class-A1
12. GT Over Current PSM-1.0
Inst. – 8.0
800/1 A TMS=0.4 Class-A1
13. GT Earth Fault PSM-0.2
TMS-0.52
800/1A 2.0 sec Class-A1
14. Impedance Protection
Stage-1
--- 10000/5A 2.0 sec Class-A2
15. Generator Over Current
During Starting
50 mA 10000/5A Inst. Class-A2
16. Generator Back-up Earth
Fault
PSM-5.4V 16.5 kV/110V TMS =0.3 Class-A2
17. Stator Earth Fault During Starting 100 mA 300/1A Inst. Class-A2
18. Low Forward Power 0.5% of FP 10000/5A 2.5 sec Class-B1
19. GT Over Fluxing Stage-1 120% --- 2 min Class-B
20. GT Over Fluxing Stage-2 135% --- 1 min Class-B
21. Negative Sequence 5% 10000/5A Inverse Class-B
22. Field Failure --- 10000/5A Inverse Class-A2
23. Under Frequency 47.77 Hz 16.5 kV/110V 4 sec Class-C
24. GT Winding Temp. High 130O C --- --- Class-C
25. GT Oil Temp. High 90O C --- --- Class-C
• What is arc and what is spark?
Spark - the heat produced that ignites, due to the rubbing of two metals is called the
spark.
Arc – the electrical discharge between two electrodes is called the arc. Arc is the
self-sustained discharge of electricity between electrodes in a gas or vapour, which
has a high voltage discharge at the cathode.
• What precautions should be taken while meggering main generator?
All PT’s are racked out.
Earthing transformer grounding terminals disconnected.
Barring gear shall be kept off.
Stator water shall be drained fully and hot air blown through conductors.
Generator flexible lines shall be disconnected to isolate GT/UT.
• What is the speed equation for AC machine?
N = 120 f / P
N – Speed in RPM
f - Frequency in Hz
P – Number of poles
• What is emf equation of alternator?
Emf = 4.44 kc kd f ∅ T volts.
Kd = Distribution factor = sin m β/2
m sin β/2
kc/kp = Coil span factor /Pitch factor = cos α/2
kf = Form factor = 1.11
∴Average emf induced / Cycle = ∅ N P/ 60
= ∅ P ∗120 f
60 * P
=2 f ∅ volt
If Z is the number of conductors = 2T (T = two sides of conductor)
emf induced = 2 f ∅ Z =2 f ∅ 2T = 4 f ∅ T
∴ RMS value of emf induced = form factor * emf
= 1.11 * 4 f ∅ T
= 4.44 f ∅ T volts.
• What is the emf equation for DC generator?
P * ∅ * Z * N
60 * A
A = number of parallel paths. That is for lap winding it is equal to Z and for wave
winding it equal to 2.
• What are the factors, which varies terminal voltage of generator?
a) Voltage drop due to resistance (Ra drop). This is negligible.
b) Voltage drops due to leakage reactance (XL).
c) Voltage drops due to armature reaction.
• What is meant by Armature reaction?
The effect of armature flux on the main field flux is called Armature reaction, where
armature flux weakens the main field flux. In Alternator power factor contributes
more importance in Armature reaction.
a) In Unity power factor field strength is average and effect is distortional. So
voltage variation will not be too much.
b) In lagging power factor armature flux is directly opposite to the main field flux.
That is armature flux is lagging 90ο by main field flux. So the result is
demagnetizing the field. Due to less field flux less voltage at the alternator
terminals and excitation required is more.
c) In leading power factor armature flux is leading by 90ο to the main field flux. The
result is additive and main field strength is more and excitation has to be reduced.
Otherwise end parts or overhang portion of the generator will heat.
• What is meant by voltage regulation?
If there is a change in load, there is a change in terminal voltage. This change not
only depends upon the load but also on power factor. The voltage regulation is
termed as the rise in voltage when full load is removed divided by rated terminal
voltage (Excitation and speed remains constant).
∴ Regulation in % = E0 – V
V
In case of leading power factor terminal voltage will fall and regulation is negative.
PF leading
Terminal
Voltage PF unity
PF lagging
Load current
Generator voltage characteristics
• Why double squirrel cage motor used in barring gear? Why?
