SECTIONAL TRANSACTIONS.— A. 327 



a uniform (electrical and thermal) field, however great the thickness. This 

 voltage might be regarded as a constant of the material. Secondly, the con- 

 ception of ionisation and the possible existence of statistical ' ionisation 

 potentials ' is considered in analogy with ionisation in gases. Thirdly, the 

 determination of the field strength which causes electrical failure of the 

 structure is examined from the theoretical and practical aspect. It is 

 indicated that in general these limitations do not, of themselves, restrict 

 unduly the voltages and stresses applied in practice and that a considerable 

 increase in the severity of present working conditions would be possible 

 without involving these ultimate limits. 



Mr. C. W. Marshall. — Some electrical discharge phenomena on high 

 voltage systems. 



Discharge phenomena encountered in operating high voltage transmission 

 systems may be classified as follows : 



(a) Corona. 



(b) Surface discharges on porcelain. 



(c) Internal discharges in solid insulation. 



(d) Arc discharges. 



(a) Corona. — Corona or brush discharge takes the form of a steady violet- 

 coloured discharge in the vicinity of the high voltage conductor. The 

 discharge current is very small, of the order of microamperes. Corona 

 usually occurs on outdoor high voltage plant and transmission lines. It is 

 due to the local breakdown of the air owing to high voltage gradient, and is 

 intensified by humid weather conditions. 



Slide 1 : Corona discharge on 132 kv. insulators. 



(b) Surface discharges on porcelain. — Surface discharges often take place 

 on the porcelain insulators of overhead lines and outdoor plant. These 

 discharges are of the nature of intermittent spark discharges ; they are 

 audible and visible. The discharge current is of the order of tens of milli- 

 amperes. Such discharge phenomena are due to polluted and humid 

 atmospheric conditions. 



Slide 2 : Surface discharges on insulators in fog. 

 Slide 2a : Oscillogram of discharge current. 



(c) Internal discharges in solid insulation. — Internal discharges occur in 

 voids in solid insulation, e.g. cable dielectric. The air is ionised owing to 

 its low dielectric constant and to the high stress (up to 16 kv./mm.) at which 

 the dielectric is worked. These discharges give rise to an increase in the 

 power factor of the insulation, and this fact is utilised to detect them. 



Slide 3 : Cable end showing discharge tracking. 



(d) Arc discharges. — Arc discharges or flashovers involve currents of the 

 order of 1,000 amperes which persist until the supply is cut off. Any of 

 the types of discharge mentioned may develop into arc discharges involving 

 complete breakdown of the insulation. Arc discharges are often produced 

 by lightning. The high voltage lightning discharge initiates an arc which 

 is sustained by the power supply. 



Slide 4 : Arc discharge on 132 kv. line. 



General Discussion on The production and technical applications of 

 high voltages. 



