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  • Power Transformer Diagnostics: Novel Techniques and their ApplicationCharles SweetserTechnical Service Manager

  • Transformers

  • Topics of Discussion

    Introduction to Power Transformers Life Expectancy Failure Modes Diagnostic Characteristics Diagnostic Characteristics Diagnostic Measurements Standard Measurements Advanced Measurements SFRA - Sweep Frequency Response Analysis DFR - Dielectric Frequency Response

  • Transformer Considerations

    Transformer Types and Classifications Transformer Configurations Vector Groups Oil Preservation Systems Insulating Materials and Fluids Insulating Materials and Fluids Construction Forms Core Steel Ratings Cooling Schemes Tap Changers (OLTC, DETC) Bushings Surge Arresters

  • Transformer Types and Classifications

    Distribution Power Rectifier Arc-Furnace Arc-Furnace Network Regulating (Voltage Regulators) Phase Shifting Reactors*

  • Vector Groups

  • Winding Types

    1. Disk Winding2. Pancake Winding3. Helical Winding4. Cylindrical or Layer Winding4. Cylindrical or Layer Winding

    Courtesy of Delta Star, San Carlos, CACourtesy of Delta Star, San Carlos, CA

  • Construction FormsCore Form Concentric Less Iron More CU

    Shell Form Interleaved More Iron Less CU

    Core Form Shell FormCore Form Shell Form

  • Life Expectancy

    180,000 hrs or 20.55 years 110 C Hottest Spot for 65 C Temp Rise insulation Degree of Polymerization (200 -1200 DP) 1200 DP - New Paper 1200 DP - New Paper 200 DP at 150,000 hrs (end of life)

    Heat Moisture Oxygen

  • Failure Modes

  • Failure Modes

  • Core Failure Modes Over-Heating Bulk Movement Multiple Core Grounding

    Lamination Gaps Shorted Laminations Ungrounded Core

  • Oil Analysis DGA and Oil Screen

    Hydrogen (H2) Methane (CH4) Ethane (C2H6) Ethylene (C2H4)

    Rate of Gas Generation Partial Discharge Arcing Electrical Heating Ethylene (C H )

    Acetylene (C2H2) Carbon Monoxide (CO) Carbon Dioxide (CO2) Oxygen (O2) Nitrogen (N2)

    Electrical Heating Metal Heating Decomposition of Paper

  • Oil Analysis DGA and Oil Screen Dielectric Breakdown IFT Color Acidity Power Factor Power Factor Moisture Specific Gravity Viscosity

    Degree of Polymerization DP (Paper) Furans (Oil)

  • Transformer Tests

    Dielectric Thermal Mechanical

    DGA DGA SFRAOil Screen Oil Screen Leakage ReactancePF/TD CAP IR PF/TD CAPExciting Ima DC Winding RES Exciting ImaTurns Ratio Tests DC Winding RESDFR

    Insulation Resistance

  • Transformer Test Protocol

    1. Overall Power Factor and Capacitance (Tip-Up, Variable Freq)

    2. Bushings (C1, C2, Energized Collar) (Tip-Up, Variable Freq)

    3. Exciting Current

    4. Surge Arresters4. Surge Arresters

    5. Insulating Fluids

    6. Leakage Reactance (Frequency Response of Stray Losses)

    7. Turns Ratio Test

    8. DC Winding Resistance (Slope, Ripple)

    9. Insulation Resistance

  • Overall PF and Capacitance

  • Bushings Test Taps kV < 72 kV (C1 and C2)

    Potential Taps kV > 72 kV (C1 and C2)

    No Tap Use Energized Collar at 10 kV

  • Bushings

  • Surge Arresters

    Analyzed on the basis of Watts

  • Advanced Diagnostics

    1. Advanced Power FactorTip-Up: Voids in Insulation

    Variable Frequency (15 Hz to 400 Hz): Moisture and Aging2. Advanced DC Winding Resistance 2. Advanced DC Winding Resistance

    Ripple and Slope

    3. Advanced Leakage ReactanceFRSL (Frequency Response Stray Losses)

    4. SFRA - Sweep Frequency Response Analysis5. DFR - Dielectric Frequency Response

  • Variable Frequency Losses

    1.50E-03

    2.00E-03

    2.50E-03SerialParallelSum

    0.00E+00

    5.00E-04

    1.00E-03

    1.50E-03

    0 50 100 150 200 250 300 350 400

  • Normal Power Factor

  • Power Factor Influenced by Moisture

  • Slope and Ripple

    Ripple

    Slope D

    Ripple

  • Advanced DC Resistance - Slope

    Slope

    -0.2A/s

    -0.1A/s

    0.0A/s

    A UPA DOWN

    -0.6A/s

    -0.5A/s

    -0.4A/s

    -0.3A/s

    -0.2A/s

    000 005 010 015 020 025 030 Taps

    A DOWNB UPB DOWNC UPC DOWN

  • Advanced DC Resistance - Slope

    Slope

    -0.2A/s

    0.0A/s0 2 4 6 8 10 12

    A UP

    -1.2A/s

    -1.0A/s

    -0.8A/s

    -0.6A/s

    -0.4A/s

    Taps

    A UPA DOWNB UPB DOWNC UPC DOWN

  • Advanced DC Resistance - Ripple

    Ripple

    35.0%40.0%45.0%50.0%

    A UPA DOWN

    0.0%5.0%

    10.0%15.0%20.0%25.0%30.0%35.0%

    0 2 4 6 8 10 12Taps

    A DOWNB UPB DOWNC UPC DOWN

  • FRSL - Good

    R(f)