To have high starting torque.
In AC motors torque is directly proportional to φ (flux), I2 and cosφ2.
i.e T ;φ (flux* I2 * cos φ2.
∴ T = k * φ (flux)* I2 * cos φ2.
Rotor at standstill E2;φ (flux)
∴ T = k * E2 * I2 * cos φ2.
In double squirrel cage motor inner cage is low resistive and high inductive. The
outer cage is high resistive and low inductive. In case of inner cage Z (impedance) is
less (XL = 2 f L). If the rotor is having high inductance at starting I2 will lag E2 by
large and cos φ2 (Rotor PF = R2 / Z2) is very less. So torque
is less.
At staring rotor torque is proportional to the rotor
resistance. At starting inductance is high and the Z is--
--------
• What are the logics adopted to close the field breaker?
a) Turbine speed 2880 rpm.
b) Class A, B and BG trip reset.
c) Auto/manual reference minimum.
d) Auto/manual channels supply normal.
e) FB closing circuit healthy.
• What you mean by positive sequence, negative sequence and zero sequence of
voltage?
Positive phase sequence
A system of vectors is said to have positive sequence if they are all of equal
magnitude and are displaced by 120° with same time interval to arrive at fixed axis
of reference as that of generated voltage. The positive phase sequence is represented
below and the vectors arrive along X-axis in order 1, 2, 3 and conscript P has been
used to designate as positive sequence.
E3P
Anti clock direction
120°
E3P
E3P
Negative phase sequence
A system of vectors is said to have a negative phase sequence if they are of equal
magnitude displaced at an angle of 120° but arrive at the axis of reference at the
regular interval same as that of positive phase sequence but in order of 1, 3, 2. That
is the order is reversed.
E3N
Clock direction
120°
E3N
E3N
Zero sequence
A system of vectors in a phase system is said to have zero phase sequence if all the
three vectors are not displaced from each other and there will be no phase sequence
in such cases. The current or voltages in the 3-phase circuit vary simultaneously in
all the 3- phases. Such phase sequence is zero phase sequence.
E1O
E2O
E3O
• What is rotor and stator resistance values?
Rotor resistance = 98.1 m
Stator winding resistance’s
R φ = 3.1/3.1
Y φ = 3.1/3.1
B φ = 3.1/3.1
• What is the rating of generator PT fuse?
24 kV, 3.15 Amps.
• What is the wearing rate of generator Slipring?
Generator Slipring wearing rate is 0.025 mm /1000 hrs.
• What is the brush pressure on Slipring?
Recommended brush pressure in the Slipring is 150 to 200 gms/cm2 (0.9 to 1 kg).
• What are the properties of hydrogen and DM water?
Hydrogen
a. Windage losses are less. Hence efficiency increased.
b. Heat transfer is more. Hence output per volume is increased.
c. No corona discharge, which makes insulation life long.
d. Lesser denser and penetration and cooling more.
e. No fire risk at purity 4% to 74%.
DM Water
a. Non toxic and low viscosity.
b. High thermal conductivity.
c. Low conductivity.
d. Freedom from fire risk.
e. External heat exchanger used.
• What are the chemical tests on hydrogen and DM water?
Hydrogen
a) Hydrogen purity in % (volume/volume).
b) Relative humidity in % (30% is nominal).
DM water
a) PH of DM water (less than 6.5 is acidic and more than 7 is alkaline where oxygen
is not forming). PH is also called IP (isotopic purity).
b) Conductivity.
c) Copper traces.
d) Dissolved oxygen (to trace corrosion and 1.2% is more).
• What are the logic’s adopted in barring gear motor?
For start permission
a) Local or remote start.
b) JOP is running.
c) Motor hand barring is permissive.
d) 42 contactor in MCC is off.
e) Turbine speed is <100 rpm.
Start permission (42S of MCC)
a) All above
b) Bearing oil pressure is >0.35 kg/ cm2.
c) No thermal over load of 42S.
d) No one-DG condition.
Start permission (42 of MCC)
a) Start permissive of 42S.
b) Barring gear engage or motor speed reached to 1475 rpm.
c) Turbine speed is <100 rpm.
d) Bearing oil pressure is >0.35 kg/ cm2.
e) No thermal over load of 42.
f) No one-DG condition.
