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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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