MAIN GENERATOR AND IPBD
• How output of the generator is depends?
Output of the generator is the function of volume, length, dia, airgap, and speed.
• What you mean by Gas pickup method?
Sucking cooled hydrogen gas from the air gap, circulating in canals and removing
the heat from the rotor. While sucking the gas, gas comes through the stator parts
also and removes the heat from the stator parts. The heated gas circulated to the
hydrogen for cooling purpose by the fans installed at rotor shaft. Thus the stator and
rotor of the generator is cooled. This method is called Gas pickup method.
• When the hydrogen explosion will take place in main generator?
When hydrogen concentration in air is more than 4% and less than 74% causes the
explosion.
• Can we excite generator without hydrogen?
No
• What are the advantages of stator water?
a. High thermal capacity
b. Low electrical conductivity (Good insulator)
c. Low viscosity
d. Free of fire risk and non-toxic
e. Simple heat exchanger i.e. it can be circulate easily and cooled by heat exchanger
• How rotor windings are held in position against centrifugal force?
Rotor windings are held by duraluminium wedges and by non-magnetic steel
retaining rings in the overhang portion.
• What is the purpose of current carrying bolts in rotor?
Feeding DC current from slip ring to rotor winding.
• How rotor cooled?
Hydrogen picked up from stator core backspace, passes through ventilation canals on
rotor and comes out through adjacent canals. Shaft fans aid the hydrogen flow. Heat
from the hydrogen removed by 4 nos. of hydrogen coolers. (NAPW)
• What is rotor E/F relay setting?
1.0 mA
• Are we using DCCB in the plant?
Yes, generator field breaker
• How arc is quenched in Generator Field Breaker?
By magnetic blow out coils, arc is elongated very fastly, so resistance of arc
increases, soon becoming unstable and quenched by arc chutes.
• How generator is protected from switching surges and lightning surges?
Surge capacitor and lightning arrestor.
• What is the use of generator PT’s?
AVR, Protection & Metering.
• What is the difference between PT and normal transformer?
Burden of PT is less and burden of normal transformer is high.
• What is the % overload allowed for TG and DG?
For TG nil
For DG 110% for 2 hours.
• When TG works as induction generator?
When excitation alone lost.
• Why motoring should be prevented in TG and DG?
In TG motoring prevented due to the turbine limitation otherwise last stage blade
will fails.
In DG motoring prevented, because of unburned fuel catches fire in DG.
• Why GFB closed only after reaching rated speed?
To prevent over fluxing of transformers or generator.
Emf = 4.44 f φ Z A
If f frequency is reduced due to less speed,
φ = V / f Z A
And flux will be more to saturate the core of transformers or generator.
• What are the protective parameters to changeover AVR auto to manual?
a. PT supply fail.
b. Auto pulse fail.
c. Supply of limiter fail.
d. Supply of auto channel fail.
e. Regulated supply fail.
f. High auto reference.
• Why AVR changes over to manual on AVR PT fuse failure?
Because loss of feedback to voltage corrector.
• Will main generator differential relay pick up for generator earth faults?
No earth fault current limited to 5A, while differential setting is 10%.
• Why generator differential provided?
For generator phase to phase faults and 3 phase faults.
• What is the effect of loss of excitation on generator?
Large induced currents in rotor leads to rotor end part over heating.
Leading VAR taken from grid leads to severe voltage dips in grid, if grid is weak.
Stability of machine lost.
Stator overheating.
Machine speed rises slightly.
• What are the limiters provided in excitation system?
a. Rotor current limiter (3000 A)
b. Rotor angle limiter (75° lag)
c. Under excitation limiter.
d. Stator current limiter (lead 10000 A)
e. Stator current limiter (lag 10000 A)
f. N – 2 limiter.
• What is the effect of unbalance currents in generator?
Double frequency currents are induced in metal parts of rotor and overheating of
retaining rings and non-magnetic wedges.
• Why low forward power relay used in parallel to reverse power relay?
A small steam leak through CIES valves will keep the machine floating on to grid at
low power. So even if turbine trip, relay picks up, there is no trip actually. So low
forward power relay (0.54%) used to detect the condition.
• Why earth fault current of generator should not be reduced below 5A?
To limit over voltage due to neutral shift.
• Why not high resistance for earth fault than using grounding transformer & resistor
0.45 ohms?
It is mechanically unwide. Difficult to manufacture.
• Why starting resistor is provided in barring gear motor?
Starting resistance at stator reduces voltage at stator terminals and accelerates the
motor very slowly so as to allow smooth gear engagement.
Other methods are
a) Auto transformer.
b) Star-delta starter.
• Will rotor earth fault relay operate for earth fault in main exciter winding or RCU
Diode Bridge also?
Yes.
• Why neoprene rubber bellows in generator IPBD?
Prevent vibrations transmitted from generator to IPBD.
• How moisture entry is prevented into bus duct?
Silicagel breathers at either end.
• Why aluminium bus duct is used?
Aluminium is nonmagnetic material.
Short circuit forces are less.
• Where fault level is more, whether in generator bus duct or UT bus duct? Why?
Fault level is more in UT bus duct. Because the fault currents fed by the both
generator and GT adds up within UT bus duct in case there is a fault in UT bus duct.
• Which is better, whether
a) Phase segregated bus duct or
b) Common bus duct?
Phase segregated bus duct is better, since phase to phase faults are avoided.
• Why cannot we have cables instead of bus duct in main generator?
Very large number of cables in parallel required problems of sealing the
terminations.
• What is the material of slip ring?
Alloy steel
• Why rotor impedance testing done during static and running condition?
