High Voltage and Wave Phenomena: - For Electrical Power System Engineers and Students by Aldo Karlmann

Author:Aldo Karlmann [Karlmann, Aldo]
Language: eng
Format: azw3, pdf
Published: 2016-06-03T16:00:00+00:00

However, corona discharges can also be very useful since they form the basis of such practical devices as Geiger counters and dust precipitators.

The corona mechanism is very difficult to treat quantitatively but may be explained and discussed on the basis of the corona pattern observed in a point- plane electrode arrangement. Such a system provides a high degree of non-uniformity in the vicinity of the point and only one source of corona is present.

a) Negative Corona: With a negative point, the corona current flows in very regular pulses (Trichel pulses) the repetition frequency of which increases with voltage, increases with the point sharpness and decreases with increasing pressure.

The generally accepted explanation of this corona is as follows:-

A random positive ion approaches the point and is accelerated so that on impact, it releases an electron, The emitted electron creates an avalanche by the Townsend mechanism in the vicinity of the point. Photons from the ionizing region liberate further electron from the point and cause further avalanches, This process continues and gives rise to a rapid increase of current. The electrons move away into regions of lower field strength, slow down and form negative ions by attachment, The dense, cloud of positive ions left near the point move gradually towards the point. This positive space charge reduces the effective high field region and the negative space charge reduces the field intensity in the ionizing region. These effects prevent the electrons being accelerated sufficiently to produce ionization and the discharge is therefore, choked off. As the last few ions reach the points, the field recovers to its original value and ‘accelerates one of the naming ions. The process therefore repeats itself in a regular manner.

b) Positive Corona: In the case of positive corona, the discharge may he pulsating or pulseless. The type of corona is difficult to predetermine and depends upon such factors as sharpness of the point. However, positive corona which changes from pulse to pulseless may be explained as follows:

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