| Summary: | This study presents the discharge process on rod–rod electrodes under oscillating lightning impulse voltage. For this purpose, a measurement system has been developed to record the discharge currents at both rod electrodes simultaneously and to photograph the spatial propagation of the discharges. The measurement results of the discharge process are shown for an air gap of [inline-formula] and for a superimposed frequency of [inline-formula]. Like the standardised lightning impulse voltage, three consecutive phases of discharge have been identified before a breakdown occurs. These are streamer discharge phase, channel transition phase and channel formation phase. The three phases are described, interpreted and their correlation is shown. On the basis of the oscillations, the discharge processes recur periodically if the momentary voltage is higher than the mean curve of the oscillating lightning impulse voltage. The streamer discharge emerges foremost at the positive rod electrode and propagates toward the grounded rod electrode. Its propagation depends on the voltage crest value. Arriving at the grounded rod electrode, the channel transition phase begins. There the discharge moves back to the positive rod electrode due to recombination and generation. Then the discharge constricts to the positive rod electrode, which forms a channel. Simultaneously, at the grounded rod electrode, a channel is also emerging. In each period, the channel will build up stepwise. The channel leads to the main discharge of the oscillating lightning impulse voltage.
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