To detect rotor earth fault.
• Which parameter indicates the rotor short-circuited turns (Not involving earthfault)?
Vibration increases.
• How stator water purity is held?
Filters, Vacuum pumps, Expansion tank, and magnetic filter.
• What are the routines checks on slip rings?
a. Correct mV drops brush to be used.
b. Brush tension adjustment.
c. Air cleaning to reduce leakage current.
d. Brush bedding before use.
e. Field polarity change every 6 months.
• Can we trip GFB from control room during unit operation?
No only if generator breaker is off.
• What is the voltage and frequency limit of generator?
±5%, ±5%
• What is the negative sequence capability?
I2 = 5% max,
I2
2 t = 7
• Why should we keep the brushes released during long shutdown?
Brushes wear out unevenly, when run on barring gear speed.
• How hydrogen purity reduces?
Due to seal oil vapour mixing.
• What is the purpose of back up impedance protection in main generator?
Covers inter-phase faults both externals to the GT and in GT. This also covers
partially faults inside generator, time delayed to coordinate with 230 kV-distance
protection.
• Why alternator rotor is made of solid iron?
Because, rotor flux = DC continuous
No iron loss problems.
• Why 50 Hz chosen?
Earlier 25 Hz generally used.
After developing of the high-speed turbine, 50/60 Hz standardized.
• Why oil cannot be used instead of water in generator stator?
Oil has high thermal capacity than gas, but low thermal capacity than water. Ability
to absorb heat is also less than waters.
High viscosity of oil causes linear flow and poor surface heat transfer in small ducts.
Large pumping power required.
• What is the purpose of JOP?
Lifts the rotor by injection of oil at high pressure, when BGM is in service. Outlet
pressure of JOP is 140 kg/cm2.
• What is the purpose of lubrication oil?
Keep oil film in bearings, avoid metal to metal contact between bottom of journal
and bearings avoid damage to bearings by lubricating the bearings. This also
removes heat from that part.
• What are the purposes of barring gear?
Start rotor from rest.
Eliminates sag in rotor - straighten and avoid rubbing at glands.
Avoid direct contact journals and bearings.
Avoid differential temp between top and bottom of cylinder due to convection of
Steam or hot air inside turbine cylinders.
• How shaft voltage produced by turbine?
Due to un-symmetry in the flux path of core, non-uniform air gap, un-symmetry in
the rotor magnetic field during short circuit in the rotor winding, causes voltage to
develop across the ends of rotor shaft.
• Why shaft-earthing brush is different from normal brush?
The contact resistance should be very low for shaft earthing brush, to prevent even
small current through the oil film, so used copper magnite brush or silver carbon
brush.
• What is the necessity of shaft voltage measurement?
It is to observe insulation of bearing 6 & 7 and hydrogen seal assembly. It requires
minimum leakage current (<100mA) through bearings and shaft seals to avoid pitting
of bearings.
If the leakage current >100mA, clean the insulation provided between bearing
pedestal and seal housing with earth.
• Why generator stator having alternate arrangements of hollow and solid conductor?
It ensures an optimum solution for increasing current and to reduce losses.
• What type of insulation is done for stator bars?
Bar insulation is done with epoxy mica thermosetting insulation. This insulation is
void free and possesses better mechanical properties. This insulation is more reliable
for higher voltages. Conductors are provided with glass lapped strand insulation.
After curing the insulation the epoxy resin (glue) fill all voids in the insulation.
• How carona discharge is prevented in generator insulation?
To prevent carona discharges between insulation and the wall of the slot, the
insulation in slot portion is coated with semi conducting varnish. This eliminates the
formation of creepage sparks during operation and during HV test.
• Why Generator should run within capability region?
Operating the Generator in excess of the capability curves will causes increase in
copper temperature, thermal expansion and higher insulation stresses.
• How cooling is done for slip ring and brush gear?
A centrifuge fan is mounted on the shaft in between two slip rings for ventilation of
the slip rings and brush gear.
• What is the type of brush used in brush gear?
Low co-efficient of friction and self-lubricating morganite grade carbon HM100.
Now a day we are using LFC554 for economical reasons.
• What is the name of instrument used to measure conductivity?
Gas chromato graph.
• What is the need of staggering of brushes and helical grooves?
The need of staggering is for uniform wear of brush and slip rings. The helical
grooved are provided to improve the brush performance by breaking air pockets. The
forced ventilation fan removes carbon dust from the helical grooves.
• What is the purpose of shaft earthing and bearing insulation?
The voltage generated in the shaft due to the leakage fluxes can circulate current
through the shaft. If shaft earthing is not done the leakage current will flow through
the bearings to ground and pitting of bearings will result. Hence bearing foundation
and pipelines are insulated.
• What is the purpose of POLARIZATION INDEX (PI) value?
It is used to assess the degree of dryness of windings. It depends on free ions in
insulating material. Initially for a new insulator free ions are less and hence more
resistance will be more. For old insulation initially free ions will be more depends on
age and material and hence resistance will be less. So the PI value for new insulation
will be more and for old insulation it will be less.
• What is the requirement of stator water electrical conductivity?
The cooling water must have an electrical conductivity less than 2.5 micro mho/cm.
One portable polishing unit consisting of mixed bed is also provided in the system to
remove impurities and maintain stator water conductivity at a less value.
• What is the necessity of Seal oil system?
The annular gap between stator and rotor of the generator are to be sealed to prevent
hydrogen leak from the casing.
Type of seal – ring type shaft seals
Pressure of seal oil – 4 kg/cm2
• What is the pressure of rotor gas (hydrogen)?
