PREVENTIVE MAINTAINANCE AND CONDITION MONITORING FOR POWER TRANSFORMERS

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PREVENTIVE MAINTAINANCE AND CONDITION MONITORING
FOR POWER TRANSFORMERS
SEMINAR ON

• PRESENTED BY -
• M/s VISHVAS POWER
• ENGINEERING SERVICES PVT. LTD. NAGPUR

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INFORMATION ABOUT COMPANY

• NAMES OF DIRECTORS Mr. R. M. Bhave
  Mr.S.V.Dmello
  Mr.Kiran Joharapurkar
• PRODUCT SKILLS OF DIRECTORS HT switchgrars
  Transformers (all make) On load tap
  changers (all type)
• No. of engineers/site managers10
• Office staff04
• Other staff/skilled workers80

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INFORMATION ABOUT COMPANY

• AREAS OF BUSINESS INTEREST
• Capital overhaul/servicing of power transformers
• Servicing of SF6 breakers
• Servicing of on load tap changers.
• Annual maintenance contracts for substation and
  control room maintenance.
• Testing of oil and bushings.
• RLA studies on power transformers.
• Testing of oil and bushings.
• Installation services for power transformers, HT
  switchgears and substation equipments
• Retrofitting of LT air circuit breakers
• Installion of rubber balloons in conservator of
  power transformers to retard oxidation of oil
• Testing and commising of substations
• Testing of transformers

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Doubts raised by users

• Whether oil filteration or dry out
• Criterions of stopping dryout/filteration
• Whether overhauling/reinsulation
• Frequency of condition monitoring
• Oil mixing ---------- preconditions
• Dissolved Gas Analyis

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PLEASE LET ME KNOW

• WHAT YOU EXPECT FROM THIS SESSION?

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Areas of concern

• OIL
• PAPER
• Wood (treated / parmali
• WINDING
• TANK
• LEADS
• OLTC

• BUSHING
• COOLING BANKS/ HEAT EXCHANGER
• AUXILIARIES

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IMPORTANT STANDARDS TRANSFORMER USER SHOULD KNOW

• IS 335
• IS 12463
• IS 1866
• IS 2099
• IS 2026

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TRANSFORMER FOR OVERHAULING
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PROCESS OF OVERHAULING
TESTING
POST SHUT DOWN OBSERVATIONS
P.t.o
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POST SHUT DOWN OBSERVATIONS(CONT..)
OIL DRAINING
DISMANTLING
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125 MVA TOP COVER LIFTING
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GASKET REPLACEMENT
INTERNAL INSPECTION
PREPARING TRS FOR EVACUATION
N2 PRESSURE TEST
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VIEW OF LEAD TAKE OFF
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IEEE GUIDELINES INTERNAL Inspection

• Winding - Lead clamping Winding support
  system Clamping Insulation
  discoloration Lead distortion Lead
  movement Indications of hot spots

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IEEE GUIDELINES INTERNAL Inspection

• Loose connections - Tap leads
  Bushings Terminal Boards
  Spacers Core bolt nuts.
• Physical status- Contacts
  Operating mechanism Coupling shafts
  Shielding

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IEEE guidelines for Internal Inspection

• Oil- Odour Color
  Moisture Free Water
• Discoloration- Tank walls Bushing
  Terminals Corona shields Copper
  connectors Bus bars

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Vacuum drying

• Vacuum drying is based on the principle of
  Thermodynamics
• Boiling point of moisture in insulation is
  reduced by way of Pressure Reduction Principle
• Thermal energy needs to be imparted externally
  for achieving faster and better moisture
  extraction
• 
  

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Vacuum drying( continued)

• Under vacuum (pressure reduction)boiling point of
  water reduces to 500c
• medium of heating can be
• Old oil
• Nitrogen
• Hot air
• other methods of heating are also under use.