• What is the equation for resistance measurement of PT 100 thermocouple?
°C = (R-100) / 0.39
• What are the requirements for synchronization and setting for generator?
a) Same phase sequence.
b) Voltage should in-phase and angle should not be more than 10°.
c) Voltage value must be same and difference of 5% is allowed.
d) Frequency should be same and difference of 0.1% i.e. 0.05 Hz is allowed.
• What is the recommended IR value for generator?
Main generator is class B insulated machine. Without stator water recommended
insulation value for the generator is R60 = kV + 1 MΩ
R60 – minimum recommended IR in MΩ of entire winding at 40°C of 60 Sec.
kV – rated voltage.
For the IR measurement 1 kV megger should be used.
• What is the type turbine installed in KGS?
Tandem compounded to expansion of steam, impulse reaction type.
• State HP & LP turbine steam values.
HP LP
Pressure Flow Temp Pressure Flow Temp
I/L 40 kg/cm2 1333 t/h 250°C I/L 5.664 kg/cm2 232.9°C
O/L 6.02 kg/cm2 O/L
Wetness (I/L) 0.26% Wetness (I/L)
Wetness (O/L) 11.058% Wetness (O/L)
• State turbine governor setting.
On 2560 rpm turbine governor becomes effective and on 2760 rpm is turbine
governor take over speed.
• What is requirement of speeder gear assembly?
To bring the turbine to synchronous speed and get tight lock with grid by grid
frequency. BPC signal is given in Auto mode to the speeder gear motor.
• What is the purpose of LLG?
To ensure that the turbine load never exceed the reactor output, to incorporate
turbine follow reactor feature governing system.
• What is the purpose of OSLG?
This gear mainly used to control the steam flow so as to limit the machine from over
speeding. On following occasions the over speed limiting gear acts.
a) When the flow of steam corresponds to load is 2/3 and
b) Electrical power on generator falls 1/3 of full load.
• What is the logic in lubrication oil pump system?
Normally main oil pump (MOP) will feed the required lub oil to turbine governor
and lubrication. If the pressure drops to 5.3 kg/cm2 6.6 kV 373 kW Aux. Oil Pump
will start. If further pressure falls to <0.65 kg/cm2 Flushing Oil Pump will start. If
further pressure drops <0.35 kg/cm2 Emergency Oil Pump will start.
Lubricating oil inlet temperature will be 40°C and outlet temperature will be 70°C.
• What is the purpose of TOPP (turbine oil purification plant)?
The purpose of TOPP is to remove the water ingress in turbine oil system from the
gland leaks, cooler leakage, and solid metal particles, which are produced due to rust,
wear of bearings and to normalize the low quality oil.
The remove capacity of TOPP is, for solids – 5 microns and for water – 300 to 500
parts per milli.
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a) OVERALL DIFFERENTIAL PROTECTION (87 GT)
This protection which is used as the differential protection of the transformer, also covers
the generator and unit transformer. The differential transformer protection measuring
circuit is based on the well-known MERZ-PRICE circulating current principle.
Fig-1 shows the relay functional block diagram. The output from each bias resistance
transformer T3 to T5 proportional to the appropriate primary line currents, are rectified
and summed to produce a bias resistance voltage. Any resulting difference current is
circulated through the transformer T1 & T2. The output from T1 is rectified and combined
with the bias voltage to produce a signal, which is applied to the amplitude comparator.
The comparator output is in the form of pulses which vary in width depending on the
amplitude of the combined bias and difference voltages where the measurement of the
interval between these pulse indicate less than a preset time, an internal fault is indicated
and a trip signal initiated after a short time delay (1/f sec), level set by the bias.
An unrestrained high set circuit, which monitors the differential current, will over ride the
amplitude comparator circuit and operate the relay output element when the difference
current is above the high set settings.