    3.0 Ohm3.5 Ohm4.0 Ohm4.5 Ohm

    0.0 Ohm0.5 Ohm1.0 Ohm1.5 Ohm2.0 Ohm2.5 Ohm3.0 Ohm

    0 100 200 300 400 500Frequency (Hz)

    ABC

  • FRSL Poor Result Short-Circuit Between Parallel Strands

    R(f)

    250

    300

    50

    100

    150

    200

    0 50 100 150 200 250 300 350 400Frequency [Hz]

    ABC

  • Dielectric Frequency Response (DFR)

    HV-winding

    Voltage source

    ~

    Current m eter1

    10

    P

    o

    w

    e

    r

    F

    a

    c

    t

    o

    r

    Power Factor in Frequency Domain

    ?

    Tank

    GuardLV-winding

    Main insulat ion 0,001

    0,01

    0,1

    0,0001 0,001 0,01 0,1 1 10 100 1000Frequency (Hz)

    P

    o

    w

    e

    r

    ?

  • Moisture in Transformers

    1. Reduces Dielectric Strength

    2. Promotes the Formation of Bubble2. Promotes the Formation of Bubble

    3. Ages Insulation with Heat and Oxygen

  • Moisture Terminology Paper, Pressboard, and Oil

    1. Water Content

    Pressboard & Paper (percentage of total mass)

    Oil (PPM)

    2. Moisture Saturation P,P,O (Relative, Humidity)

  • Moisture Fact in Transformers

    1. If mass of oil equals paper-pressboard, then at equilibrium the water content is (2000:1)

    2. In transformers there is 10X more oil mass than paper and pressboard, so the water content is (200:1)

  • Oil Ratio to Paper : Pressboard

    Mass of the oil:100,000 kg = 220,000 Lbs

    Water content at 60 C:40 ppm

    Mass of the solid insulation:13,000 kg = 20,000 Lbs

    Water content at 60 C:4 %

    Mass of the water, dissolved in the oil:

    4 kg = 8.8 LbsMass of the water contained in the

    paper:

    520 kg = 1200 Lbs

  • .DUO)LVFKHU7LWUDWLRQDQG(TXLOLEULXP&XUYHV

    9

    [%]11

    20 C 30 C 40 C

    50 C

    Curves are only valid for new oil and new paper, for aged oil/paper different curves are necessary

    Balance between water

    0

    1

    2

    3

    4

    5

    6

    7

    8

    0 10 20 30 40 50 60 70 [ppm] 90

    W

    a

    t

    e

    r

    C

    o

    n

    t

    e

    n

    t

    i

    n

    t

    h

    e

    P

    a

    p

    e

    r

    Water Content in the Oil

    60 C

    80 C

    100C

    Temperature

    low

    high

    Balance between water content in the paper and in oil needs constant temperatures over a long period

    Only average measurement

  • Water Content RecommendationsCategory Moisture content in %

    Dry below 2.2

    Moderately wet 2.2-3.7 Moderately wet 2.2-3.7

    Wet 3.7-4.8

    Extremely wet above 4.8

  • Measurement Characteristics Current in wide frequency

    range, e.g. 1 mHz 1 kHz

    Display as dissipation factor or complex capacitance or complex permittivity

    )()()()()(0

    00

    UjCjI

    ++=

    )()(

    )(

    )()(

    )()()(tan 0

    0

    +

    +

    =

    =

    =

    CC

    D

    i

    s

    s

    i

    p

    a

    t

    i

    o

    n

    f

    a

    c

    t

    o

    r permittivity

    Interpretation Slope - oil conductivity Hump - insulation geometry Low frequencies

    - moisture and aging- long test duration

    0.001

    0.01

    0.1

    1

    10

    0.0001 0.001 0.01 0.1 1 10 100 1000Frequency (Hz)

    D

    i

    s

    s

    i

    p

    a

    t

    i

    o

    n

    f

    a

    c

    t

    o

    r

    high

    low

    high

    high

    low

    low

    m

    o

    i

    s

    t

    u

    r

    e

    o

    f

    c

    e

    l

    l

    u

    l

    o

    s

    e

    a

    n

    d

    a

    g

    i

    n

    g

    i

    n

    s

    u

    l

    a

    t

    i

    o

    n

    g

    e

    o

    m

    e

    t

    r

    y

    oil conductivity

    moisture of cellulose, aging

  • Measurement Examples

    0.050.10.2

    0.51.02.0

    5.0

    New Moderate Aged 0.12

    6

    0

    H

    z

    Freq/Hz0.0001 0.010 0.10 1.0 10.0

    0.0050.010.02

    0.05

    10000.001

    0.0024 0.0036

    New, dry, cold: 1kHz - 0.1 mHz

    Moderate: 1kHz - 1 mHz

    Very aged: 1kHz - 10 mHz

  • 3.1 % at 9 qC Oil Sample Yielded 5.0%

  • What is SFRA?

    Powerful and sensitive tool to assess the mechanical and electrical integrity of power transformers active part

    Measurement of the transfer function over a wide frequency range

  • Diagnostic Category

    Dielectric Thermal Mechanical Mechanical

    Use SFRA:1. Transportation2. Post Fault

  • Life Cycle

    Delivery PortManufacturer Workshop

    Quality AssuringAfter Short Circuit Test

    Transport CheckingTruck Transport 1