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Process of dryout
Attend leakages
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PRESSURE TEST
NITROGEN PRESSURE
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Continue to heat till Core_lt 70 c Med _lt 75c Wdg
_lt70c
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COMPARISON OF HEATING METHODS

• OI L
• REQUIRES OLD OIL FOR HEATING
• REQUIRES FILTER PLANT OIL RESERVIORS
• REQUIRES LESS MONITORING

• NITROGEN
• REQUIRES NITROGEN GAS
• REQUIRES EXTRNAL HEATING ELEMENTS SUCH AS
  HEATERS
• REQUIRES MORE VIGILLANT MONITORING

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• OIL SOAKED INSULATION DOES NOT RELEASE MOISTURE
  VERY FAST
• IF OIL IS ACIDIC THEN IT CAN AFFECT INSULATION
  PAPER
• THIS METHOD IS NOT CONSIDERED PRESENTLY AS USER
  FRIENDLY METHOD

• DRY AND NON SOAKED INSULATION RELEASES MORE
  MOISTURE AND IN FASTER WAY
• IF NITROGEN IS PURE IT CANNOT AFFECT INSULATION
  PAPER COPPER

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• OIL
• MONITORING PARAMETERS ARE OIL BDV MOISTURE
  CONTENT OF OIL USED FOR HEATING PURPOSE
• OIL WILL LOSE/REDUCE ITS PARAMETERS SUCH AS
  RESISTIVITY, TANDELTA WHEN HEATED ABOVE 600C

• NITROGEN
• THIS METHOD IS PRESENTLY CONSIDERED AS USER
  FRIENDLY AND TECHNICALLY BETTER .
• MONITORING PARAMETERES ARE CONPARISON OF IR VALUE
  AND DRYNESS OF NITROGEN GAS USED FOR HEATING
  PURPOSE
• NITROGEN HEATING DOES NOT INVOLVE OIL HEATING
  ABOVE 600C

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Details of nitrogen heating

• Nitrogen gas 99.6 pure
• Dew point-40c
• better
• Pressure gauge 0.1 To 1.0KG/SQ.CM
• Vacuum gauge MM scale Bourden
• torr scale Macleoid
• M-bar scale

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HEATING ARRANGEMENT
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VIEW OF HEATING ARRANGEMENT
HALOGEN HEATING OF TANK
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VIEW OF SHED FOR HEATING
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DRY OUT IN PROGRESS
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PT 100 SENSOR FOR TEMPERATURE MONITORING
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Heating cycle duration will depend upon size of
trs
72 hrs 48 hrs 24hrs
100-250 MVA 50 MVA 25 MVA
Vacuum cycles 1 for 60/72 hrs
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Heating cycles 2 36/48 hours
Vacuum cycle 2 48/60 hrs
Heating cycle 3 24/36 hrs
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Vacuum cycle 3 36 hrs
Heating cycle 4 36 hrs
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Vacuum cycle iv
N2 filling Ir. value measurement dew pt
measurement
Take vacuum and fill up oil
While filling oil fill under vacuum and fill from
top filler valve
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Break vacuum over oil by N2/ dry
air




Oil toppling up oil circulation
Oil testing at site for BDV gt_ 60 kV moisture
content lt- 10 ppm
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Stop filtration if oil parameters are
satisfactory as per voltage class of trs.
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VIEW OF FILTERPLANT OIL TANK AND DG SET
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TRANSFORMER AFTER OVERHAULING
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TRENDS IN OIL

• Napthene based oil
• 1. Low pour point enabling the use in low ambient
  temp.areas
• 2. Oxidation products are soluble in oil.
• 3. High resistivity.
• 4. High Viscosity.
• Paraffin based oil
• 1. High pour point but no problem in country like
  INDIA.

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• ALKYL BENZENE BASED OIL
• 1. Low Tan Delta
• 2. High resistivity.
• 3. No sludge.
• 4. Low viscosity.
• 5. Very low pour point.
• 6. Non hygroscopic and does not form gum and
  sludge.

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OIL COMPARISION
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• SYNTHETIC OIL
• Used due to fire resistance property
• 2. Contain polychlorinated biphenyl.
• 3. Hazardous for dumping in earth or otherwise.