Fig-2 shows the basic circuit diagram of the differential protection and fig-3 shows the
current direction of the restraint/differential transformers in the relay. The currents I1, I2,
& I3 are the output of generator CT, UT CT and GT CT respectively. These currents is
passing through the star connected restraint transformer, the algebraic sum of vector
(I1+I2+I3 = I4) is passing through the differential transformer, which will give the output
for operating the relay (87).
b) OVER FLUXING PROTECTION (59V/F)
This is designed to protect the transformer from damages caused by the flux density in the
core exceeds the designed value. The excessive flux can cause serious overheating of
metallic parts and in extreme case can cause localized rapid melting of generator and
transformer core laminations. Over fluxing can be caused by regulator failure, load
reduction or excessive excitation with generator off-line it can also result from decreasing
speed while the regulator or the operator attempts to maintain rated stator voltage. Its
main application is to protect the transformers where, unless considerable care is taken,
the flux density can become excessive during the running up or running down sequence.
The flux density in the core depends on the ratio of terminal voltage (V) divided by the
frequency (f). Normally the over fluxing withstand characteristics of the transformer are
120% over fluxing for 2 minutes
135% over fluxing for 1 minutes
140% over fluxing for 5 seconds.
Whenever the v/f ratio of the transformer exceeds the pre-set time, the relay will operate
and initiate
• Running down the AVR if the machine is off the bus bar.
• Tripping the GT breaker.
OPERATING PRINCIPLE:
The basic principle of the relay is to produce an alternating voltage, which is proportional
to the ratio of voltage & frequency, and to compare this with a fixed voltage. When the
peak of the alternating voltage exceeds the fixed dc reference, the first timer is started. At
the end of the fixed timer cycle the second adjustable timer is initiated.
To obtain the correct measuring quantity the applied voltage V is converted to a current
by means of a resistor R. This V/R is arranged to flow through a capacitor C to produce
an output voltage
V/2 π f RC.
Over fluxing relay which consists of Voltage/Frequency measuring circuit, which output
is given to a comparator, compares with dc reference and to give an output after a fixed
time delay of 0.5 to 1.0 seconds. After the end of fixed time delay, the 2nd variable timer
initiates. The fixed time auxiliary has one of its two pairs of contact wired out which is
normally arranged to operate a follower.
c) GENERATOR TRANSFORMER RESTRICTED EARTH FAULT PROTECTION (64)
In addition to overall differential protection, a restricted earth fault protection covering
the transformer HV winding only is provided. The zone of protection extends from CT
provided on the transformer neutral end to the CT provided on the transformer bushings.
The relay is high impedance type and high speed of operation. A non-linear resistance is
connected across the relay terminal to limit the voltage developed during serial internal
fault. This protection energizes Class-A trip of the turbo-generator.
REF relay
Transformer
SCHEME OF RESTRICTED E/F PROTECTION
d) GENERATOR TRANSFORMER BACK-UP OVER CURRENT PROTECTION FOR PHASE FAULT (51)
This protection consists of a 3 phase over current relay. The relay is 3-pole version of
very inverse time over current relay plus high set instantaneous over current relay. This
will act as the back up protection for the transformer fault due to the fault current flowing
from system side. This may also serve limited back up protection function for fault
external to the transformer. This will energize Class-A trip.
R
e) GENERATOR TRANSFORMER BACK UP EARTH FAULT PROTECTION (51N)
This is a simple inverse type over current relay connected to the neutral CT of
transformer. This relay provided back up function for fault both internal and external to
the transformer, This protection energizes Class-A trip.
f) OVER LOAD MONITORING (49)
Measuring oil temperature and winding temperature indirectly monitors the loading of the
generator transformer. The oil and winding temperature indicators are provided with
contacts for initiating alarms as a first stage and tripping as the second stage whenever the
oil and winding temperature limits are exceeded. The oil temperature /winding
temperature trips are routed through Class-C trip.
g) GAS PROTECTION (63)
A Buchholz relay is supplied along with the transformer. The relay has two contacts one
closes on slow gas formation and initiate alarm. The second contact closes of sudden
surge of oil flow in case of severe internal fault and this contact is wired for tripping the
unit in Class-A trip.
The relay consists of two float switches contained in a closed housing, which is located in
the pipe from transformer to conservator tank. Any internal fault in the transformer
comes, the oil decomposes and the generating gases which passes up the pipe towards the
conservator and is trapped in the relay. In this two float relay the top float responds the
slow accumulation of gas due to mild and incipient fault, the lower float being deflected
by the oil surges caused by a major fault. The float control contacts, in the first stage give
an alarm and second case to isolate the transformer.
