Preventive maintainance and condition monitoring for power transformers
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SEMINAR ON. PREVENTIVE MAINTAINANCE AND CONDITION MONITORING FOR POWER TRANSFORMERS. PRESENTED BY - M/s VISHVAS POWER ENGINEERING SERVICES PVT. LTD. NAGPUR. INFORMATION ABOUT COMPANY.

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Preventive maintainance and condition monitoring for power transformers

SEMINAR ON

PREVENTIVE MAINTAINANCE AND CONDITION MONITORING FOR POWER TRANSFORMERS

PRESENTED BY -

M/s VISHVAS POWER

ENGINEERING SERVICES PVT. LTD. NAGPUR


Information about company

INFORMATION ABOUT COMPANY

  • NAMES OF DIRECTORS :Mr. R. M. Bhave : Mr.S.V.D’mello :Mr.Kiran Joharapurkar

  • PRODUCT SKILLS OF DIRECTORS:HT switchgrars : Transformers (all make):On load tap changers (all type)

  • No. of engineers/site managers:10

  • Office staff:04

  • Other staff/skilled workers:80


Information about company1

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


Doubts raised by users

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


Please let me know

PLEASE LET ME KNOW

WHAT YOU EXPECT FROM THIS SESSION?


Areas of concern

OIL

PAPER

Wood (treated / parmali

WINDING

TANK

LEADS

OLTC

BUSHING

COOLING BANKS/ HEAT EXCHANGER

AUXILIARIES

Areas of concern


Important standards transformer user should know

IMPORTANT STANDARDS TRANSFORMER USER SHOULD KNOW

  • IS 335

  • IS 12463

  • IS 1866

  • IS 2099

  • IS 2026


Preventive maintainance and condition monitoring for power transformers

TRANSFORMER FOR OVERHAULING


Process of overhauling

PROCESS OF OVERHAULING

TESTING

POST SHUT DOWN

OBSERVATIONS

P.t.o


Post shut down observations cont

POST SHUT DOWN OBSERVATIONS(CONT..)

OIL DRAINING

DISMANTLING


Preventive maintainance and condition monitoring for power transformers

125 MVA TOP COVER LIFTING


Preventive maintainance and condition monitoring for power transformers

GASKET REPLACEMENT

INTERNAL INSPECTION

PREPARING TRS FOR EVACUATION

N2 PRESSURE TEST


Preventive maintainance and condition monitoring for power transformers

VIEW OF LEAD TAKE OFF


Ieee guidelines internal inspection

IEEE GUIDELINES INTERNAL Inspection

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


Ieee guidelines internal inspection1

IEEE GUIDELINES INTERNAL Inspection

  • Loose connections - Tap leads Bushings Terminal Boards Spacers Core bolt nuts.

  • Physical status- Contacts Operating mechanism Coupling shafts Shielding


Ieee guidelines for internal inspection

IEEE guidelines for Internal Inspection

  • Oil- Odour Color Moisture Free Water

  • Discoloration- Tank walls Bushing Terminals Corona shields Copper connectors Bus bars


  • Vacuum drying

    Vacuumdrying

    Vacuum drying is based on the principle of Thermodynamics

    Boiling point of moisture in insulation isreduced by way of PressureReductionPrinciple

    Thermal energy needs to be impartedexternally for achieving faster and better moisture extraction


    Preventive maintainance and condition monitoring for power transformers

    DRY OUT PROCESS INVOLVING CYCLES


    Vacuum drying continued

    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.