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EFFECT OF MOISTURE CONTENT IN OIL ON IMPULSE
STRENGTH

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70 IMPULSE STRENGTH
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Oil color Interpretation ASTM- color ASTM -
D1500-91

• Clear New condition
• Pale yellow Good condition
• Yellow Service condition
• Bright yellow Marginal condition
• Amber Bad condition
• Brown Severe bad condition

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OIL COLOUR INTERPRITATION

• Dark brown - Extremely bad condition
• Sludge - To detect presence of pentane
  insoluble sludge in serviced oil
• THE TEST IS not Performed unless
• IFT lt26 dynes/cm ACIDITYgt 0.15 mg of KOH
  gm of oil

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IS 335

• Density o.89 gm/cm3 at 29.5º C
• Viscosity 27 MAX at 27º C
• IFT 0.04 N/M
• Flash point 1400c
• Pour point -60c
• BDV 30 KV unfiltered 60
  KV filtered
• TAN DELTA 0.002
• RESISTIVITY 351012 ohm-cm AT 90º c
  15001012 ohm-cm AT 27º c

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IS 335

• Acidity 0.4(max)
• Sludge 0.1 (max)
• Corrosive sulphur No traces
• oxidation inhibitor 0.05 (max)
• Water content 50 PPM max
  (unfiltered) 15 PPM max
  (filtered)

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DISSOLVED GAS ANALYSIS

• Most widely used and effective technique. Require
  s skill to draw inference.
• FAULTS
• Thermal aging - overhead metallic parts
• Partial discharge
• Arcing

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FACTORS INFLUENCING GAS PRODUCTION

• Meaningful assessment of a Transformer on basis
  of DGA is possible if information and data is
  considered on following factors which influence
  gassing rate gassing comparison
• Oxygen content of oil
• Oil temperature
• Ambient temperature

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Dissolved Gas AnalysisMethodology

• Gas tight syringe with a 3 way stop cock
• Stainless steel tubular bottle
• Bottle capable of being air/gas tight with
  screwed cap

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Labeling

• Details of transformers
• Date of sampling
• History of transformers
• Temp of transformer
• Charged/service condition
• Ambient Temp.

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• STAGES
• Collection of oil sample.
• Extraction of gases in dissolved oil.
• Separation identification and quantitative
  determination of gas
• Interpretation.

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Key gas and fault pattern

• C2H2 (Acetylene) Arcing
• H2 (Hydrogen) Partial discharge
• C2H4 (Ethylene) Oil overheating.
• Co/Co2 (Carbon monoxide Carbon Dioxide)
  Conductor overheating

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• Various h2 ch2 c2h6 c2h4 c2h2 co co2
• sources
• IEEE 140 160 115 190 11 580 --
• GTR 100 120 65 50 35 350 2500
• CIGRE 28.6 42.2 85.6 74.6 -- 289 3771
• DOBLE 100 100 60 100 5 250 --
• New Trs 200 100 100 150 15 500 ----
• Old trs 250 200 200 300 35 1000 --
• Oil overheating c2h4 c2h6 ch4
• Cellulose overheating co
• Partial discharge in oil h2 ch4
• Arcing in oil c2h2 h2

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• KEY GAS Hydrogen
• ARCING Between leads. Leads to coil.
• KEY GAS Acetylene , Hydrogen

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• IEEE
• Arcing Acetylene C2H2
• Hydrogen H2
• Carona H2
• Overheated oil Ethylene C2H2
• Methane CH4

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• Thermal degradation of insulation material
• Co, Co2, H2o
• Healthy ratio of Co/Co2 5
• If ratio is below 3, it indicates severe
  overheating of paper.
• Oil degradation Methane(CH4),
  Ethane(C2H6) Ethylene(C2H5)
• Partial Discharge Occurs due to ionization in
  high stressed areas where gas/vapour filled
  voids.