3.5 kg/cm2
• What is the paint used in the surface and interior of enclosure and why it is?
Matt black paint, for efficient heat dissipation.
• What is type of nut and bolts are used in IPBD?
Non-magnetic stainless steel nut and bolts are used in IPBD to restrict magnetic
effect at joints.
• Why flexible expansion joints are used in IPBD?
To cater thermal expansion and contraction due to heating and to eliminate
mechanical vibrations to the equipment.
• What are the salient features of IPBD?
a. This for a vital link between generator, GT, UAT, SPPT, SET and neutral
grounding transformer.
b. The continuous enclosure operating at ground potential limits the leakage flux
outside the enclosure to a very low value thereby eliminating the problem of
inductive heating of magnetic materials in the vicinity of the busduct.
c. Shielding effect of the enclosure reduced the electromagnetic forces under fault
conditions between bus to bus to a great.
d. The IPBD consists of high purity aluminium alloy bus supported by high strength
porcelain insulator (24 kV class) within enclosure separates adjacent conductor
by air. This eliminates phase to phase faults to a great extent.
e. Practically negligible inductive heating on adjacent steel structure.
f. High current carrying capacity. Because the conductors are of circular type
having very little skin effect and has a very large cooling surface.
g. Conductors are painted with epoxy Matt black paint results in heat dissipation and
the temperature rise is small and current carrying capacity is improved.
h. High dielectric strength as conductors are supported on porcelain insulators.
i. Air tight, watertight and dust free bus conductors. Hence maintenance is nil.
j. Separate parts erected IPBD. Hence changing parts makes it easy.
• Why neoprene rubber bellows are used in IPBD?
Neoprene rubber bellows are used near the terminals of the equipment and also at
building wall from indoor to outdoor area to allow thermal expansion and to
minimise vibrations.
• Why aluminium bus bars are silver-plated in IPBD?
Aluminium bus bars are silver-plated at flexible connection to prevent the galvanic
corrosion ant also for low contact resistance.
• Why seal-off bushings are used in IPBD?
To prevent interchange of air at different temperature and leakage of hydrogen or
infiltration of dust into the bus duct.
• What are the precautions to be taken while working at SPPT?
PT trolley should be isolated very carefully so as to isolate secondary terminals first
and primary (HT) terminals next. When primary isolated the arrangement in the
trolley make ground connection and HT terminal will be discharged at the drawn-out
position.
When fuse is blown the temporary earth should be done at the HT side of the fuse to
replace the fuse. Because PT may be energised through secondary side.
• What is use of hot air blower in IPBD?
To remove moisture and to prevent moisture condensation inside the duct at
commissioning time or in long shutdown periods.
• What are the temperature limits for UAT and SPPT bus bar?
2 kA (UAT) & 1 kA
Ambient temp 45°C 45°C
Maximum temp 60°C 60°C
Short circuit for 1 second temp 200°C (max load) 200°C(max load)
Bus material Al alloy Al alloy
Thickness 15 mm 6 mm
Dia 12.7 cm
• Specification of NGT & NGR.
NGT – 1 phase, natural cooled, indoor dry type, 16.5 kV / 250V, 50 kVA.
NGR – natural cooled, stainless steel grid type, 0.5Ω, 250V, 288A (continuous) and
temperature rise allowed to 375 °C.
• Surge protector and potential transformer cubicle specification.
Surge protector – non-inflammable, synthetic liquid impregnated and hermetically
sealed, 24KV, 0.25μ f (micro farad).
PT – 16500/√3 /110/√3 volts. Fuse – 24kv, 3.15A.
• How the power of the Generator can be varied?
Injecting inlet steam to the prime mover can vary active power. Reactive power can
vary by the Generator main field voltage variation. An excitation change PF at which
load is delivered.
Active power is produced by source and used effectively. VAR is the power used for
magnetization of core of transformers, motors, generators, overhead transmission
lines (capacitive), household appliances etc.
• What is the protection for IPBD?
Generator – GT overall differential protection.
• How the liquid in generator can be detected?
There are three liquid detection devices provided for the same purpose.
• Why and where the magnetic filter is provided in stator water circuit?
Magnetic filter is provided to catch the metal particles in stator water circuit, which
are produced in the pipelines. This is mounted at the end of the circuit nearer to the
inlet of the generator.
• What are the isolations required for working on IPBD/ Generator?
a. Generator field breaker open and tagged.
b. GT breaker open and earth switch closed.
c. Barring gear motor stopped and tagged.
d. Generator PT’s isolated and tagged.
e. CB 472 and CB 474 open and PT’s are isolated and tagged.
f. Before doing any work on brush gear 64F1 relay to be taken out.
• What are the futures of turbine generator?
a. Low heat drop
b. Moisture control (HP-0.26%, LP-3%.)
c. Turbine governing system
d. 70% steam dumping to the condenser to avoid reactor trip.
e. Gland sealing
f. LP exhaust hood cooling
g. Generator stator and rotor cooling
h. Hydrogen sealing
i. Static excitation
• What are the intervals for generator overhauling?
a. 1st inspection after 8000 hrs of working
b. 2nd inspection after 8000 hrs of 1st inspection
c. 3rd inspection after 24000 hrs of 1st inspection
d. 4th inspection after 48000 hrs of 1st inspection
• Write nameplate details of the main generator.
Type THW-235
kW 237700
kVA 264000
Voltage 16500 V
Amps 9240 A
Power factor 0.9 lag.
Field voltage 326 V
Field current 2755 A
Insulation Class-F
Speed 3000 rpm
50 Hz, 3, double star connection.
• What are the torque settings used in IPBD connection?
