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Title: PREVENTIVE MAINTAINANCE AND CONDITION MONITORING FOR POWER TRANSFORMERS


1
PREVENTIVE MAINTAINANCE AND CONDITION MONITORING
FOR POWER TRANSFORMERS
SEMINAR ON
  • PRESENTED BY -
  • M/s VISHVAS POWER
  • ENGINEERING SERVICES PVT. LTD. NAGPUR

2
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

3
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

4
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

5
PLEASE LET ME KNOW
  • WHAT YOU EXPECT FROM THIS SESSION?

6
Areas of concern
  • OIL
  • PAPER
  • Wood (treated / parmali
  • WINDING
  • TANK
  • LEADS
  • OLTC
  • BUSHING
  • COOLING BANKS/ HEAT EXCHANGER
  • AUXILIARIES

7
IMPORTANT STANDARDS TRANSFORMER USER SHOULD KNOW
  • IS 335
  • IS 12463
  • IS 1866
  • IS 2099
  • IS 2026

8
TRANSFORMER FOR OVERHAULING
9
PROCESS OF OVERHAULING
TESTING
POST SHUT DOWN OBSERVATIONS
P.t.o
10
POST SHUT DOWN OBSERVATIONS(CONT..)
OIL DRAINING
DISMANTLING
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125 MVA TOP COVER LIFTING
24
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

47
IEEE GUIDELINES INTERNAL Inspection
  • Loose connections - Tap leads
    Bushings Terminal Boards
    Spacers Core bolt nuts.
  • Physical status- Contacts
    Operating mechanism Coupling shafts
    Shielding

48
IEEE guidelines for Internal Inspection
  • Oil- Odour Color
    Moisture Free Water
  • Discoloration- Tank walls Bushing
    Terminals Corona shields Copper
    connectors Bus bars

49
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.

52
Process of dryout
Attend leakages
53
PRESSURE TEST
NITROGEN PRESSURE
54
Continue to heat till Core_lt 70 c Med _lt 75c Wdg
_lt70c
55
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

56
  • 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

57
  • 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

58
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

59
HEATING ARRANGEMENT
60
VIEW OF HEATING ARRANGEMENT
HALOGEN HEATING OF TANK
61
VIEW OF SHED FOR HEATING
62
DRY OUT IN PROGRESS
63
PT 100 SENSOR FOR TEMPERATURE MONITORING
64
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
65
Heating cycles 2 36/48 hours
Vacuum cycle 2 48/60 hrs
Heating cycle 3 24/36 hrs
66
Vacuum cycle 3 36 hrs
Heating cycle 4 36 hrs
67
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
68
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
74
TRANSFORMER AFTER OVERHAULING
75
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.

76
  • 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.

77
OIL COMPARISION
78
  • SYNTHETIC OIL
  • Used due to fire resistance property
  • 2. Contain polychlorinated biphenyl.
  • 3. Hazardous for dumping in earth or otherwise.

79
EFFECT OF MOISTURE CONTENT IN OIL ON IMPULSE
STRENGTH

100
70 IMPULSE STRENGTH
80
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

81
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

82
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

83
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)

84
DISSOLVED GAS ANALYSIS
  • Most widely used and effective technique. Require
    s skill to draw inference.
  • FAULTS
  • Thermal aging - overhead metallic parts
  • Partial discharge
  • Arcing

85
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

86
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

87
Labeling
  • Details of transformers
  • Date of sampling
  • History of transformers
  • Temp of transformer
  • Charged/service condition
  • Ambient Temp.

88
  • STAGES
  • Collection of oil sample.
  • Extraction of gases in dissolved oil.
  • Separation identification and quantitative
    determination of gas
  • Interpretation.

89
Key gas and fault pattern
  • C2H2 (Acetylene) Arcing
  • H2 (Hydrogen) Partial discharge
  • C2H4 (Ethylene) Oil overheating.
  • Co/Co2 (Carbon monoxide Carbon Dioxide)
    Conductor overheating

90
  • 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

91
  • KEY GAS Hydrogen
  • ARCING Between leads. Leads to coil.
  • KEY GAS Acetylene , Hydrogen

92
  • IEEE
  • Arcing Acetylene C2H2
  • Hydrogen H2
  • Carona H2
  • Overheated oil Ethylene C2H2
  • Methane CH4

93
  • 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.

94
  • STRATEGIC END OF LIFE
  • ECONOMIC END OF LIFE
  • TECHNICAL END OF LIFE
  • DEGREE OF POLYMERIZATION
  • INITIAL 1200
  • POOR 300
  • AFFECTING PARAMETERS
  • TEMPERATURE
  • MOISTURE
  • ACIDITY

95
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

98
CASE III
99
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

100
  • 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)
101
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.

102
DETERIORATION OF PAPER
900 c
Degree of polymerization
1050c
1200c
150
Days
103
Source documents / forums
  • IEEE
  • ANSI
  • I.S.
  • C.B.I.P.s INT. Conference
  • CIGREs working group
  • Our own field experience

104
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

105
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.

106
BUSHING
  • TYPE
  • SRBP/OIP/RIP
  • TEST TAP
  • FOR MEASUREMENT
  • MUST BE EARTHED
  • PROPER EARTHING
  • CORROSION OF THREADS ON CONNECTION CAUSES HIGH
    RESISTANCE AND SPARKING

107
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.

108
1
109
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.

110
DGA OF OIL FOR BUSHING IEC-36A
111
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.

112
Predictive maintenance(cont...)
  • Changes in switching time weak springs,
  • Overhauling
  • Contact wear

113
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

114
  • SETTLING TIME
  • KV Time
  • 420 KV 72 hours
  • 245 KV 48 hours
  • 145 KV 36 hours

115
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
116
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.

117
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.

118
  • 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.

119
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.

120
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

121
  • INDICATIONS
  • Shift in response of winding
  • Differences in response between phases of same
    transformer.
  • Difference between responses of transformer of
    same design.

122
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.

123
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.

124
  • 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

125
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.

126
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.

127
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

128
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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