B. Cathode Streamers
Fig 4a shows a typical pulse train sequence recorded during a negative bush-like streamer. This behavior is not displayed in the IEC 60270 waveforms due to its bandwidth limitations. The UHF sensor results show PD activity contains many pulses ascending in magnitude. The first pulse in this sequence appears to have a different characteristic and smaller magnitude than later pulses. As mentioned previously, bush- like negative streamer initiation can be described by the generation of initial cavities due to electron avalanches in the liquid phase [2]. The hypothesis is that the first pulse of this
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particular event is related to a discharge in liquid nitrogen and is responsible for initiating a phase change streamer. Once the initial bubble has been created, the later pulses showed discharges inside the bubbles, e.g. gaseous nitrogen. As the size of the cavities increased, the discharge currents increased. This might explain the increase in magnitude of the UHF sensor pulses. When the streamers achieve to a certain length, the voltage across the phase change channels is not sufficient to cause breakdown. If no further discharges occur, streamers cannot be sustained and dissolve into micro bubbles as shown in Fig 4b. Results from the RFCT are not displayed for cathode streamers due to the high noise from the earth connection.
B. Cathode Streamers
Fig 4a shows a typical pulse train sequence recorded during a negative bush-like streamer. This behavior is not displayed in the IEC 60270 waveforms due to its bandwidth limitations. The UHF sensor results show PD activity contains many pulses ascending in magnitude. The first pulse in this sequence appears to have a different characteristic and smaller magnitude than later pulses. As mentioned previously, bush- like negative streamer initiation can be described by the generation of initial cavities due to electron avalanches in the liquid phase [2]. The hypothesis is that the first pulse of this
69
particular event is related to a discharge in liquid nitrogen and is responsible for initiating a phase change streamer. Once the initial bubble has been created, the later pulses showed discharges inside the bubbles, e.g. gaseous nitrogen. As the size of the cavities increased, the discharge currents increased. This might explain the increase in magnitude of the UHF sensor pulses. When the streamers achieve to a certain length, the voltage across the phase change channels is not sufficient to cause breakdown. If no further discharges occur, streamers cannot be sustained and dissolve into micro bubbles as shown in Fig 4b. Results from the RFCT are not displayed for cathode streamers due to the high noise from the earth connection.
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