TABLE OF CONTENTS Transmission lines sometimes produce radio interference often heard as the crackling, buzzing noise on a car radio when a car passes beneath a power distribution or transmission line. Radio interference (RI) is essentially a random radio signal radiated from some source. Depending on the relative strengths of the broadcast signal and the interfering noise, the signal to noise ratio (SNR) - or perhaps more accurately defined as the 'protection ratio' - varies from being a minor annoyance to a level which results in the total blotting out of a radio station. RI can have serious consequences if essential services involving air or marine radio navigation equipment are affected. Equally, power line carrier (PLC) equipment which generally operates at frequencies of 50 kHz to a few 100 kHz must not be affected by RI.
The main source of RI radiated by overhead lines are corona discharges formed on the conductor, hardware or insulators. Corona discharges form when the electric field intensity exceeds the breakdown strength of air and localised ionisation of the air occurs. There are a number of factors which affect the voltage at which corona forms, including the air density, the air humidity, the amount of photo-ionisation and to a limited degree the material of the electrode.
Corona will form when an electrode is charged to a sufficiently high voltage from a power frequency voltage source. The mechanism of corona production is different for each half-cycle of voltage. The negative half-cycle tends to...
