Experiance Sharing for Best Maintenance-L.K.jain

7/28/2019 Experiance Sharing for Best Maintenance-L.K.jain

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EXPERIENCE SHARING

ADOPTING BEST PRACTICES&

MARCH TOWARDS ZEROFORCED OUTAGE

By L.K.JAIN/NCR-OS


2/44

220 KV BREAKERS

While opening of 220 kv breaker one pole gotstuck up in closed condition, two poles opened.

Aux contacts operated and breaker open statuswas available

One pole operating rod disengaged fromcoupling of interrupter

Generator continued to operate drawing currentfrom one pole which was closed Immediate action was taken to isolate generator


3/44

ACTION PROPOSED

CARE TO BE TAKEN DURING ERECTIONSTAGE/OVERHAULING STAGE( To be

included in FQA Check list) Breaker to be overhauled after 10 years of

operation to over cum problems of pole

stucking, contacts bouncing


4/44

OPERATION OF220/400 KVBREAKER DURING FOGGY SEASON

Whenever any running unit desynchronized orbeing synchronized in foggy weather, it should

not be delayed as 220/400 kv asynchronousvoltage appears across breaker terminals andsituation becomes worse in foggy weather ,itmay lead to flash over across pole insulators

If there is delay line isolator should be openedor unit excitation should be made off


5/44

ICT OVERLOADING SCHEME

To avoid ICTs Tripping on over current due totripping of other ICT,to avoid total black outEngg./OS developed current sensing over loadscheme

On sensing overloading of ICT it will give tripcommand to two unit after time delay

Scheme has been implemented at DGPS/DADRI

COAL & AuGPS It is proposed that scheme may be implemented

to other stations where ICTs are provided


6/44

FAULTS IN SWITCHYARD

Unit trips have been reported due toswitchyard faults caused due to hanging

metal wires/metallic threads of Kites It is proposed to carry out daily walk downsurvey to detect such incidences andcorrective action be taken

Switchyard gantries and insulators bechecked


7/44

HOUSE LOAD OPERATION OF GASTURBINES V94.2

High Frequency -51.5 HZ, Time delay-2/3 sec Low Frequency -47.5 HZ, Time delay-2sec Unit 220KV C.B opens, which initiates change of

load mode to speed mode and sudden loadthrow off is taken care and GT continue to runat set speed with available house load

220 KV Switch yard is connected to 400 KV

S/yard through ICTS with 220/400kv C.B


8/44

In case of Grid disturbance /opening of 400/220 KVC.B of ICTS Power evacuation is Interrupted

Gas Turbines continue to operate in load control atload set point and load rejection /House load operationdoes not take place and GT Trips on Mechanical overspeed which is very dangerous for Gas Turbines

It is mandatory to go for inspection of combustionchambers for checking intactness of CC Tiles, whichrequires outage of Unit for 20-24 Hrs

It reduces Machine availability/Generation loss andUnit is not available in time of crisis


9/44

In past Gas Turbines Tripped on actual over speed

during Grid disturbance

On 10.10.95 at 22.49Hrs-GT#3

On 12.01.96 -GT#3,GT#4

On JAN 97Running GT Tripped

On 06.07.05 GT#2/ GT#4 Speed reached up to54.62/54.50 HZ

GT#1/GT#3 Survived on house load but reached tohigher speed(220 KV Railway feeders helped inachieving house load operation)


10/44

ON 06.07.05 During Grid DisturbanceICT3/4 tripped at 01.23.43/01.23.46hrs prior to trip Frequencywas around 50.50H

TIME GT#2 GT#4

01.23.48 50.66 50.53

01.23.49 53.48 52.54

01.23.50 53.48 52.54

01.23.52 54.62 54.71


11/44

ACTION PROPOSED

House load operation settings 51.5Hz with time delayof 2/3 sec be reviewed. These setting were in Pre

ABT Regime when freq use to very high

In present scenario frequency does not reach up to

51.5 HZ. It is proposed to keep time delay as 100 m sec.

which will change control to speed mode from loadmode without any delay.

Lower side setting at 47.5 HZ may be kept as earlier.


12/44

Speed signal at 53.0 HZ be initiated to tripGas Turbine to avoid actual over speeding. Corporate guide lines have also been

issued to introduce speed sensing tripbefore mechanical over speed device trip.


13/44

ACTION TAKEN

Matter was referred to Engineering byNCROS.

After approval it was implemented at site It is proposed to review schemes of other

gas stations to safely change over to

house load in case of Grid disturbances


14/44

POWER TRANSFORMERS

Lowering of setting of start of oil coolingfans/ Pumps to keep better cooling

Mostly start is taken from breakercontacts, but if it is taken from tempsensing it will result in saving of AuxPower

Replacement of Fuses with MCBs to avoidsingle phase operation of pumps/fans


15/44

Tripping of L.V.Transformers throughthermister sensing

At Dadri Gas Power Station, Dry type castresign transformer are installed.

