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    First Pole To Clear

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    ashutosh2009
    The first pole to clear factor is the ratio of the recovery voltage across the first circuit breaker pole to clear during a fault to the normal phase-to-ground voltage. It depends on the system grounding - for non-effectively grounded systems it is 1.5, and for effectively grounded systems it is 1.3. The first pole to clear factor must be considered in circuit breaker design to ensure it can withstand the transient recovery voltage experienced by the first pole to open during a fault.

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    First Pole To Clear

    Uploaded by

    ashutosh2009
    154 views4 pages
    The first pole to clear factor is the ratio of the recovery voltage across the first circuit breaker pole to clear during a fault to the normal phase-to-ground voltage. It depends on the system grounding - for non-effectively grounded systems it is 1.5, and for effectively grounded systems it is 1.3. The first pole to clear factor must be considered in circuit breaker design to ensure it can withstand the transient recovery voltage experienced by the first pole to open during a fault.

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    FIRST POLE TO CLEAR

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    The first pole to clear factor is the ratio of the recovery voltage across the first circuit breaker pole to clear during a fault to the normal phase-to-ground voltage. It depends on the system grounding - for non-effectively grounded systems it is 1.5, and for effectively grounded systems it is 1.3. The first pole to clear factor must be considered in circuit breaker design to ensure it can withstand the transient recovery voltage experienced by the first pole to open during a fault.

    Copyright:

    © All Rights Reserved
    Download as DOCX, PDF, TXT or read online from Scribd
    Download as docx, pdf, or txt
    154 views4 pages

    First Pole To Clear

    Uploaded by

    ashutosh2009
    The first pole to clear factor is the ratio of the recovery voltage across the first circuit breaker pole to clear during a fault to the normal phase-to-ground voltage. It depends on the system grounding - for non-effectively grounded systems it is 1.5, and for effectively grounded systems it is 1.3. The first pole to clear factor must be considered in circuit breaker design to ensure it can withstand the transient recovery voltage experienced by the first pole to open during a fault.

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    First Pole to Clear Factor of a Circuit Breaker is the ratio of power frequency recovery

    voltage across the first pole to clear the arc to the normal phase to ground voltage
    when all the three poles of breaker are open. It is denoted as k pp. If Vcb be the recovery
    voltage across the first pole of CB and Vph be the phase to ground voltage then

    First Pole to Clear Factor = Vcb / Vph

    The value of kpp depends on the system grounding. For system with non-effectively
    neutral earthing, kpp = 1.5 while for system with effectively neutral earthing, kpp =
    1.3

    Let us now understand the concept of first pole to clear factor in detail. We know that
    a circuit breaker is used to make or break circuit or part of circuit in power system.
    Since the power system is mostly inductive in nature, therefore the current flowing in
    the network is lags the voltage. This definitely means that whenever the voltage is
    zero, current is at its peak and when voltage is at its peak, current is zero. In circuit
    breaker, the basic philosophy used to break the circuit current is to open the breaker
    contacts at natural current zero. But the three phases current are separated from
    each other by an angle of 120 degree. This means that the natural zero of the three
    phases will take place at three different points of time. Figure below shows the
    current interruption for an assumed fault.

    As clear from the figure, the current of Y phase first comes to zero at 53.5 ms
    followed by R and B phases (phase current first coming to natural zero is dependent
    on the time of occurrence of fault). This means that first Y pole will be opened and
    then R and B pole of breaker. When Y poles opens in response to any fault, the
    remaining two poles are closed. This gives rise to high recovery voltage across the
    breaker contacts when compared with phase to ground voltage when all the three
    poles are open.

    Let us consider a three phase fault as shown in figure below. The figure shows the
    simplified equivalent circuit of effectively earthed neutral with three phase fault
    without involving earth. When the very first pole (here it is assumed that R pole
    opens first) interrupts the fault, the symmetry of the system is lost and the system
    goes into a pure a phase to phase fault condition. Due to this, the voltage of short
    circuit point shifts.

    The power frequency recovery voltage across the first opening pole i.e. R pole
    (assumed) is nothing but the difference between the source side voltage and network
    side voltage. In our case, this will be different between the source side voltage Ua and
    network side voltage Vp (voltage at short circuit point). Thus,

    VR =   Recovery Voltage across the first opening pole

         = Ua – Vp   …………..(1)

    From the symmetry of the system when the system goes into pure phase to phase
    fault, the voltage at the short circuit point will be average of the U b and Uc. The
    phasor diagram is shown below.
    Vp = (phasor sum of Ub and Uc) / 2

         = 0.5Ub

    But magnitude of Ua, Ub and Uc are same, hence

    Vp = 0.5Ua

    Therefore from (1),

    VR = Ua + 0.5Ua

         = 1.5Ua

    Hence from the definition,

    First Pole to Clear Factor, kpp = 1.5Ua / Ua

                                                     = 1.5

    Thus we see that, value of first pole to clear factor for effectively gounded neutral
    system with ungrounded fault is 1.5.

    For three phase to earth faults with effectively earth neutral, the value of kpp depend
    on the positive sequence and zero sequence reactance. The value of kpp for such
    condition is given as

    kpp = 3 / [(X1 / X0) + 2]

    where X1 = Positive Sequence Reactance

    X0 = Zero Sequence Reactance

    For grounded system as is the case with transmission line, the ratio of (X 1 / X0) is 1/3
    and therefore the value of first pole to clear factor is 1.3.
    Significance of First Pole to Clear Factor:

    First Pole to Clear Factor is basically the impact of system grounding on


    the Transient Recovery Voltage (TRV). Apart from system grounding, TRV depends
    on many other factors. All the three poles of circuit breaker can never open
    simultaneously; therefore first pole to clear factor must be taken care for the design
    of breaker. The dielectric strength of breaker must be sufficient enough to sustain the
    TRV across the first pole to clear the fault.

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