M12 (Nut bolt) 4506 100 kg-cm or 55 NM
M16 (Nut bolt) 9006 250 kg-cm or 80 NM
M20 (Nut bolt) 18006300 kg-cm or 100 NM
• Write critical speeds of turbine generator?
Generator rotor
1st critical speed 1283 rpm
2nd critical speed 3600 rpm
Combined turbine generator
1st critical speed 1938 rpm
2nd critical speed 2120 rpm
3rd critical speed 2385 rpm
4th critical speed 2837 rpm
• What type of governing system used in turbine and what are the purposes of the
system?
Hydraulic governing system of centrifugal (speed) governer type is adopted in
turbine.
Sensitive oil pressure to actuate centrifugal governer is 6.1 kg/cm2 (max). At 6.7
kg/cm2 relief valve is attached for on load testing.
Relay oil at pressure 21 kg/cm2 (max) is used to actuate HP CIES valve, governer
valve, LP CIES valve and LP governer valves.
At speed of 2560-rpm governer system becomes effective and starts draining of
sensitive oil to 2.81 kg/cm2 as speed is 2760 rpm and this is the governer take over
speed. Once the speed takes over by governer, governer valves position comes to
closing side and then CIES valves are opening fully. At this stage further opening of
CIES valve does not change any speed of system and the speed depends only on
governer valve opening position and speeder gear system.
HP speeder gear controls HP governer valves and LP speeder gear controls LP
governer valves and closes fully when 6% over speed which starts when 3% over
speeding.
The main purposes are as follows.
a. Bring the TG to rated (synchronous speed) speed from rest.
b. Loading and unloading when synchronised.
c. Responding with grid frequency variations within design rage and loading and
unloading the machine so that grid frequency remains stable.
d. Limiting the load as per reactor load.
e. Protecting the machine from over speed and from sudden large load thrown off or
trip.
f. Tripping the machine and bringing it on barring gear when event for not operation
happens.
g. When synchronised the speed is regulated by speeder gear from BPC signal. Once
synchronised the grid frequency and speeder gear controls the speed.
• Why inter-turn protection is provided along with differential protection in generator?
Inter-turn protects two separate windings from the fault of the generator.
• How patina formation is done.
By injecting low excitation current of 50 Amps for half an hour interval to 250
Amps. (Epoxy insulation in the stator winding absorbs no moisture).
• What are the tests to be carried during PM checks of IPBD?
a) Physical inspection of bus for any spark or overheating or discoloration.
b) Physical inspection of copper braided flexibles for discoloration.
c) Physical inspection of inspection window gaskets, seal off bushings, supporting
insulators, CT’s, painting of IPBD.
d) Torque tightness of flexibles.
e) Connection tightness of CT’s, SPPT cubicle, NGT cubicle, CT’s master JB, and
Generator terminal bushing connection.
f) Tightness of supporting insulator, seal off bushing, inspection windows
g) Inspection of rubber bellows
h) Electrical checks on SPPT, NGT, CT, mVDT of copper flexible connection.
i) Capacitance measurement of surge capacitor.
j) Healthiness checks of lightning arrestor.
k) HV test of IPBD
l) Tan-delta test of IPBD
m) IR value measurement
n) Cleanliness checks entire IPBD.
• What are the works to be done in generator in major overhauling?
Works on stator
a) Hydro test (DM water at 5 kg/cm2 pressure, leak acceptable is 5% for 24 hrs).
b) Hydro test of H2 coolers (DM water at 4 kg/cm2 for 30 seconds no leak is
allowed)
c) Pneumatic test with mask air.
d) Drying out of stator conductor. Hot air blower is used.
e) IR value check.
f) Stator overhang portion inspection.
g) Inspection of Teflon tubes and rubber grummets.
h) Stator wedge tightness test with 200 grams hammer.
i) Inspection of RTD’s.
j) Maintenance of end shields.
k) Hot air and hot water test of stator conductors to check whether flow through all
stator conductors is uniform.
l) Measurement of IR and PI value.
m) Capacitance and tan-delta measurements.
n) Partial discharge test.
o) Winding resistance measurement.
p) DC step voltage.
q) ELCID (electromagnetic core imperfection detection) test.
Works on Rotor
a) Nitrogen leak tightness test of CC bolts at 4 kg/cm2.
b) Inspection of rotor slots.
c) Purge test of rotor ventillation canals.
d) DP test on slip-ring hub to detect micro crack.
e) DP and Ultrasonic test on retaining rings to detect any cracks.
f) Measurement of IR and PI value.
g) Impedance measurement.
h) Recurrence surge oscillograph.
i) Winding resistance measurement.
j) Slip-ring groove cutting and machining.
k) Patina formation. Then OCC test.
• What is the purpose of tan-delta measurement?
Insulation in electrical system has parameters such as Capacitance, Die-electric loss,
and Power factor. By detecting the changes in these parameters failures can be
revealed. In this tan-delta test measured quantities are dissipation factor, power
factor, capacitance and dielectric power loss.
The very purpose of this test is to detect moisture content in the insulation. This
detects moisture and void in the insulation. This indicates amount of ionization.
• What is meant by partial discharge? How can be tested?
Partial discharges are electrical sparks, which occur in gas voids within the insulation
when the voltage is high enough. The discharges are partial since there is some
insulation remaining to prevent a complete breakdown. Partial discharge can erode
the insulation and therefore contribute to insulation ageing.
This can be tested by electromagnetic probe, which is a detector that is sensitive to
the radio frequency signals produced by the partial discharges within the winding.