    Preventive maintainance and condition monitoring for power transformers

    Prepare TRS for evacuation

    Fill N2over air to get pressure 0.2KG/CM2

    Process of dryout

    Attend leakages


    Preventive maintainance and condition monitoring for power transformers

    PRESSURE TEST

    NITROGEN PRESSURE


    Preventive maintainance and condition monitoring for power transformers

    Continue to heat till

    Core_< 70 c

    Med _< 75c

    Wdg _<70c


    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

    COMPARISON OF HEATING METHODS


    Preventive maintainance and condition monitoring for power transformers

    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


    Preventive maintainance and condition monitoring for power transformers

    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


    Details of nitrogen heating

    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


    Preventive maintainance and condition monitoring for power transformers

    HEATING ARRANGEMENT


    Preventive maintainance and condition monitoring for power transformers

    VIEW OF HEATING ARRANGEMENT

    HALOGEN HEATING OF TANK


    Preventive maintainance and condition monitoring for power transformers

    VIEW OF SHED FOR HEATING


    Preventive maintainance and condition monitoring for power transformers

    DRY OUT IN PROGRESS


    Preventive maintainance and condition monitoring for power transformers

    PT 100 SENSOR FOR TEMPERATURE MONITORING


    Heating cycle duration will depend upon size of trs

    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


    Preventive maintainance and condition monitoring for power transformers

    Heating cycles 2 :36/48 hours

    Vacuum cycle 2 : 48/60 hrs

    Heating cycle 3 :24/36 hrs


    Preventive maintainance and condition monitoring for power transformers

    Vacuum cycle 3 :36 hrs

    Heating cycle 4 :36 hrs


    Preventive maintainance and condition monitoring for power transformers

    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


    Preventive maintainance and condition monitoring for power transformers

    Break vacuum over oil

    by N2/ dry air

    Oil toppling up oil circulation

    Oil testing at site for

    BDV >_ 60 kV

    moisture content

    <- 10 ppm


    Preventive maintainance and condition monitoring for power transformers

    Stop filtration if oil parameters are satisfactory as per voltage class of trs.


    Preventive maintainance and condition monitoring for power transformers

    VIEW OF FILTERPLANT OIL TANK AND DG SET


    Preventive maintainance and condition monitoring for power transformers

    TRANSFORMER AFTER OVERHAULING


    Trends in oil

    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.


    Preventive maintainance and condition monitoring for power transformers

    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.


    Oil comparision

    OIL COMPARISION


    Preventive maintainance and condition monitoring for power transformers

    SYNTHETIC OIL

    • Used due to fire resistance property

      2. Contain polychlorinated biphenyl.

      3. Hazardous for dumping in earth or otherwise.


    Effect of moisture content in oil on impulse strength

    30 ppm

    MOISTURE CONTENT

    EFFECT OF MOISTURE CONTENT IN OIL ON IMPULSE STRENGTH

    100%

    70%

    IMPULSE

    STRENGTH


    Oil color interpretation astm color astm d1500 91

    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


    Oil colour interpritation

    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 <26 dynes/cmACIDITY> 0.15 mg of KOH gm of oil


    Is 335

    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 35*1012 ohm-cm AT 90º c 1500*1012 ohm-cm AT 27º c


    Is 3351

    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)


    Dissolved gas analysis

    DISSOLVED GAS ANALYSIS

    • Most widely used and effective technique.Requires skill to draw inference.

      FAULTS

    • Thermal aging - overhead metallic parts

    • Partial discharge

    • Arcing


    Factors influencing gas production

    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


    Dissolved gas analysis methodology

    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


    Labeling

    Labeling

    • Details of transformers

    • Date of sampling

    • History of transformers

    • Temp of transformer

    • Charged/service condition

    • Ambient Temp.


    Preventive maintainance and condition monitoring for power transformers

    STAGES

    • Collection of oil sample.

    • Extraction of gases in dissolved oil.

    • Separation identification and quantitative determination of gas

    • Interpretation.


    Key gas and fault pattern

    Key gas and fault pattern

    • C2H2 (Acetylene) :Arcing

    • H2 (Hydrogen) :Partial discharge

    • C2H4 (Ethylene) :Oil overheating.

    • Co/Co2 (Carbon monoxide & Carbon Dioxide) :Conductor overheating


    Preventive maintainance and condition monitoring for power transformers

    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 oilc2h2 h2


    Preventive maintainance and condition monitoring for power transformers

    • KEY GAS : Hydrogen

    • ARCING : Between leads. Leads to coil.

    • KEY GAS :Acetylene , Hydrogen


    Preventive maintainance and condition monitoring for power transformers

    IEEE

    Arcing :Acetylene C2H2

    :Hydrogen H2

    Carona : H2

    Overheated oil :Ethylene C2H2

    :Methane CH4


    Preventive maintainance and condition monitoring for power transformers

    • 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.