For monitoring of excessive heating , windingtemperature High Alarm & Trip System isprovided to prevent transformer fromexcessive heating & leading to failure.

Temperature monitoring relays are inenergized conditions, when heat is generatedResistance of Thermister increases & relaydrops and normally close, contact givesAlarm/ Trip as the case way be.


16/44

PROBLEM FACED

Temperature monitoring relays onvarious transformer Panel started

failing & Alarm of High Winding

Temperature use to appear.

In past at 2-3 instances, running Unit

tripped due to mal operation of relay

(relay failure).


17/44

IMMEDIATE ACTION TAKEN

Faulty relays replaced with New

Relays which were of imported

origin. OEM is Siemens Germany.


18/44

BEST MAINTENANCE PRACTICES

Reason for failure of relay was analyses

in depth. It was observed that there areTwo Nos. 5 Watt resistor on P.C.B. of

relay which is generating heat &

damaging card. Following steps were

taken to over come problem.


19/44

STEP 1

Holes were made in relay body for

proper heat dissipation to over come

the problem of excessive heating.


20/44

STEP 2

Resistances were taken out & fixed

on relay body so that even heat

generated does not effect the card &

mal oper-ation / failure of relay can

be avoided.


21/44

STEP 3

Modification in Alarm & Trip circuit

were made as follows-

a. One trip relay contact was hooked up in

parallel to Alarm Relay.

b


22/44

b. One NC contact of Alarm Relay

connected in series to trip contact Now

trip command will be activated whenboth relays sense High Temperature.

Failure/ mal function of individual relay

will not give trip signal.

c. Failure of any relay will give alarm

signal &relay can be attended while equipment

is

Ci it Di f T f Wi di


23/44

Circuit Diagram of Transformer Winding

Temperature Alarm/ Trip Relays

ORIGINAL SCHEME

Ci it Di f T f Wi di


24/44

Circuit Diagram of Transformer Winding

Temperature Alarm/ Trip Relays

MODIFIED SCHEME

b difi i h d


25/44

Above modification has removedchances of tripping of unit due to mal

operation of relay.

Failure rate of relay has been

brought to Zero since lasts threeyears. It is a special relay and not

available in India. It is being

procured for SAG.

Cost of Relay is Rs 15000/


26/44

OTHER ISSUES

Looping of neutral/DC supply at contactorterminals DOUBLE D.C supply to D.C Seal oil pumps with

Diode coupling

Separate A.C supply to two AC seal oil pumps D.C drive supply circuit to be housed in

separate panel to keep system available in caseof fire in A.C Seal oil panel

Double D.C Control supply to Switch gearswhere DC control supply is used


27/44

PROBLEMS FACED - REMEDIES

In seal oil panel, control supply forA.C. Seal oil pump is 240 voltsA.C. Neutral of control circuit werelooped at contactor coil terminal.

In case of failure of any contactorcoil, while replacing coil loopingused to get open and both A.C.Pumps used to trip.

To over come this problem,two wires looped at contactorcoils were crimped in one lug,which eliminated problem of

disconnection of neutral andproblem of tripping of bothA.C. Seal oil pumps wassolved.


28/44

SPARKING, PITTING ON SLIPRINGS OF GENERATOR

GT GENERATOR OF 131 MW ARE AIR COOLED.ROTOR EXICITATION CURRENT IS FEDTHROUGH 8NOS CARBON BRUSHES EACH ON+VE, -VE SLIP RINGS

In spite of proper preventive maintenancesparking started & fire parabola started on slip

ring assembly, Unit was stopped immediately


29/44

DAMEGED CAUSED

It resulted in pitting on slip ring and Brushholders, carbon brushes also damaged

Following failure took place UNIT YEAR OUTAGE HRS GT#3 1993 10HRS GT#2 1999 08HRS Rated current is 846amp.each brush share about

106amp. Rated current of brush is 150amp.