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• STRATEGIC END OF LIFE
• ECONOMIC END OF LIFE
• TECHNICAL END OF LIFE
• DEGREE OF POLYMERIZATION
• INITIAL 1200
• POOR 300
• AFFECTING PARAMETERS
• TEMPERATURE
• MOISTURE
• ACIDITY

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Comparison of Furful content in various generator
transformers
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Interpretations of impedance test

• Variation in short circuit impedance of a
  transformer winding indicates possible movement
  of winding/deformation within the transformer
• Variations upto 2 --Negligible
• Variations upto 3 --Significant

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CASE III
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AGEING OF INSULATING PAPER

• With advanced instruments available for
  technical analysis of aged insulation it is
  possible to determine /measure following
  properties of insulation paper
• 1. Thickness
• 2. Water content
• 3. Tensile strength
• 4. Degree of polymerization

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• TENSILE STRENGTH
• 1. Determined as per IS 1924 determined in both
  machine direction and cross machine directions.
• 2. Deterioration of tensile strength is further
  dependent on temp. factor.
• 3. The rate of tensile strength deterioration is
  exponential with time.

Tensile strength
Ageing (days)
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Degree of polymerization

• This proves to be the most informative parameter
  for assessing the ageing or the process of ageing
  of cellulose . The connection between
  deterioration in material properties and
  formation of aging products is degree of
  polymerization.
• This is also direct decomposition of cellulose.

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DETERIORATION OF PAPER
900 c
Degree of polymerization
1050c
1200c
150
Days
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Source documents / forums

• IEEE
• ANSI
• I.S.
• C.B.I.P.s INT. Conference
• CIGREs working group
• Our own field experience

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Abnormalities observed during internal
inspection/ capital overhauling

• EXTERNAL
• Pumps running in reverse direction
• Thermosyphon blanked
• Thimble-terminal cap fusion due to heating.heat
  generated due to faulty term.connector.

• INTERNAL
• 1 Cotton tape used for gasket locating not
  removed causing leakage
• Core bolt insulation zero
• Selector switch contact arm assy not making
  adequate contact

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Abnormalities(continued)

• Bucholz pipeline slope not proper
• Lens screw of OLTC shaft not properly fitted
• Oil leakges in box type stiffner, between main
  tank and stiffner

• Core earthing melted.
• Part of laminations fused.

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BUSHING

• TYPE
• SRBP/OIP/RIP
• TEST TAP
• FOR MEASUREMENT
• MUST BE EARTHED
• PROPER EARTHING
• CORROSION OF THREADS ON CONNECTION CAUSES HIGH
  RESISTANCE AND SPARKING

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3

• Measurement of mounted bushing done in UST
  mode , using test tap connection
• 4
• Measurement of winding is done in GST mode
  using connections between bushing top terminal
  and grounded.

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2

• 10 kV is a referred as a preferred reference
  voltage although usually measurements of tan
  delta and capacitance should not vary with
  applied voltage.
• Usually measurements at 2 KV,5KV and 10KV are
  taken at site.
• Capacitance value measurement is equally
  important.
• Tan delta value measurement may get affected by
  conditions such as oil. temperature, humidity,
  surface cleanliness and inductions in the
  switchyard.

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DGA OF OIL FOR BUSHING IEC-36A
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Predictive maintenance for OLTC

• Contact resistance measurement .
• Measurement of divertor motor acoustic signals.
• Temp difference between oil in main tank and oil
  in divertor switch.
• Dissolved gas analysis.
• Deposition of solid carbon.

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Predictive maintenance(cont...)

• Changes in switching time weak springs,
• Overhauling
• Contact wear

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Precautionary steps for ensuring correct
installations

• Paper Moisture
• 420 KV 0.5 or less
• 245 KV less than1.0
• 145 KV less than 2
• Oil Moisture
• 420 KV less than 10
• 245 KV less than 15
• 145 KV less than 20

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• SETTLING TIME
• KV Time
• 420 KV 72 hours
• 245 KV 48 hours
• 145 KV 36 hours

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Magnetic balance test is like a signature
testDiagnostic tool with focus on current is
more importantQuality of core is not assessedif
voltage distribution is in order but current are
not then further analysis is required
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Partial Discharge

• Effective for comparing at factory/ at site after
  repairs.
• Advanced diagnostic system used for off line PD
  detection.
• 3 Background noise suppression, using a Spectrum
  analyzer to do selective filtration.

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FREQUENCY RESPONSE ANALYSIS

• Short circuit high fault currents can lead to
  displacement of windings.
• An already weak coil or an insulation system can
  fail, during over voltage stress.
• Background.
• Electrical measurements are not sufficient to
  indicate this type of displacement of coil or
  insulation.

