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    Sweep Frequency Response 

    Analysis

    Why  

    it  

    should  

    be  

    in  

    your  

    diagnostic toolbox

    Keith Hill  – Doble Engineering

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    SFRA History

    Sweep Frequency Response Analysis (SFRA) 

    has   been

      around

      for

      several

      years

    Once required highly specialized personnel to 

    perform  

    and  

    interpret  

    the  

    results Powerful and sensitive tool to evaluate the 

    mechanical integrity

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    SFRA History

    1960:  

    Lech  

    &  

    Tyminski,  

    Poland

    1966:  

    L&T  

    Publish  

    Results

    1969: Smith, UK (ERA testing)

    1976:  

    First  

    Doble  

    Paper:  

     A.G. Richenbacker “Frequency  Domain  Analysis 

    of   

    Responses  

     from  

    L.V.I.  

    Testing  

    of   

    Power   

    Transformers” 

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    SFRA History

    1978: Dick & Erven, Canada, IEEE first 

    SFRA  

    approach:  

    “Transformer   

    Diagnostic   Testing by  FRA”  

    1980’s:  

    CEGB,  

    UK  

     – SFRA  

    Field  

    Trials 1990’s:Application

     

    Development

    1990’s:  

    LVI  

    Commercialized 2000: SFRA Commercialized (Doble)

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    SFRA

    Does not evaluate the condition of  the 

    insulation Power Factor and low voltage TTR can be 

    acceptable  

    but  

    can  

    still  

    have  

    problems Mechanical aspects of  the unit

     Identify

     

    problems  

    after  

    a  

    fault Great tool to help identify shipping 

    damage

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    When to Perform SFRA

    At the factory Benchmark

    When   shipping

      carriers

      change

    Arrival on site

    This is not “over‐kill”

    During routine testing if  no benchmark

    After a fault (benchmark is very helpful)

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    Transformer Fire

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    Damaged Bushing and Bus

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    Tests after the Fire

    SFRA tests did not reveal a problem 

    (compared   to

      benchmark

      signatures)

    Damage was caused by a fire in the bus 

    duct

    Transformer returned to service

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    SFRA Data

    Interpretation can be challenging if  not 

    impossible   for

      the

      novice

      or

      very

     

    experienced if  benchmark data is not 

    available

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    SFRA Data

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    SFRA Data

    Performing the test not difficult as the 

    setup   is

      very

      simple

    Software and templates are often 

    supplied   by

      the

      manufacturers

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    SFRA Setup

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    Various Templates

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    Various Templates

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    Another Tool in the Toolbox

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    Component Characteristics

    Three basic components

     Resistors    Capacitors

       Inductors

    Each component has  a different  response being closely elated to their 

    geometry;   both

      internal

      and

      external

      to

     

    other components

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    Resistor

    The resistance of a resistor is not frequency

    dependent as the resistance will remain the same with a change in frequency. There will be a straight

    line from a low frequency to a higher frequency

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    Inductor

     At a low frequency the inductor will act as a short and the inductance will increase as the frequency

    increases.

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    Response of  an Inductor

    0 dB down at low

    frequency means it looks

    like a dead short

    Larger inductances start to

    roll off at lower frequencies

    Inductive roll off 

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    Capacitor

     A capacitor will be the opposite of an inductor as the

    capacitor will increase with an increase in frequency

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    Response of  a Capacitor

    Low frequency

    response is like

    an open circuit

    Capacitive climb

    back

    Knee point depends

    on the size of the

    capacitor  0 dB down at high frequency

    like a dead short

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    “Real Transformer”

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    Parallel RLC Circuit

    Constant dBs v. frequency

    Dead short at

    low frequency

    Open circuit at

    high frequency

    Open circuit at

    low frequency

    Dead short at

    high frequency

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    Response of  a Parallel RLC Circuit

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    Test Setup

    Once the nameplate and templates have been 

    selected   the

      tester

      must

      make

      the

     connections to the apparatus.

    Tester should make sure that the transformer 

    is isolated from all energy sources and should 

    be   aware

      that

      static

      voltage

      may

      be

      present

      if 

     

    not properly grounded.

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    Test Setup

    The first resonance frequency will be affected  if  the transformer is tested with insulating  fluid

      and

      without

      insulating).

      Removing

      oil

     

    lowers capacitances and resonances shift to  higher frequencies. (lower resonant with oil)

    How the neutral position of  the Load Tap 

    Changer  

    (LTC)  

    was  

    reached  

    as  

    there  

    have  

    been  

    cases in which reaching neutral position from  the raise or from the lower taps affected the 

    signatures.

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    Test Setup

    A magnetized core can affect the signatures 

    If  LTC or DETC are not on neutral the 

    signatures  

    can  

    change

     Test

     

    on  

    16R  

    if   

    only  

    one  

    test  

    is  

    performed

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    Test Setup

    Shorting of  the secondary windings for the  short circuit test can be affected if  the 

    secondary   windings

      are

      not

      “closed”

      It

      is

     

    recommended to  jumper X1  – X2, X2  – X3 and  X3  – X1 thus closing the delta or wye 

    windings.

     Make

     

    sure  

    that  

    the  

    transformer  

    being  

    tested  

    is  

    grounded

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    Test Setup

    Make sure that the test leads grounds are 

    properly   terminated

      to

      a   “good”

      ground

     Select

     

    proper  

    winding  

    configuration  

    template

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    TYPICAL

    SFRA

    SIGNATURES

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    WYE ‐ With and Without Oil

    With Oil

    Lower

    resonant

    frequencies

    Without Oil Higher

    resonant

    frequencies

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    Delta  – With and Without Oil

    With Oil

    Without Oil

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