• What are the set values of generator protection?
TYPE OF PROTECTION AND ITS SET VALUES
No. Type of Protection Set Values CT/PT Ratio Time Delay Class
1. Generator differential 0.5A(10%) 10000/5 Inst. Class-A1
2. Generator Inter turn 0.5A(10%) 5000/5 Inst. Class-A1
3 Generator reverse power 0.5% 10000/5 5 sec Class-A1
(stage 2 Tx trip)
4. 100% Stator Earth Fault ND = 5V(3r
harmonic 70
100%)
16.5 kV/110V 1.0 sec Class-A1
5. 2nd Rotor Earth Fault 1.0 mA --- --- Class-A1
6. Over Frequency 51.5 Hz 16.5 kV/110V 0.1 sec 86 BG
7. Over Voltage 120% 16.5 kV/110V 2.0 sec Class-A
8. Overall Differential 1.0A 10000/5A Inst. Class-A1
9. GT Restricted E/F 0.1A 800/1A Inst. Class-A1
10. GT Gas Protection --- ---- Inst. Class-A1
11. GT Fire --- --- Inst. Class-A1
12. GT Over Current PSM-1.0
Inst. – 8.0
800/1 A TMS=0.4 Class-A1
13. GT Earth Fault PSM-0.2
TMS-0.52
800/1A 2.0 sec Class-A1
14. Impedance Protection
Stage-1
--- 10000/5A 2.0 sec Class-A2
15. Generator Over Current
During Starting
50 mA 10000/5A Inst. Class-A2
16. Generator Back-up Earth
Fault
PSM-5.4V 16.5 kV/110V TMS =0.3 Class-A2
17. Stator Earth Fault During Starting 100 mA 300/1A Inst. Class-A2
18. Low Forward Power 0.5% of FP 10000/5A 2.5 sec Class-B1
19. GT Over Fluxing Stage-1 120% --- 2 min Class-B
20. GT Over Fluxing Stage-2 135% --- 1 min Class-B
21. Negative Sequence 5% 10000/5A Inverse Class-B
22. Field Failure --- 10000/5A Inverse Class-A2
23. Under Frequency 47.77 Hz 16.5 kV/110V 4 sec Class-C
24. GT Winding Temp. High 130O C --- --- Class-C
25. GT Oil Temp. High 90O C --- --- Class-C
• What is arc and what is spark?
Spark - the heat produced that ignites, due to the rubbing of two metals is called the
spark.
Arc – the electrical discharge between two electrodes is called the arc. Arc is the
self-sustained discharge of electricity between electrodes in a gas or vapour, which
has a high voltage discharge at the cathode.
• What precautions should be taken while meggering main generator?
All PT’s are racked out.
Earthing transformer grounding terminals disconnected.
Barring gear shall be kept off.
Stator water shall be drained fully and hot air blown through conductors.
Generator flexible lines shall be disconnected to isolate GT/UT.
• What is the speed equation for AC machine?
N = 120 f / P
N – Speed in RPM
f - Frequency in Hz
P – Number of poles
• What is emf equation of alternator?
Emf = 4.44 kc kd f ∅ T volts.
Kd = Distribution factor = sin m β/2
m sin β/2
kc/kp = Coil span factor /Pitch factor = cos α/2
kf = Form factor = 1.11
∴Average emf induced / Cycle = ∅ N P/ 60
= ∅ P ∗120 f
60 * P
=2 f ∅ volt
If Z is the number of conductors = 2T (T = two sides of conductor)
emf induced = 2 f ∅ Z =2 f ∅ 2T = 4 f ∅ T
∴ RMS value of emf induced = form factor * emf
= 1.11 * 4 f ∅ T
= 4.44 f ∅ T volts.
• What is the emf equation for DC generator?
P * ∅ * Z * N
60 * A
A = number of parallel paths. That is for lap winding it is equal to Z and for wave
winding it equal to 2.
