Principle of Streamers Theory

Principle of Streamers Theory

• With positive polarity of the point Figure A (a) an electron formed in the gap, while moving towards the point, falls in the region of strong field, starts ionizing and forms an electron avalanche.
• If voltage between the electrodes increases slowly upto the time of formation of the corona (i.e. fulfilment of condition of self sustaining of discharge sufficiently large number of such avalanches are formed.
• When each of these avalanches reaches the point electrode, electrons of the avalanches go away into point electrode, positive ions remain in the space moving slowly towards the opposite plane electrode.

Figure A Figure B

• Thus, near the point electrode positive volume charge is formed as shown in Figure A (b).
• Presence of positive volume charge decreases the field intensity in the vicinity of the point and slightly increases it in the external space. Figure A(c).
• Therefore, further ionization in the vicinity of point is weakened and this makes fulfilment of condition for self sustenance of discharge i.e. formation of corona difficult.
• Consider the rod electrode to be positive and plane electrode to be negative the voltage between these two electrodes is sufficiently high that is the energy gained is higher than the ionizing potential then an avalanche begins at the right hand side of the volume charge (as seen in Figure B (a) ) which then mixes with the positive ions and forms the head of the streamer that is attracted towards the cathode or plane electrode.
• Figure B (b) shows that the voltage or field is strong enough to cause more avalanches and thus moves ahead in direction of the cathode or plane electrode.
• When this canal of streamer has crossed 2/3^rd length of the gap it gains sufficient momentum and energy (as shown by Figure B (d) )and thus lengthens further towards the cathode (Figure B (c).

There are two probabilities at this stage :

1. Because of lengthening of the streamer the intensity of the field may increase, in which case the streamer will cross the complete gap and hence breakdown of the gap takes place.
2. Because of lengthening of the streamer the intensity of the field may decrease, in which case the streamer after reaching a finite length does not have sufficient energy to overcome the entire gap and thus only partial discharge in form of corona may take place. It can be explained in a similar manner if the electrode polarities have been changed as shown in Figure C and D.

• Similarly Figure C and Figure D shows the formation of cathode streamer when the rod electrode is negative and plane electrode is positive.
• Hence from the above theories we can conclude that development of streamer with point negative takes place with much greater difficulties.
• Therefore breakdown voltage with negative point is considerably higher than that with positive point (by 2 to 2.5 times).

Figure C

Figure D