This occurs (see Fig. 10-67) when disconnecting unloaded transformers, reactors, or compensating coils. An arc is produced between the contacts when the circuit breaker is opened.

  FIGURE 10-67 Principleof small inductive- current interruption; equivalent circuit; typical shape of current and voltage.

The arc voltage is approximately constant at higher currents, since the arc energy is removed only be convection. With small currents, the arc voltage increases as a result of arc looping and a change in the cooling mechanism.

When approaching current zero, the arc current begins to oscillate as a result of interaction with the system; that is it becomes unstable.

As a result of the high oscillation frequency, the current interruption may occur prior to the natural zero passage, can be regarded as instantaneous, and is called current chopping. The chopping current is affected not only by the properties of the circuit breaker but also to a great extent by the system parameters.

Energy at the disconnected load side (L) oscillates with the natural frequency of the capacitances local to the circuit breaker. The maximum voltage is attained at the moment when all the energy is converted into capacitive energy.

As a result of the resistive losses, the voltage on the disconnected load side decays to zero. During current chopping, the breaker is stressed by the supply-side voltage on one side and by the load voltage on the other side.

The supply side voltage is at a maximum, since the load is highly inductive. The load side voltage is the oscillating voltage as the energy exchanges from inductive energy to capacitive energy.

This load side voltage will have a high frequency of up to several thousand cycles per second. During this increasing stress, reignition across the breaker may occur. However, the arc is immediately extinguished again because of the low current and the process begins anew.

Hence, the reignition also helps reduce the energy stored in the disconnected circuit.

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