Corona discharges are low-power, luminous and audible discharges that are observed in non-uniform fields and can be transient or steady state. In high voltage engineering non- uniform fields cannot be avoided so the phenomenon is of particular importance [59]. The name comes from the Greek word ‘κορώνα’ which means ‘crown’ and it is believed to have been originally used by seafarers to describe the luminous discharges emanating from the masts of their ships during electrical storms [60].
The formation of corona discharges is attributed to an electron avalanche process that ceases before reaching ground. The avalanche is initiated by the presence of an inhomogeneous electric field sufficiently strong to ionize the neutral molecules surrounding the electrode [61]. In air, which is a mixture of mainly nitrogen (79%) and oxygen (20%), the electronegative nature of oxygen molecules significantly affects corona discharge development. The avalanche process can be impeded by the presence of negative ions formed by the ability of oxygen to capture free electrons [62]. The presence of oxygen also enhances the accumulation of ionic space charge at the vicinity of the high voltage electrode which in turn changes the distribution of the local electric field. The different equilibrium states of space charge near the high voltage electrode give rise to several corona discharge modes which are used to categorise corona either as negative or as positive [63].
3.2.1 Negative (cathode) corona
With the high voltage electrode at negative potential, the electron avalanche starts at the cathode, extending towards the anode until it reaches a boundary surface (S0) where the net ionization coefficient is zero. Two regions of ionic space charge are formed in the gap between electrodes with the higher mobility free electrons at the head and the positive ions at the tail of the avalanche (Figure 3.1). Three distinct corona modes can be observed that influence the development of negative corona that occur at different field intensities [63].
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Figure 3.1 - Negative corona
With the voltage just above the corona onset value, the first observable mode is the
Trichel streamer. It is caused be a repetitive cycle of initiation, development and suppression of the streamer along a narrow channel from the cathode. The cycle has a very short duration (tens of nanoseconds) with a few milliseconds of dead time before it is repeated. The discharge current resulting from this process appears in the form of short duration, small amplitude pulses called Trichel pulses [64, 65]. The pulsating behaviour of the discharge at this stage is a result of the suppression of the electron avalanche due to a very active recombination process which reduces the field intensity and suppresses the discharge momentarily until a new avalanche is created. The repetition rate depends on the effectiveness of the field to remove the ionic space charge and has a linear relationship with the applied voltage [66].
As the field increases it becomes more efficient in removing the space charge, preventing its accumulation near the cathode and suppressing the ionization activity. Stable emission of electrons from the cathode by ionic bombardment creates a negative pulseless glow, the second negative corona mode [67]. The transition from the Trichel streamer mode to the negative pulseless glow mode is corroborated by the reduction of the repetition rate of the Trichel streamer as the field increases, which indicates the existence of an equilibrium state between the creation and removal of space charge [68].
73 If the field is increased even further the third negative corona mode can be observed,
the negative streamer. A stable streamer channel originating from the cathode is
formed that constricts the discharge forcing it further into the gap between electrodes. The discharge current pulsates at low frequency as the streamer channel extends from the cathode into the gap and back. The discharge characteristics are similar to the negative pulseless glow since the electron emission from the cathode is also responsible albeit accompanied by an even more effective space charge removal process, hence the development of the streamer channel [66].
3.2.2 Positive (anode) corona
With the high voltage electrode at positive potential, the electron avalanche develops towards the anode originating from a point on the boundary surface with zero ionization coefficient (S0). As before, because of the lower mobility of positive ions compared to electrons, the tail of the avalanche consists of positive ion space charge. In this case however, most of the free electrons are neutralised upon reaching the anode due to the higher local field intensity which limits electron attachment. Negative ions tend to appear farther in the gap in the low-field region (Figure 3.2) [67]. For positive corona, four modes with different characteristics can be observed as the field increases, affected by the presence of space charge of both polarities near the anode.
74 As soon as the voltage exceeds the corona onset value two positive corona modes begin developing in parallel. One of them is burst corona. The highly mobile electrons lose most of their energy before being neutralised at or near the anode as the ionization activity spreads at the surface of the anode. This results in the build-up of a small positive space charge due to the presence of positive ions near the anode which supresses the discharge. The spreading of the ionization over a small area of the anode and its subsequent suppression by the space charge produces a discharge current consisting of small pulses. Burst corona appears as a lucent skin attached to the anode surface [63, 67].
The other positive corona mode that is observed at the same time as burst corona is the
onset streamer. A thin streamer channel is formed originating from the anode causing the discharge to extend radially farther into the gap than burst corona. This is the result of the accumulation of positive space charge near the anode which enhances the local field attracting electron avalanches. The creation and subsequent absorption of electron avalanches by the anode supresses the streamer discharge until the field removes the residual space charge enabling the development of a new streamer. Hence the onset streamer appears in the form of short duration, high amplitude, positive current pulses with low repetition rate [60, 63].
By raising the voltage higher, burst corona begins to dominate over the onset streamer, something that gives rise to a new mode, the positive glow. The field is effective enough at removing space charge from the anode but at the same time is not intense enough to enable streamer formation. This results in the creation and removal of positive ions in the gap that gives rise to a primarily direct current with a small pulsating component. The positive glow mode appears as a thin luminous layer very close to the anode [63, 67].
An even further increase of the applied voltage sees the reappearance of streamers which lead to the final mode of positive corona, the breakdown streamer. The nature of the discharge at this stage is similar to the onset streamer but because the field is more effective at removing the space charge near the anode the discharge can extend even further into the gap. The intensity of breakdown streamers increases as the voltage increases until the gap is fully bridged. This does not always cause a breakdown which indicates that the streamer channel is not completely conductive [63].
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3.2.3 AC Corona
At the presence of an alternating voltage, the electric field polarity and amplitude changes within a cycle. Depending on the length of the gap between electrodes, more than one corona mode can be observed in one cycle. Trichel streamers, positive onset streamers and burst corona are present just above the corona onset voltage for short gaps as the space charge, positive or negative, created during the corresponding half cycle is neutralised at the end of the half cycle. For longer gaps, Trichel streamers, negative glow, positive glow and positive streamers can be observed because the residual space charge that cannot be absorbed by the electrodes during one half cycle ends up in the high field region during the next half cycle, influencing the development of the discharge [63].