• What are the factors, which varies terminal voltage of generator?
a) Voltage drop due to resistance (Ra drop). This is negligible.
b) Voltage drops due to leakage reactance (XL).
c) Voltage drops due to armature reaction.
• What is meant by Armature reaction?
The effect of armature flux on the main field flux is called Armature reaction, where
armature flux weakens the main field flux. In Alternator power factor contributes
more importance in Armature reaction.
a) In Unity power factor field strength is average and effect is distortional. So
voltage variation will not be too much.
b) In lagging power factor armature flux is directly opposite to the main field flux.
That is armature flux is lagging 90ο by main field flux. So the result is
demagnetizing the field. Due to less field flux less voltage at the alternator
terminals and excitation required is more.
c) In leading power factor armature flux is leading by 90ο to the main field flux. The
result is additive and main field strength is more and excitation has to be reduced.
Otherwise end parts or overhang portion of the generator will heat.
• What is meant by voltage regulation?
If there is a change in load, there is a change in terminal voltage. This change not
only depends upon the load but also on power factor. The voltage regulation is
termed as the rise in voltage when full load is removed divided by rated terminal
voltage (Excitation and speed remains constant).
∴ Regulation in % = E0 – V
V
In case of leading power factor terminal voltage will fall and regulation is negative.
PF leading
Terminal
Voltage PF unity
PF lagging
Load current
Generator voltage characteristics
• Why double squirrel cage motor used in barring gear? Why?
To have high starting torque.
In AC motors torque is directly proportional to φ (flux), I2 and cosφ2.
i.e T ;φ (flux* I2 * cos φ2.
∴ T = k * φ (flux)* I2 * cos φ2.
Rotor at standstill E2;φ (flux)
∴ T = k * E2 * I2 * cos φ2.
In double squirrel cage motor inner cage is low resistive and high inductive. The
outer cage is high resistive and low inductive. In case of inner cage Z (impedance) is
less (XL = 2 f L). If the rotor is having high inductance at starting I2 will lag E2 by
large and cos φ2 (Rotor PF = R2 / Z2) is very less. So torque
is less.
At staring rotor torque is proportional to the rotor
resistance. At starting inductance is high and the Z is--
--------
• What are the logics adopted to close the field breaker?
a) Turbine speed 2880 rpm.
b) Class A, B and BG trip reset.
c) Auto/manual reference minimum.
d) Auto/manual channels supply normal.
e) FB closing circuit healthy.
• What you mean by positive sequence, negative sequence and zero sequence of
voltage?
Positive phase sequence
A system of vectors is said to have positive sequence if they are all of equal
magnitude and are displaced by 120° with same time interval to arrive at fixed axis
of reference as that of generated voltage. The positive phase sequence is represented
below and the vectors arrive along X-axis in order 1, 2, 3 and conscript P has been
used to designate as positive sequence.
E3P
Anti clock direction
120°
E3P
E3P
Negative phase sequence
A system of vectors is said to have a negative phase sequence if they are of equal
magnitude displaced at an angle of 120° but arrive at the axis of reference at the
regular interval same as that of positive phase sequence but in order of 1, 3, 2. That
is the order is reversed.
E3N
Clock direction
120°
E3N
E3N
Zero sequence
A system of vectors in a phase system is said to have zero phase sequence if all the
three vectors are not displaced from each other and there will be no phase sequence
in such cases. The current or voltages in the 3-phase circuit vary simultaneously in
all the 3- phases. Such phase sequence is zero phase sequence.
E1O
E2O
E3O
• What is rotor and stator resistance values?
Rotor resistance = 98.1 m
Stator winding resistance’s
R φ = 3.1/3.1
Y φ = 3.1/3.1
B φ = 3.1/3.1
• What is the rating of generator PT fuse?
24 kV, 3.15 Amps.
• What is the wearing rate of generator Slipring?
Generator Slipring wearing rate is 0.025 mm /1000 hrs.
• What is the brush pressure on Slipring?
Recommended brush pressure in the Slipring is 150 to 200 gms/cm2 (0.9 to 1 kg).
• What are the properties of hydrogen and DM water?
Hydrogen
a. Windage losses are less. Hence efficiency increased.
b. Heat transfer is more. Hence output per volume is increased.
c. No corona discharge, which makes insulation life long.
d. Lesser denser and penetration and cooling more.
e. No fire risk at purity 4% to 74%.