With this probe test it is possible to locate specific
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• How output of the generator is depends?
Output of the generator is the function of volume, length, dia, airgap, and speed.
• What you mean by Gas pickup method?
Sucking cooled hydrogen gas from the air gap, circulating in canals and removing
the heat from the rotor. While sucking the gas, gas comes through the stator parts
also and removes the heat from the stator parts. The heated gas circulated to the
hydrogen for cooling purpose by the fans installed at rotor shaft. Thus the stator and
rotor of the generator is cooled. This method is called Gas pickup method.
• When the hydrogen explosion will take place in main generator?
When hydrogen concentration in air is more than 4% and less than 74% causes the
explosion.
• Can we excite generator without hydrogen?
No
• What are the advantages of stator water?
a. High thermal capacity
b. Low electrical conductivity (Good insulator)
c. Low viscosity
d. Free of fire risk and non-toxic
e. Simple heat exchanger i.e. it can be circulate easily and cooled by heat exchanger
• How rotor windings are held in position against centrifugal force?
Rotor windings are held by duraluminium wedges and by non-magnetic steel
retaining rings in the overhang portion.
• What is the purpose of current carrying bolts in rotor?
Feeding DC current from slip ring to rotor winding.
• How rotor cooled?
Hydrogen picked up from stator core backspace, passes through ventilation canals on
rotor and comes out through adjacent canals. Shaft fans aid the hydrogen flow. Heat
from the hydrogen removed by 4 nos. of hydrogen coolers. (NAPW)
• What is rotor E/F relay setting?
1.0 mA
• Are we using DCCB in the plant?
Yes, generator field breaker
• How arc is quenched in Generator Field Breaker?
By magnetic blow out coils, arc is elongated very fastly, so resistance of arc
increases, soon becoming unstable and quenched by arc chutes.
• How generator is protected from switching surges and lightning surges?
Surge capacitor and lightning arrestor.
• What is the use of generator PT’s?
AVR, Protection & Metering.
• What is the difference between PT and normal transformer?
Burden of PT is less and burden of normal transformer is high.
• What is the % overload allowed for TG and DG?
For TG nil
For DG 110% for 2 hours.
• When TG works as induction generator?
When excitation alone lost.
• Why motoring should be prevented in TG and DG?
In TG motoring prevented due to the turbine limitation otherwise last stage blade
will fails.
In DG motoring prevented, because of unburned fuel catches fire in DG.
• Why GFB closed only after reaching rated speed?
To prevent over fluxing of transformers or generator.
Emf = 4.44 f φ Z A
If f frequency is reduced due to less speed,
φ = V / f Z A
And flux will be more to saturate the core of transformers or generator.
• What are the protective parameters to changeover AVR auto to manual?
a. PT supply fail.
b. Auto pulse fail.
c. Supply of limiter fail.
d. Supply of auto channel fail.
e. Regulated supply fail.
f. High auto reference.
• Why AVR changes over to manual on AVR PT fuse failure?
Because loss of feedback to voltage corrector.
• Will main generator differential relay pick up for generator earth faults?
No earth fault current limited to 5A, while differential setting is 10%.
• Why generator differential provided?
For generator phase to phase faults and 3 phase faults.
• What is the effect of loss of excitation on generator?
Large induced currents in rotor leads to rotor end part over heating.
Leading VAR taken from grid leads to severe voltage dips in grid, if grid is weak.
Stability of machine lost.
Stator overheating.
Machine speed rises slightly.
• What are the limiters provided in excitation system?
a. Rotor current limiter (3000 A)
b. Rotor angle limiter (75° lag)
c. Under excitation limiter.
d. Stator current limiter (lead 10000 A)
e. Stator current limiter (lag 10000 A)
f. N – 2 limiter.
• What is the effect of unbalance currents in generator?
Double frequency currents are induced in metal parts of rotor and overheating of
retaining rings and non-magnetic wedges.
• Why low forward power relay used in parallel to reverse power relay?
A small steam leak through CIES valves will keep the machine floating on to grid at
low power. So even if turbine trip, relay picks up, there is no trip actually. So low
forward power relay (0.54%) used to detect the condition.
• Why earth fault current of generator should not be reduced below 5A?
To limit over voltage due to neutral shift.
• Why not high resistance for earth fault than using grounding transformer & resistor
0.45 ohms?
It is mechanically unwide. Difficult to manufacture.
• Why starting resistor is provided in barring gear motor?
Starting resistance at stator reduces voltage at stator terminals and accelerates the
motor very slowly so as to allow smooth gear engagement.
Other methods are
a) Auto transformer.
b) Star-delta starter.
• Will rotor earth fault relay operate for earth fault in main exciter winding or RCU
Diode Bridge also?
Yes.
• Why neoprene rubber bellows in generator IPBD?
Prevent vibrations transmitted from generator to IPBD.
• How moisture entry is prevented into bus duct?
Silicagel breathers at either end.
• Why aluminium bus duct is used?
Aluminium is nonmagnetic material.
Short circuit forces are less.
• Where fault level is more, whether in generator bus duct or UT bus duct? Why?
Fault level is more in UT bus duct. Because the fault currents fed by the both
generator and GT adds up within UT bus duct in case there is a fault in UT bus duct.
• Which is better, whether
a) Phase segregated bus duct or
b) Common bus duct?
Phase segregated bus duct is better, since phase to phase faults are avoided.
• Why cannot we have cables instead of bus duct in main generator?
Very large number of cables in parallel required problems of sealing the
terminations.
• What is the material of slip ring?
Alloy steel
• Why rotor impedance testing done during static and running condition?