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• Principle
• Deformation results in minor changes of internal
  inductance and capacitance of winding.
• These deformations can be noted by noting a
  difference in response of particular winding on
  network analyzer when 1 signal (sinusoidal)
  having frequency band 50 Hz to 500 Hz is
  connected to bushing and response is measured
  through neutral bus.

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Procedure for FRA

• Identify two ends of winding.
• Connect two cables from measuring
• Unit giving sinusoidal output of two V rms
  and one measuring input
• Other end of winding is connected to measuring
  input.
• Other windings are grounded.
• Voltage transfer function is measured for each
  winding for four frequency scans between 5 Hz to
  2 MHz.

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FREQUENCY RESPONSE ANALYSIS

• BENEFITS
• Immune to Electro-magnetic interference.
• Easy to perform on site.
• Identical results for similar transformers.
• DRAW BACKS
• Above data required effectively used more as
  signature

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• INDICATIONS
• Shift in response of winding
• Differences in response between phases of same
  transformer.
• Difference between responses of transformer of
  same design.

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CONTROL OF LEAKAGE FLUX IN POWER TRANSFORMER

• NETT FLUX WHICH IS NOT LINKED BY BOTH COILS
• Magnetic flux which energizes core abd induces
  required flux density.
• Load flux which is balanced by load flux of
  secondary winding.
• Leakage flux which links the winding conductors
  and returns through a path outside the core.

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EFFECT OF LEAKAGE FLUX

• Leakage flux will follow a path of low
  resistance
• Will be attracted to magnetic steel.
• This induces Eddy Currents and hence generate
  heat.
• REMEDY
• Increase the distance of tank wall from source
  of leakage flux.
• Continuously transposed conductor (CTC).
• To provide shunt plates to carry leakage flux.

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• CASE STUDY New 390 MVA generator transformer
• PROBLEM Core overheating Yoke
  overheating Tank top cover overheating
• VISIBLE EFFECTS
• Due To overheating caused by eddy currents
  flowing in top cover or bell cover visible
  deterioration of gasket was observed
• DESIGN REMEDY
• To subdivide the plate width of outermost pockets
• Use of nonmagnetic material for frames.
• Use of flux shunt plates at LV side of the tank

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CONCEPT OF AIR CELL

• 1. Oil Deteriorates majorly because of Oxidation.
• 2. Atmospheric breathing is a continuous process.
• 3. To avoid atmospheric breathing, rubber
  cell/balloon/separator is inserted in
  conservator.
• 4. The balloon / air cell is INFLATED INITIALLY.
  The breathing of transformer is then through this
  cell.
• 5. When air expands, air cell contracts.
• 6. When oil contracts , air cell expands
• 7. There can be a substantial reduction in oil
  deterioration, preserving or at least retarding
  its increase parameters like acidity moisture
  content and dielectric dissipation factor.

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THERMOSYPHON FILTER CHAMBERS

• On line acidity Reduction
• Acidity of oil is bad for insulation.
• Aluminum granules can reduce acidity and Fullers
  earth can reduce acidity.
• Above materials are contained in cylinder
  chamber/chambers in wire mesh.
• Cylinder/ cylinders are fitted to transformer
  tank and connected through valves.
• Oil passing through the column of Alumina or
  fullers earth reduces the acidity
• Continuous use of facility can help in
  maintaining acidity low since commissioning.

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Hydran 2011 system

• Continuos monitoring of key gases
• Alarm facility
• Hourly and daily changes in level
• Adjustable alarm levels
• Self test system
• Possibility of connection to SCADA systems

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SF6 INSULATED TRANSFORMERS

• MANUFACTURING RANGE 275KV300MVA
• COMPACT SIZE
• REDUCED SIZE
• EXTRA LIFE
• HIGHER COST ALMOST DOUBLE
• RATE OF LOWERING DP VALUE IS RELATIVELY MUCH
  SLOWER IN SF6 THAN NITROGEN AND OIL

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THANK YOU
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FILTER PLANT HIGH VACUUM WITH DOUBLE DEGASSING
CHAMBER
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TRANSFORMER ASSEMBLY IN PROGRESS
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