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Transformers

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  • Transformers

  • Faradays Law of Induction

  • 30:1

    7200 V 240 V

    60:1

    120 V

    15:1

    480 v

    26:1

    277 v

  • The only relationship between the

    primary and secondary winding is the

    ratio and polarity.

    Either winding can be left ungrounded,

    or grounded at any point without

    affecting the grounding or the voltage

    of the other winding.

  • Transformers can be

    interconnected together in a

    variety of ways on both the

    primary and secondary sides to

    get a variety of three phase

    secondary voltages.

  • A Phase

    B Phase

    C Phase

    B Phase

  • A Phase

    C Phase

    B Phase

  • A Phase

    B Phase

    C Phase

    B Phase

  • Distribution Systems

  • 24

    Typical Substation

    Transformer C A N B

    7200V

    7200V 7200V

  • Multi-Grounded Wye Distribution Most common Type

    Neutral Phase Voltage 7200V and 14,400V most common

  • Delta Distribution System

  • Various Transformer

    Connections May Be Used

    On Multi-Grounded

    Distribution System.

  • Four Wire Multi-Grounded

    System Allowable Transformer

    Connections

    Ungrounded Wye Primary Transformer

    Bank

    Grounded Wye Primary Transformer Bank

    Delta Primary Transformer Bank

    Open Wye (Primary) Open Delta

    (Secondary) Transformer Bank

  • 4 Wire Multigrounded Wye Ungrounded Wye Transformer Grounded Wye Transformer

  • 4 Wire Multigrounded Wye Ungrounded Wye Delta Transformer

  • 4 Wire Multigrounded Wye Ungrounded Wye Delta Transformer

    Open Primary Phase

  • 4 Wire Multigrounded Wye Grounded Wye Delta Transformer

    Grounding Bank

  • 4 Wire Multigrounded Wye Delta Connected Transformer

  • 4 Wire Multigrounded Wye Delta Connected Transformer

  • 4 Wire Multigrounded Wye Open Wye-Open Delta Transformer Bank

  • The secondary (low voltage) side of the

    three phase banks can be interconnected in a

    variety of manners, independently of the

    connection on the primary (high voltage)

    side.

  • Probability of Ferroresonance

    From Westinghouse Distribution Transformer Guide.

  • Intended to examine the condition

    of the distribution primary neutral

    and the farm grounding system by

    measuring the resistance of the farm

    grounding system and primary

    neutral. It also measures the cow contact voltage in response to the

    primary neutral voltage.

  • Inject a current into the

    neutral system, measure

    the voltage rise, measure

    the current into the farm

    and the primary, and

    determine the resistance of

    each with the formula

    R = V / I

  • RPRI RFARM

    LOAD

    BOX

    CURRENT

    REMOTE

    +

    Vp

    _

  • One Bushing Single Phase Transformer

  • Load

    Box

  • 3 Phase Delta - Delta 120/240 Volt

  • Load

    Box

  • Load

    Box

  • 3 Phase Floating Wye - Delta 120/240 Volt (4 wire)

  • Load

    Box

  • Load

    Box

  • 3 Phase Delta - Grd Wye 120/208 Volt

  • Load

    Box

  • Load

    Box

  • 3 Phase Grd Wye - Grd Wye 120/208 Volt

  • Load

    Box

  • Load

    Box

  • 2 Phase Open Wye - Open Delta 120/240 Volt (4w)

  • Load

    Box

  • Load

    Box

  • 2 Phase Open Wye - Open Delta 240 Volt (3w)

  • Load

    Box

  • Load

    Box

  • Important Considerations are:

    A balanced three phase load on any system using three transformers will NOT cause any neutral current in the primary

    If the transformer bank has no primary winding connection to the primary neutral, the load (at the farm) CAN NOT cause a primary neutral current.

    On most systems, with the exception of open wye/open delta connection, a balanced three phase load box will do nothing to load up the primary neutral.

  • Rf = Vs / Isec(farm)

    Rp = Vp / Ip ?

    Rt = Vp / Itotal ?

    K = Vcc (hi) / Vs(hi) x 100

    CR = (IPrimary Neutral / ITotal)x 100 ?

  • Rupstream

    Rdownstream

  • RPRI RFARM

    LOAD

    BOX

    CURRENT

    REMOTE

    +

    Vp

    _

  • ITotal = Iload box /transformer ratio

    Inet = I sec(farm)

    Ipri neutral = Itotal - Inet

  • OR

    Ipri neutral =

    Ipri n upstream

    +/-

    Ipri n downstream

  • Rt = Vp / Itotal

    Rf = Vs / Isec(farm)

    Rp = Vp / Ipneutral

    K = Vcc (hi) / Vs(hi) x 100

    CR = (IPrimary Neutral / ITotal)x 100

  • Vp / Ipneutral

    In Each Direction

    Rp =

    Parallel Combination of

    OR

  • I(net)

  • RPRI RFARM

    LOAD

    BOX

    CURRENT

    REMOTE

    +

    Vp

    _

  • ITotal = Iload box /xfrmr ratio

    Inet = I sec(farm)

    Ipri n = Itotal - Isec(farm)

  • 5

  • Rt = Vtotal / Ipri

    Rf = Vs / Isec(farm)

    Rp = Vp / Ipneutral

    K = Vcc (hi) / Vs(hi) x 100

    CR = (IPrimary Neutral / IPhase)x 100

  • Ifarm

    Itotal

  • A Phase

    B Phase

    C Phase

  • 90

    Single phase

    20A

    20A

    Substation

  • 3 Wire Ungrounded Delta