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Life extension of old designed capital electrical equipments
of power plant
110 MW Generators Type TGP 234260 / 2H
Presented by
NTPC Ltd., Tanda, India
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110 MW Generator TGP 234260 / 2H
Old generators faced severe electrical
conditions:
• Due to poor grid discipline.
• Protection technology was not as fast and
accurate as today.
• Variations in grid frequency and voltage.
• High no. of trips.
All these uncontrolled parameters stressed
electrical equipment.
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110 MW Generator TGP 234260 / 2H
• The main tests during RLA are:
• NDT of rotor forging & fan blades.
• UT & Metallographic examination of
components.
• Hardness measurement of mech. parts. (to
assess the mechanical integrity) Besides
above, elect. tests are done to assess rotor
and stator windings insulation & healthiness of
stator core.
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110 MW Tanda Generators • Tanda is a coal based station of 4 x110 MW,
commissioned in 80’s. Since Year 2000, this
plant has become part of NTPC.
• RLA study of all four sets were conducted ,
no major deviation was found.
• Rotor cooling mechanism modified from
symphonic to four flute cooling system to
enhance the cooling of rotor winding under
retaining rings.
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110 MW Tanda Unit -1 A major failure occurred in unit 1 on 26th
May 2011. After rotor thread out,
observations were as follows:
• All six bushings insulation were charred.
Bushing connecters and bolts were melted
• No visible core damage was observed.
• Sprinkles of copper deposits observed on
Rotor Body and stator windings on Turbine
Side.
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Shorting of bars 30 - 31
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Melting of bushing connecting bolts
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Melting of bushing connecting bolts
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110 MW Tanda U–1
• Bushings were disconnected from winding
Stator was cleaned but IR at 2.5 KV was
found almost zero.
• Due to conductive soot deposit in rotor slots,
the rotor winding also got earthed. Due to
unapproachable locations, removal of soot was
not possible. Cleaning was possible only
after removal of retaining rings. Rotor was
therefore sent to works.
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Carbon soot in rotor
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Tanda unit-1 Cause of Failure
• Breaking of strips of coil bars due to high
vibration of stator winding in over hang
portion was the main cause of flash over at
coil no 30 & 31. Due to breaking of strips of
bars, the insulating material got damaged,
ohmic resistance and localized heating
increased and insulation got burnt. Copper
strips got melted, vaporized and mixed with
carbon soot generated due to burning of
insulation.
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Tanda unit-1 Cause of Failure
• This conductive mixture spread inside the
generator along with circulating H2 gas
leading to stator winding earthed & operation
of earth fault relay.
• Insulating nature of H2 got converted to
conductive, reaching up to bushing area.
Conductive soot in this area created short
circuits. With rotation and field energy,
stator voltage continued for almost 1.5
seconds feeding the fault.
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Unit 1 - Fault event record
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Tanda unit-1 Cause of Failure
• 50 to 60 kA short circuit fault current flown.
Which created large electromech. forces &
high vibration in stator windings, creating
cracks in insulation , especially at slot exit
and in overhang portion of the winding.
• Partial repair with replacement of three bottom
bars (slot no 31,32,33) was decided.
110 MW Tanda U–1 Repair
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Constraints were as follows:
• No spares were available with OEM.
• Sending 110 Ton stator to OEM workshop was not
practical.
• OEM was not ready to take the job at site.
• Only 83 spare bars were available.
• Alternate vendor was required to take the job at site.
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110 MW Tanda U–1 Repair
• After removal of 30 top bars & 3 bottom bars, insulation of bottom bars no. 1, 2 , 5 & 6 was found damaged at slot exit portions. For their removal all the top bars had to be removed.
• 40 bottom bars remained in slot, but during final HV testing they failed, due to carbon deposits. Hence complete rewinding was decided.
• After physical inspection and ELCID test of core, some minor surface defects and few deep rooted defects were observed.
• The defective locations were repetition of previous records
• The surface defects were rectified to the extent possible.
• Based on past experience, it was
considered that deep rooted defect shall
not cause any trouble.
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110 MW Tanda U–1 Repair
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110 MW Tanda U–1 Repair
• Displacement of semi conducting fillings were
observed during bar removal.
• Displaced fillings creates, air pocket between
bar surface and slot wall.
• Due to high potential gradient, partial
discharges occur in these pockets & may lead
to earth faults.
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110 MW Tanda U–1 Repair
• These displacements were caused due to
vibration of bars inside the slots.
• Complete rewinding of stator in situ was the
only solution.
• Out of 83 available bars, 72 were used and balance 48 old bars were reconditioned & used.
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110 MW Tanda U–1 Repair
• Rewinding done at site with modifications:-
• The ‘C’ shape slot exit spacers were changed
to straight to avoid future rubbing
• The quality of overhang braiding was
improved.
• Old braiding loosened due to aging, new
material will get tightened with aging, reducing
possibility of looseness.
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Damage at slot exit portion due to rubbing of C-shaped spacer
• Braiding design of overhang is now
similar to 500MW generators.
• Slot thermocouple and core
thermocouple were replaced with
silicone extension lead thermocouples.
• NFT tests on stator winding done.
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110 MW Tanda U–1 Repair
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Tanda Unit-1: Wound Stator
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110 MW Tanda U–1Failure II • HV tests were done on finished winding.
• Bottom bar of slot 53 (Phase –B) failed at 11
KV.
• 27 top and 1 bottom bar again removed and
defective bar replaced with new bar.
• The generator was assembled with renovated
rotor.
• During final synchronization renovated rotor
failed. Duel earth fault occurred in lead.
110 MW Tanda U–1Failure II
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110 MW Tanda U–1Failure II
• Finally the unit was re-assembled
with the rotor borrowed from
‘KBUNL’ and synchronized with
grid on 6th Oct 2011 successfully.
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Conclusions • Electrical tests during RLA study are not
enough to ensure mechanical integrity of winding insulation. To some extent NFT can convey the condition with regard to overhang vibration thus enabling to take corrective measures and hence need to be done on old TGs during overhauls.
• Insulation at slot exit is to be inspected in every major overhaul for erosion and if noticed, corrective actions to be taken.
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Conclusions
• Slot liners between wedges and bars be
replaced by top ripple springs to minimize bar
vibrations in slots. Wedges be locked at ends
to avoid unwanted displacement.
• Possibility may be explored to install on line
vibration pickups in overhang portion.
• On line PD monitoring may be installed to
monitor the partial discharge.
BHEL MAKE 110 MW
Generators Spares position ( Major) :
Customer No. of sets Customer
No. of sets
Ennore One Panki TPS Two
DPL, Durgapur One Barauni Two
Harduagunj One Talcher Two
Patratu TPS Four Kota Two
GNDTP Four Paricha Two
AECO Three Muzzafarpur Two
BHEL MAKE 110 MW
Generators
Position of spare stator, rotor & rewinding of rotor
Customer No. of sets
KTPS Four
Panipat Four
Tanda Four
Customers with two tier drum type rotor :
Customer No. of sets
GNDTP,Bhatinda Two
AECO One
Panipat Two
Tanda Two
Paricha One
Four Flute cooling system for
Rotor
ROTOR TEMPERATURE DISTRIBUTION
THIS CURVES ARE ONLY INFORMATIVE AND MAY VARY FROM MACHINE TO MACHINE
TE
MP
COOLING GAS FLOW PATH FOR THERMO
SYPHONIC COOLING SYSTEM
Gen#1 from exciter end
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