To detect rotor earth fault.
• Which parameter indicates the rotor short-circuited turns (Not involving earthfault)?
Vibration increases.
• How stator water purity is held?
Filters, Vacuum pumps, Expansion tank, and magnetic filter.
• What are the routines checks on slip rings?
a. Correct mV drops brush to be used.
b. Brush tension adjustment.
c. Air cleaning to reduce leakage current.
d. Brush bedding before use.
e. Field polarity change every 6 months.
• Can we trip GFB from control room during unit operation?
No only if generator breaker is off.
• What is the voltage and frequency limit of generator?
±5%, ±5%
• What is the negative sequence capability?
I2 = 5% max,
I2
2 t = 7
• Why should we keep the brushes released during long shutdown?
Brushes wear out unevenly, when run on barring gear speed.
• How hydrogen purity reduces?
Due to seal oil vapour mixing.
• What is the purpose of back up impedance protection in main generator?
Covers inter-phase faults both externals to the GT and in GT. This also covers
partially faults inside generator, time delayed to coordinate with 230 kV-distance
protection.
• Why alternator rotor is made of solid iron?
Because, rotor flux = DC continuous
No iron loss problems.
• Why 50 Hz chosen?
Earlier 25 Hz generally used.
After developing of the high-speed turbine, 50/60 Hz standardized.
• Why oil cannot be used instead of water in generator stator?
Oil has high thermal capacity than gas, but low thermal capacity than water. Ability
to absorb heat is also less than waters.
High viscosity of oil causes linear flow and poor surface heat transfer in small ducts.
Large pumping power required.
• What is the purpose of JOP?
Lifts the rotor by injection of oil at high pressure, when BGM is in service. Outlet
pressure of JOP is 140 kg/cm2.
• What is the purpose of lubrication oil?
Keep oil film in bearings, avoid metal to metal contact between bottom of journal
and bearings avoid damage to bearings by lubricating the bearings. This also
removes heat from that part.
• What are the purposes of barring gear?
Start rotor from rest.
Eliminates sag in rotor - straighten and avoid rubbing at glands.
Avoid direct contact journals and bearings.
Avoid differential temp between top and bottom of cylinder due to convection of
Steam or hot air inside turbine cylinders.
• How shaft voltage produced by turbine?
Due to un-symmetry in the flux path of core, non-uniform air gap, un-symmetry in
the rotor magnetic field during short circuit in the rotor winding, causes voltage to
develop across the ends of rotor shaft.
• Why shaft-earthing brush is different from normal brush?
The contact resistance should be very low for shaft earthing brush, to prevent even
small current through the oil film, so used copper magnite brush or silver carbon
brush.
• What is the necessity of shaft voltage measurement?
It is to observe insulation of bearing 6 & 7 and hydrogen seal assembly. It requires
minimum leakage current (<100mA) through bearings and shaft seals to avoid pitting
of bearings.
If the leakage current >100mA, clean the insulation provided between bearing
pedestal and seal housing with earth.
• Why generator stator having alternate arrangements of hollow and solid conductor?
It ensures an optimum solution for increasing current and to reduce losses.
• What type of insulation is done for stator bars?
Bar insulation is done with epoxy mica thermosetting insulation. This insulation is
void free and possesses better mechanical properties. This insulation is more reliable
for higher voltages. Conductors are provided with glass lapped strand insulation.
After curing the insulation the epoxy resin (glue) fill all voids in the insulation.
• How carona discharge is prevented in generator insulation?
To prevent carona discharges between insulation and the wall of the slot, the
insulation in slot portion is coated with semi conducting varnish. This eliminates the
formation of creepage sparks during operation and during HV test.
• Why Generator should run within capability region?
Operating the Generator in excess of the capability curves will causes increase in
copper temperature, thermal expansion and higher insulation stresses.
• How cooling is done for slip ring and brush gear?
A centrifuge fan is mounted on the shaft in between two slip rings for ventilation of
the slip rings and brush gear.
• What is the type of brush used in brush gear?
Low co-efficient of friction and self-lubricating morganite grade carbon HM100.
Now a day we are using LFC554 for economical reasons.
• What is the name of instrument used to measure conductivity?
Gas chromato graph.
• What is the need of staggering of brushes and helical grooves?
The need of staggering is for uniform wear of brush and slip rings. The helical
grooved are provided to improve the brush performance by breaking air pockets. The
forced ventilation fan removes carbon dust from the helical grooves.
• What is the purpose of shaft earthing and bearing insulation?
The voltage generated in the shaft due to the leakage fluxes can circulate current
through the shaft. If shaft earthing is not done the leakage current will flow through
the bearings to ground and pitting of bearings will result. Hence bearing foundation
and pipelines are insulated.
• What is the purpose of POLARIZATION INDEX (PI) value?
It is used to assess the degree of dryness of windings. It depends on free ions in
insulating material. Initially for a new insulator free ions are less and hence more
resistance will be more. For old insulation initially free ions will be more depends on
age and material and hence resistance will be less. So the PI value for new insulation
will be more and for old insulation it will be less.
• What is the requirement of stator water electrical conductivity?
The cooling water must have an electrical conductivity less than 2.5 micro mho/cm.
One portable polishing unit consisting of mixed bed is also provided in the system to
remove impurities and maintain stator water conductivity at a less value.
• What is the necessity of Seal oil system?
The annular gap between stator and rotor of the generator are to be sealed to prevent
hydrogen leak from the casing.
Type of seal – ring type shaft seals
Pressure of seal oil – 4 kg/cm2
• What is the pressure of rotor gas (hydrogen)?
