The effect of high direct voltage on the formation and dislodging of water drops hanging from a smooth conductor will be shown and used to explain the resulting corona characteristics, such as pulse amplitude and repetition rate, audible noise, radio influence (RI) and loss current, and corona current waveforms. Phenomena taking place during the conductordrying period are also explained. The relationship between the number and spacing of points on a smooth conductor, on one hand, and RI and loss current, on the other, is demonstrated. Results obtained from water-drop experiments are compared with those from experiments using metal points of various shapes.

Corona Phenomena from Water Drops on Smooth Conductors under High Direct Voltage

The conditions for threshold inception of positive DC corona in the coaxial cylindrical electrode arrangement are investigated. The establishment of the self-sustained corona discharge in glow or streamer form, as influenced by conductor radius and atmospheric conditions, is evaluated through detailed computations that consider space charge field effects on avalanche growth. The salient corona characteristics at threshold inception are assessed and discussed through comparisons with literature experimental data. In a wide range of inner conductor radius and atmospheric conditions corona initiates in streamer form when a critical avalanche number per unit length of about 108 cm-1 is attained. Accurate formulation of the critical field strength required to sustain ionization allowed for the derivation of approximate expressions accounting well for the interactive effects of conductor radius and atmospheric conditions on the corona inception field strength in both glow and streamer regimes.

Threshold inception conditions for positive DC corona in the coaxial cylindrical electrode arrangement under variable atmospheric conditions

This study was made to identify and photograph the foul-weather coronas occurring on a high-voltage transmission-line conductor and to establish their radio-noise importance. On the most tested 230-kV line, snow impingement plumes caused radio interference (RI) to 90 times, and water drop spray plumes to 75 times, that of the base fair-weather noise level. Foul-weather RI caused by snow, rain, fog, ice, and condensate associated with the conductor overrode RI generated on insulators, accessories, hardware, and weathered conductor protrusions. H/sub 2/0 particles in different forms were present in the conductor environment for one-fifth the yearly time. The conductor surface gradient, the rate of decay outward from the conductor, the total conductor surface area, H/sub 2/0 particle size and formation, and air velocity over the conductor are important to both particle attachment to the conductor and to the severity and form of corona. This corona occurs as plumes, impingement plumes, spray plumes, glows, and microsparks. Confirming laboratory tests were made at 230 and 345 kV. Confirming observations were made on 345-and 500-kV lines. Corona losses were caused by impingement plumes during snowstorms and principally by spray plumes in rain.

Corona and RI Caused by Particles on or Near EHV Conductors: II - Foul Weather

Radio Noise Propagation and Attenuation Tests on Bonneville Power Administration McNary-Ross 345-Kv Line

Power transmission of high-voltage direct current (HV dc) by overhead lines presents a new concept with which the industry has had little experience. Radio noise and corona loss values were recorded on a full-scale test line and statistically analyzed in relation to prevailing weather conditions. Comparison was made to radio noise readings taken on a nearby ac line. Results cover the analysis of approximately six months' data.

Progress Report on BPA HV DC Test Line Radio Noise and Corona Loss

1. When a quasipeak-type RN meter is used for the measurement of a corona noise, it is advisable to calibrate the meter with a pulse generator. 2. A procedure of calibration and a calibration chart are given. Consistent readings have been obtained for all the meters so calibrated. 3. Within the range of measurements the RN meter readings are linearly proportional to the voltage of square pulses and to the pulse width between 2 and 200 millimicroseconds above which the increase is no longer linear. 4. A given RI level can result from a variety of corona patterns characterized by the number, magnitude polarity, width, and distribution of pulses. These characteristics can sometimes be used to differentiate between the types of corona noise. 5. A good correlation between corona pulses and RI values can be obtained for a given setup if the repetition rate is known for each group of pulses of equal magnitude 6. Sudden jumps of RI were observed when an electrode with sharp points was subjected to a critical voltage. The instability was associated with the onset of corona pulses in the positive half cycle of the applied voltage. 7. Since moist insulating points cause much higher corona pulses and RI than metal points, it is possible that vegetation deposits on transmission-line conductors are more liable to cause RI than are surface imperfections. 8.

D. R. Powell

Papers

Relationship Between Corona and Radio Influence on Transmission Lines, Laboratory Studies I-Point and Conductor Corona