DM Water
a. Non toxic and low viscosity.
b. High thermal conductivity.
c. Low conductivity.
d. Freedom from fire risk.
e. External heat exchanger used.
• What are the chemical tests on hydrogen and DM water?
Hydrogen
a) Hydrogen purity in % (volume/volume).
b) Relative humidity in % (30% is nominal).
DM water
a) PH of DM water (less than 6.5 is acidic and more than 7 is alkaline where oxygen
is not forming). PH is also called IP (isotopic purity).
b) Conductivity.
c) Copper traces.
d) Dissolved oxygen (to trace corrosion and 1.2% is more).
• What are the logic’s adopted in barring gear motor?
For start permission
a) Local or remote start.
b) JOP is running.
c) Motor hand barring is permissive.
d) 42 contactor in MCC is off.
e) Turbine speed is <100 rpm.
Start permission (42S of MCC)
a) All above
b) Bearing oil pressure is >0.35 kg/ cm2.
c) No thermal over load of 42S.
d) No one-DG condition.
Start permission (42 of MCC)
a) Start permissive of 42S.
b) Barring gear engage or motor speed reached to 1475 rpm.
c) Turbine speed is <100 rpm.
d) Bearing oil pressure is >0.35 kg/ cm2.
e) No thermal over load of 42.
f) No one-DG condition.
• What is the equation for resistance measurement of PT 100 thermocouple?
°C = (R-100) / 0.39
• What are the requirements for synchronization and setting for generator?
a) Same phase sequence.
b) Voltage should in-phase and angle should not be more than 10°.
c) Voltage value must be same and difference of 5% is allowed.
d) Frequency should be same and difference of 0.1% i.e. 0.05 Hz is allowed.
• What is the recommended IR value for generator?
Main generator is class B insulated machine. Without stator water recommended
insulation value for the generator is R60 = kV + 1 MΩ
R60 – minimum recommended IR in MΩ of entire winding at 40°C of 60 Sec.
kV – rated voltage.
For the IR measurement 1 kV megger should be used.
• What is the type turbine installed in KGS?
Tandem compounded to expansion of steam, impulse reaction type.
• State HP & LP turbine steam values.
HP LP
Pressure Flow Temp Pressure Flow Temp
I/L 40 kg/cm2 1333 t/h 250°C I/L 5.664 kg/cm2 232.9°C
O/L 6.02 kg/cm2 O/L
Wetness (I/L) 0.26% Wetness (I/L)
Wetness (O/L) 11.058% Wetness (O/L)
• State turbine governor setting.
On 2560 rpm turbine governor becomes effective and on 2760 rpm is turbine
governor take over speed.
• What is requirement of speeder gear assembly?
To bring the turbine to synchronous speed and get tight lock with grid by grid
frequency. BPC signal is given in Auto mode to the speeder gear motor.
• What is the purpose of LLG?
To ensure that the turbine load never exceed the reactor output, to incorporate
turbine follow reactor feature governing system.
• What is the purpose of OSLG?
This gear mainly used to control the steam flow so as to limit the machine from over
speeding. On following occasions the over speed limiting gear acts.
a) When the flow of steam corresponds to load is 2/3 and
b) Electrical power on generator falls 1/3 of full load.
• What is the logic in lubrication oil pump system?
Normally main oil pump (MOP) will feed the required lub oil to turbine governor
and lubrication. If the pressure drops to 5.3 kg/cm2 6.6 kV 373 kW Aux. Oil Pump
will start. If further pressure falls to <0.65 kg/cm2 Flushing Oil Pump will start. If
further pressure drops <0.35 kg/cm2 Emergency Oil Pump will start.
Lubricating oil inlet temperature will be 40°C and outlet temperature will be 70°C.
• What is the purpose of TOPP (turbine oil purification plant)?
The purpose of TOPP is to remove the water ingress in turbine oil system from the
gland leaks, cooler leakage, and solid metal particles, which are produced due to rust,
wear of bearings and to normalize the low quality oil.
The remove capacity of TOPP is, for solids – 5 microns and for water – 300 to 500
parts per milli.
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