3.5 kg/cm2
• What is the paint used in the surface and interior of enclosure and why it is?
Matt black paint, for efficient heat dissipation.
• What is type of nut and bolts are used in IPBD?
Non-magnetic stainless steel nut and bolts are used in IPBD to restrict magnetic
effect at joints.
• Why flexible expansion joints are used in IPBD?
To cater thermal expansion and contraction due to heating and to eliminate
mechanical vibrations to the equipment.
• What are the salient features of IPBD?
a. This for a vital link between generator, GT, UAT, SPPT, SET and neutral
grounding transformer.
b. The continuous enclosure operating at ground potential limits the leakage flux
outside the enclosure to a very low value thereby eliminating the problem of
inductive heating of magnetic materials in the vicinity of the busduct.
c. Shielding effect of the enclosure reduced the electromagnetic forces under fault
conditions between bus to bus to a great.
d. The IPBD consists of high purity aluminium alloy bus supported by high strength
porcelain insulator (24 kV class) within enclosure separates adjacent conductor
by air. This eliminates phase to phase faults to a great extent.
e. Practically negligible inductive heating on adjacent steel structure.
f. High current carrying capacity. Because the conductors are of circular type
having very little skin effect and has a very large cooling surface.
g. Conductors are painted with epoxy Matt black paint results in heat dissipation and
the temperature rise is small and current carrying capacity is improved.
h. High dielectric strength as conductors are supported on porcelain insulators.
i. Air tight, watertight and dust free bus conductors. Hence maintenance is nil.
j. Separate parts erected IPBD. Hence changing parts makes it easy.
• Why neoprene rubber bellows are used in IPBD?
Neoprene rubber bellows are used near the terminals of the equipment and also at
building wall from indoor to outdoor area to allow thermal expansion and to
minimise vibrations.
• Why aluminium bus bars are silver-plated in IPBD?
Aluminium bus bars are silver-plated at flexible connection to prevent the galvanic
corrosion ant also for low contact resistance.
• Why seal-off bushings are used in IPBD?
To prevent interchange of air at different temperature and leakage of hydrogen or
infiltration of dust into the bus duct.
• What are the precautions to be taken while working at SPPT?
PT trolley should be isolated very carefully so as to isolate secondary terminals first
and primary (HT) terminals next. When primary isolated the arrangement in the
trolley make ground connection and HT terminal will be discharged at the drawn-out
position.
When fuse is blown the temporary earth should be done at the HT side of the fuse to
replace the fuse. Because PT may be energised through secondary side.
• What is use of hot air blower in IPBD?
To remove moisture and to prevent moisture condensation inside the duct at
commissioning time or in long shutdown periods.
• What are the temperature limits for UAT and SPPT bus bar?
2 kA (UAT) & 1 kA
Ambient temp 45°C 45°C
Maximum temp 60°C 60°C
Short circuit for 1 second temp 200°C (max load) 200°C(max load)
Bus material Al alloy Al alloy
Thickness 15 mm 6 mm
Dia 12.7 cm
• Specification of NGT & NGR.
NGT – 1 phase, natural cooled, indoor dry type, 16.5 kV / 250V, 50 kVA.
NGR – natural cooled, stainless steel grid type, 0.5Ω, 250V, 288A (continuous) and
temperature rise allowed to 375 °C.
• Surge protector and potential transformer cubicle specification.
Surge protector – non-inflammable, synthetic liquid impregnated and hermetically
sealed, 24KV, 0.25μ f (micro farad).
PT – 16500/√3 /110/√3 volts. Fuse – 24kv, 3.15A.
• How the power of the Generator can be varied?
Injecting inlet steam to the prime mover can vary active power. Reactive power can
vary by the Generator main field voltage variation. An excitation change PF at which
load is delivered.
Active power is produced by source and used effectively. VAR is the power used for
magnetization of core of transformers, motors, generators, overhead transmission
lines (capacitive), household appliances etc.
• What is the protection for IPBD?
Generator – GT overall differential protection.
• How the liquid in generator can be detected?
There are three liquid detection devices provided for the same purpose.
• Why and where the magnetic filter is provided in stator water circuit?
Magnetic filter is provided to catch the metal particles in stator water circuit, which
are produced in the pipelines. This is mounted at the end of the circuit nearer to the
inlet of the generator.
• What are the isolations required for working on IPBD/ Generator?
a. Generator field breaker open and tagged.
b. GT breaker open and earth switch closed.
c. Barring gear motor stopped and tagged.
d. Generator PT’s isolated and tagged.
e. CB 472 and CB 474 open and PT’s are isolated and tagged.
f. Before doing any work on brush gear 64F1 relay to be taken out.
• What are the futures of turbine generator?
a. Low heat drop
b. Moisture control (HP-0.26%, LP-3%.)
c. Turbine governing system
d. 70% steam dumping to the condenser to avoid reactor trip.
e. Gland sealing
f. LP exhaust hood cooling
g. Generator stator and rotor cooling
h. Hydrogen sealing
i. Static excitation
• What are the intervals for generator overhauling?
a. 1st inspection after 8000 hrs of working
b. 2nd inspection after 8000 hrs of 1st inspection
c. 3rd inspection after 24000 hrs of 1st inspection
d. 4th inspection after 48000 hrs of 1st inspection
• Write nameplate details of the main generator.
Type THW-235
kW 237700
kVA 264000
Voltage 16500 V
Amps 9240 A
Power factor 0.9 lag.
Field voltage 326 V
Field current 2755 A
Insulation Class-F
Speed 3000 rpm
50 Hz, 3, double star connection.
• What are the torque settings used in IPBD connection?
