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    C.T Knee Point Voltage Calculation

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    kapil
    The document provides calculations to determine the minimum knee point voltages for transformer current transformer (CT) cores used for differential and restricted earth fault (REF) protection in a 66kV substation. For differential protection CTs using an ABB RADSB relay, the calculated knee point voltage is 280V. For REF protection CTs using an Alstom CAG14 relay, the calculated knee point voltage is 120V. Formulas are provided using parameters like CT secondary resistance, lead resistance, and equivalent fault currents to determine the knee point voltages.

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    C.T Knee Point Voltage Calculation

    Uploaded by

    kapil
    3K views2 pages
    The document provides calculations to determine the minimum knee point voltages for transformer current transformer (CT) cores used for differential and restricted earth fault (REF) protection in a 66kV substation. For differential protection CTs using an ABB RADSB relay, the calculated knee point voltage is 280V. For REF protection CTs using an Alstom CAG14 relay, the calculated knee point voltage is 120V. Formulas are provided using parameters like CT secondary resistance, lead resistance, and equivalent fault currents to determine the knee point voltages.

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    CT_knee

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    The document provides calculations to determine the minimum knee point voltages for transformer current transformer (CT) cores used for differential and restricted earth fault (REF) protection in a 66kV substation. For differential protection CTs using an ABB RADSB relay, the calculated knee point voltage is 280V. For REF protection CTs using an Alstom CAG14 relay, the calculated knee point voltage is 120V. Formulas are provided using parameters like CT secondary resistance, lead resistance, and equivalent fault currents to determine the knee point voltages.

    Copyright:

    © All Rights Reserved
    Download as DOC, PDF, TXT or read online from Scribd
    Download as doc, pdf, or txt
    3K views2 pages

    C.T Knee Point Voltage Calculation

    Uploaded by

    kapil
    The document provides calculations to determine the minimum knee point voltages for transformer current transformer (CT) cores used for differential and restricted earth fault (REF) protection in a 66kV substation. For differential protection CTs using an ABB RADSB relay, the calculated knee point voltage is 280V. For REF protection CTs using an Alstom CAG14 relay, the calculated knee point voltage is 120V. Formulas are provided using parameters like CT secondary resistance, lead resistance, and equivalent fault currents to determine the knee point voltages.

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    © All Rights Reserved
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    of 2

    C.

    T KNEE POINT VOLTAGE CALCULATION

    CLIENT : M/s SWIL, JHAGADIA


    PROJECT : 66 Kv SWITCHYARD
    CONSULTANT : M/s ENGINEERS INDIA LTD.

    1) Calculation of Knee point voltage for Transformer C.T ( Differential Protection)


    We are using RADSB relay of ABB make for differential protection . However , we
    are calculating the Knee point voltage using formula of Alstom make DTH-31 relay
    as it is much on higher side..RADSB being a low impedence relay it will require still
    lower knee point voltage.

    Knee point voltage is derived by the following formula.

    Vk = 40 I ( RCT + 2 RL)
    Imag = 0.03I at Vk/4

    Where ,
    Vk = Knee point voltage required for C.T
    Imag = Exciting Current
    I = Relay rated Current
    = 1A

    RCT = C.T Secondary Winding resistance at 75 degree centigrade


    = 3 ohms
    RL = One way lead resistance between CT and relay
    = Length of conductor in Km x resistivity of conductor per Km
    = 0.25 x 8 ohm/km ( Considering 250 mtr of cable length)
    = 2 ohm
    Vk = 40 x I ( RCT + 2RL)
    = 40 x 1 (3 + 2 x 0.25)
    = 40 ( 3 + 4)
    = 280 Volts

    CT core shall have minimum knee point voltage of 280 volts.

    Imag = 0.03 I at Vk/4


    = 0.03 x 1 at Vk/4
    = 0.03 A at Vk/4
    Imag = 30 mA at Vk/4
    2) Calculation for Knee point voltage for Transformer Neutral C.T. ( Restricted Earth
    Fault Protection )

    We are using Alstom make CAG 14 relay , wherein knee point voltage is derived by the
    following formula.

    Vk = 2 If ( RCT + 2 RL )

    Where
    If = Equivalent secondary pilot current of max fault current
    RCT = C.T secondary winding resistance at 75 degree centigrade
    = 4 ohms ( assumed)
    RL = One way lead resistance between C.T and Relay
    = Cable length in km x resistance of cable per km.
    = 0.1 x 8 ohm/ km ( Considering 100 mtr of Cable length)
    = 0.8 ohms

    If shall be calculated as under:

    Transformer Capacity = 8 / 10 MVA


    Transformer rating = 66 / 6.9 Kv
    Full load current on = (10 x 10^3)/ 1.732 x 6.9
    L.V side
    = 836.76 Amps
    % Impedence = 8.35 %
    Fault Current on L.V = 836.76 / 0.0835
    Primary side
    = 10021.13 Amp
    = 10.02 KA

    Fault current on L.V ( If) = 10.02 A ….. ( as C.T ratio is 1000/1 A)


    secondary

    Vk = 2If ( RCT + 2 RL)


    = 2 x 10..02 ( 4 + 2 x 0.8 )
    = 112.224 volts

    CT core shall have minimum Knee point voltage of 120 volts.

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