Direct measurement of electric field at line conductors during a.c. corona

An electrostatic-fluxmeter probe has been used to obtain a direct measurement of the electric field at a cylindrical conductor during a.c. corona. For conductor diameters of 1.59, 2 and 2.75 cm, the resultant electric field falls below the onset level for both positive and negative corona. For the positive discharge, the field becomes independent of the applied voltage, a maximum fall of 9%, being observed. In contrast, the average field in the negative corona discharge decreases with increasing applied voltage, the effective field being reduced by up to 15% of the onset value. The assumption in many theoretical analyses of the discharge that the surface field remains at the onset value is thus invalid. It is suggested that increased secondary ionisation processes account for the maintenance of corona at reduced field strength. The measurements also fail to support the hypothesis that field enhancement occurs during positive corona owing to a negative-ion sheath. Field enhancement is observed to occur in a.c. corona prior to onset, because of remnant space charge from the preceding half cycle, but the effect is small. The field immediately following negative corona extinction is reduced, but, after positive corona, is enhanced because of electron attachment near the anode surface. The method, suitably modified, would appear to offer a useful means of study of bundle-conductor corona and precipitator assemblies of parallel line conductors.