M12 (Nut bolt) 4506 100 kg-cm or 55 NM
M16 (Nut bolt) 9006 250 kg-cm or 80 NM
M20 (Nut bolt) 18006300 kg-cm or 100 NM
• Write critical speeds of turbine generator?
Generator rotor
1st critical speed 1283 rpm
2nd critical speed 3600 rpm
Combined turbine generator
1st critical speed 1938 rpm
2nd critical speed 2120 rpm
3rd critical speed 2385 rpm
4th critical speed 2837 rpm
• What type of governing system used in turbine and what are the purposes of the
system?
Hydraulic governing system of centrifugal (speed) governer type is adopted in
turbine.
Sensitive oil pressure to actuate centrifugal governer is 6.1 kg/cm2 (max). At 6.7
kg/cm2 relief valve is attached for on load testing.
Relay oil at pressure 21 kg/cm2 (max) is used to actuate HP CIES valve, governer
valve, LP CIES valve and LP governer valves.
At speed of 2560-rpm governer system becomes effective and starts draining of
sensitive oil to 2.81 kg/cm2 as speed is 2760 rpm and this is the governer take over
speed. Once the speed takes over by governer, governer valves position comes to
closing side and then CIES valves are opening fully. At this stage further opening of
CIES valve does not change any speed of system and the speed depends only on
governer valve opening position and speeder gear system.
HP speeder gear controls HP governer valves and LP speeder gear controls LP
governer valves and closes fully when 6% over speed which starts when 3% over
speeding.
The main purposes are as follows.
a. Bring the TG to rated (synchronous speed) speed from rest.
b. Loading and unloading when synchronised.
c. Responding with grid frequency variations within design rage and loading and
unloading the machine so that grid frequency remains stable.
d. Limiting the load as per reactor load.
e. Protecting the machine from over speed and from sudden large load thrown off or
trip.
f. Tripping the machine and bringing it on barring gear when event for not operation
happens.
g. When synchronised the speed is regulated by speeder gear from BPC signal. Once
synchronised the grid frequency and speeder gear controls the speed.
• Why inter-turn protection is provided along with differential protection in generator?
Inter-turn protects two separate windings from the fault of the generator.
• How patina formation is done.
By injecting low excitation current of 50 Amps for half an hour interval to 250
Amps. (Epoxy insulation in the stator winding absorbs no moisture).
• What are the tests to be carried during PM checks of IPBD?
a) Physical inspection of bus for any spark or overheating or discoloration.
b) Physical inspection of copper braided flexibles for discoloration.
c) Physical inspection of inspection window gaskets, seal off bushings, supporting
insulators, CT’s, painting of IPBD.
d) Torque tightness of flexibles.
e) Connection tightness of CT’s, SPPT cubicle, NGT cubicle, CT’s master JB, and
Generator terminal bushing connection.
f) Tightness of supporting insulator, seal off bushing, inspection windows
g) Inspection of rubber bellows
h) Electrical checks on SPPT, NGT, CT, mVDT of copper flexible connection.
i) Capacitance measurement of surge capacitor.
j) Healthiness checks of lightning arrestor.
k) HV test of IPBD
l) Tan-delta test of IPBD
m) IR value measurement
n) Cleanliness checks entire IPBD.
• What are the works to be done in generator in major overhauling?
Works on stator
a) Hydro test (DM water at 5 kg/cm2 pressure, leak acceptable is 5% for 24 hrs).
b) Hydro test of H2 coolers (DM water at 4 kg/cm2 for 30 seconds no leak is
allowed)
c) Pneumatic test with mask air.
d) Drying out of stator conductor. Hot air blower is used.
e) IR value check.
f) Stator overhang portion inspection.
g) Inspection of Teflon tubes and rubber grummets.
h) Stator wedge tightness test with 200 grams hammer.
i) Inspection of RTD’s.
j) Maintenance of end shields.
k) Hot air and hot water test of stator conductors to check whether flow through all
stator conductors is uniform.
l) Measurement of IR and PI value.
m) Capacitance and tan-delta measurements.
n) Partial discharge test.
o) Winding resistance measurement.
p) DC step voltage.
q) ELCID (electromagnetic core imperfection detection) test.
Works on Rotor
a) Nitrogen leak tightness test of CC bolts at 4 kg/cm2.
b) Inspection of rotor slots.
c) Purge test of rotor ventillation canals.
d) DP test on slip-ring hub to detect micro crack.
e) DP and Ultrasonic test on retaining rings to detect any cracks.
f) Measurement of IR and PI value.
g) Impedance measurement.
h) Recurrence surge oscillograph.
i) Winding resistance measurement.
j) Slip-ring groove cutting and machining.
k) Patina formation. Then OCC test.
• What is the purpose of tan-delta measurement?
Insulation in electrical system has parameters such as Capacitance, Die-electric loss,
and Power factor. By detecting the changes in these parameters failures can be
revealed. In this tan-delta test measured quantities are dissipation factor, power
factor, capacitance and dielectric power loss.
The very purpose of this test is to detect moisture content in the insulation. This
detects moisture and void in the insulation. This indicates amount of ionization.
• What is meant by partial discharge? How can be tested?
Partial discharges are electrical sparks, which occur in gas voids within the insulation
when the voltage is high enough. The discharges are partial since there is some
insulation remaining to prevent a complete breakdown. Partial discharge can erode
the insulation and therefore contribute to insulation ageing.
This can be tested by electromagnetic probe, which is a detector that is sensitive to
the radio frequency signals produced by the partial discharges within the winding.
With this probe test it is possible to locate specific
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