30/44

ANALYSIS OF PROBLEM

After checking brush holders it wasobserved that for most of the brush

holders current lead was found burnt,which can be due to excessive currentflow

It was decided to measure D.C.Current ofeach brush for all four generators throughD.C.Tong tester on daily basis to see thecurrent loading pattern on brushes


31/44

OBSERVATION

It was observed that current on somebrushes increased beyond 175 amp due toreduction of current on some brushes

Inspire of adjustment problem continued Brush which takes high current use to fail

at lead which use to increase current onother brushes and failure use to take placein cascade fashion


32/44

ACTION PLAN

It was decided to increase nos. of brushesfrom 8 to 12 brushes by fixing 4 nos.addition brushes on slip rings due to thismodification current on brushes reducedto 50-60 amps. Failure rate reduced tozero

no failure took place after year2000 No brush lead failure observed


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OTHER ISSUES

Use of DC Control supply for future project. Ithas advantage of survival of drives from trippingin case of heavy voltage dips

Alternately we can use A.C control supply fromUPS System. Improvement in Environment for L.T/H.T

Switchgears & Excitation panels by providing A.CSystem

CT SWGR/OFF Site areas are mostly neglectedAreas

Compressor MCC should be in separate room toavoid excessive noise and heat effect


34/44

ACHIEVEMENTS OF NCR OS AFTERSTRENGTHNING

After Reorganization of OS-NCR more stress was givenfor Station support to improve operationalperformance,Improve quality of Overhauls and render

ALL ASSISTANCE needed by Stations. OS-NCR took lead and Station support group/Maint

Management& efficiency group reviewed all pasttripping and started their involvement in all

overhaul/inspection activities ,Efficiency activities Apart From Routine activities some specific contributions

are as follows


35/44

RECTIFICATION OF WRONG C.T.CONNECTION AT AuGPS

GT#2,3,4 Tripped on Generator Diff. Protectionon faults outside its zone in the Year 2003-4,Total 07 Tripping were observed

Site took up matter with M/S M.H.I whosuggested to Demagnetize CTs to remove effectof saturation

MHI asked for detailed testing of CTs andreplacement of C.T which would have cost Rs23.0 lakh


36/44

RECTIFICATION OF WRONG C.T.CONNECTION AT AuGPS

NCR OS decided to witness Demagnetization test ofGT#4 which was under shutdown in March 05

During Demagnetization test it was observed that

class1.0 CT was used for Gen. Diff protection instead ofclass PS,which was root cause for spurious trip

C.T, connections were normalized and C.Ts tested formagnetizing current and found O.K

C.T connections were checked in GT#2,3 and foundnormal


37/44

RECTIFICATION OF WRONG C.T.CONNECTION

During Testing of C.Ts of other Units forMagnetizing current , deviation has beenobserved from original curves

It is being studied and matter will be discussedwith Engg.

If signs of deteoration is observed, than C.Tswill be required to be replaced

It is proposed to refer matter to OEM M/SMELCO JAPAN


38/44

DYNAMIC TESTING OF GENERATOR/EXCITATION SYSTEM AT AuGPS

Dynamic testing of Generator protection system ,ieSCC/OCC was not in practice since 1997 at Gas Stations

As per recent OS guidelines it should be done during

recommissioning after overhauling to eliminate false tripsafter overhauls

At AuGPS this test was not done after initialcommissioning. Matter was taken up with M/S MHIJAPAN by site for GT#4 , M/S MHI offered for testing

which involved financial implication of 7.8 millionYen with time schedule of 8 days


39/44


NCR OS and SITE studied the excitation system anddecision was taken to carryout SCC/OCC Tests withoutOEM supervision for GT#4 in April05

After detailed study procedure was made for interlockingand manual operation of Excitation system

To save Time required for testing, preparations weredone in advance and Excitation system was tested in

manual mode before rolling of machine


40/44


During GT#4 commissioning SCC/OCC was donesuccessfully in April 05 in record period of 6.30 Hrs

SCC/OCC Tests were also carried out in GT#2 afteroverhauling on 05.08.05 SCC/OCC Tests are to be done at ANGPS/ DGPS/ FGPS Sites are making procedure for conducting above tests

Test will be carried out in coming capital overhauls atNCR stations


41/44

TESTING /TUNNING OF AVR&LIMITORS

As per OS guidelines testing/tunings ofAVR/Limiters to be checked for correct operationof Excitation system

At ANGPP it is not done since commissioning At AuGPS it was also not done since

commissioning

matter was discussed in March ORTatAuGPS,andit was decided to get one Unit tuned byOEM,GT#2 was checked by OEM in Aug05,


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TESTING /TUNNING OF AVR&LIMITORS

Next Units will be tuned/checked by Site/NCROS without OEM supervision

DGPS has completed checking in OEMsupervision FGPS to check limiters/AVR Tuning Testing has to be included in scope of work

during overhauling


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CONCLUSION

Best Maintenance Practices resulted in Eliminationof Problem.

Identification of problems and their immediateresolution will help in improvement in system,

thereby marching towards ZERO forced outage We must adopt transparency in Unit trip analysisand problems should be analyzed and referred toOS/ Engg for concurrence of any modification

It will motivated all of us to think for bestpractices/aiming for ZERO forced outages


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THANKINGYOU

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Experiance Sharing for Best Maintenance-L.K.jain