    Preventive maintainance and condition monitoring for power transformers

    STRATEGIC END OF LIFE

    ECONOMIC END OF LIFE

    TECHNICAL END OF LIFE

    DEGREE OF POLYMERIZATION

    INITIAL : 1200

    POOR : 300

    AFFECTING PARAMETERS

    TEMPERATURE

    MOISTURE

    ACIDITY


    Comparison of furful content in various generator transformers

    Comparison of Furful content in various generator transformers


    Interpretations of impedance test

    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


    Case iii

    CASE III


    Ageing of insulating paper

    AGEINGOF INSULATING PAPER

    With advanced instruments available fortechnical 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


    Preventive maintainance and condition monitoring for power transformers

    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)


    Degree of polymerization

    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.


    Deterioration of paper

    DETERIORATION OF PAPER

    900 c

    Degree of polymerization

    1050c

    1200c

    150

    Days


    Source documents forums

    Source documents / forums

    • IEEE

    • ANSI

    • I.S.

    • C.B.I.P.’s INT. Conference

    • CIGRE’s working group

    • Our own field experience


    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

    Abnormalities observed during internal inspection/ capital overhauling


    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.

    Abnormalities(continued)


    Bushing

    BUSHING

    TYPE

    • SRBP/OIP/RIP

      TEST TAP

    • FOR MEASUREMENT

    • MUST BE EARTHED

    • PROPER EARTHING

    • CORROSION OF THREADS ON CONNECTION CAUSES HIGH RESISTANCE AND SPARKING


    Preventive maintainance and condition monitoring for power transformers

    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.


    Preventive maintainance and condition monitoring for power transformers

    1


    Preventive maintainance and condition monitoring for power transformers

    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.


    Dga of oil for bushing iec 36a

    DGA OF OIL FOR BUSHING [IEC-36A]


    Predictive maintenance for oltc

    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.


    Predictive maintenance cont

    Predictive maintenance(cont...)

    • Changes in switching time weak springs,

    • Overhauling

    • Contact wear


    Precautionary steps for ensuring correct installations

    Precautionary steps for ensuring correct installations

    Paper Moisture

    420 KV0.5 or less

    245 KVless than1.0

    145 KVless than 2

    Oil Moisture

    420 KVless than 10

    245 KVless than 15

    145 KVless than 20


    Preventive maintainance and condition monitoring for power transformers

    SETTLING TIME

    KVTime

    420 KV 72 hours

    245 KV 48 hours

    145 KV36 hours


    Preventive maintainance and condition monitoring for power transformers

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


    Partial discharge

    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.


    Frequency response analysis

    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.


    Preventive maintainance and condition monitoring for power transformers

    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.


    Procedure for fra

    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.


    Preventive maintainance and condition monitoring for power transformers

    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


    Preventive maintainance and condition monitoring for power transformers

    INDICATIONS

    • Shift in response of winding

    • Differences in response between phases of same transformer.

    • Difference between responses of transformer of same design.


    Control of leakage flux in power transformer

    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.


    Effect of leakage flux

    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.


    Preventive maintainance and condition monitoring for power transformers

    CASE STUDYNew 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


    Concept of air cell

    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.


    Thermosyphon filter chambers

    THERMOSYPHON FILTER CHAMBERS

    On line acidity Reduction

    • Acidity of oil is bad for insulation.

    • Aluminum granules can reduce acidity and Fuller’s 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 fuller’s earth reduces the acidity

    • Continuous use of facility can help in maintaining acidity low since commissioning.


    Hydran 2011 system

    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


    Sf6 insulated transformers

    SF6INSULATED 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


    Thank you

    THANK YOU


    Preventive maintainance and condition monitoring for power transformers

    FILTER PLANT HIGH VACUUM WITH DOUBLE DEGASSING CHAMBER


    Preventive maintainance and condition monitoring for power transformers

    TRANSFORMER ASSEMBLY IN PROGRESS


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