MV AC Switchgear

Direct started asynchronous MV motor switching by mv circuit breaker characters acc to IEC

  1. Starting current:
    • high current(I =6…7 × motor rated current).
    • low power factor(cosϕ≈0,1…0,2).
    • infrequent operation.
    • due to the higher interrupted current the choppingcurrent can higher values.
    • the arcing time for some switching devices may be longer than when the motor is running.
    • overvoltages when breaking the starting current.

    are usually higher than when breaking no-load or load-current

  2. No-load current:
    • the no-load current of a small HV motor may be lower than the chopping current level of the circuit breaker.
    • the power-frequency recovery-voltage is very small compared to the case of breaking the starting current.
    • re-ignitions are not likely as the recovery voltage is low.
    • low powerfactor(cosϕ≈0,1).
  3. Load current:
    • high power factor(cosϕ=0,7 – 0,9).
    • power-frequency recovery voltage is low.
    • low overvoltages, no re-ignitions.

    With regard to breaking capability and overvoltage generation, it is clear that the breaking of starting current is the most severe case.
    Motor switching is analogous to the non-effectively earthed shunt reactor switching case.
    The motor switching equivalent circuit is shown in Figure.

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