The issue of corona is mainly a maintenance issue. The reasons to locate corona:
Corona causes damage to insulators, especially to polymer insulators
- Corona generates corrosive materials (ozone and nitric acid under conditions of high humidity) that shorten the life span of various components of lines and substations
- Corona, if not always a problem by itself, is often an indicator of a fault
- Corona generates audio noise and radio interference (RI) leading to complaints by people living near the lines. These complaints must be handled by the utility even if the corona is not a problem to the utility
- On new installations, like new lines and new substations, corona may indicate faulty installation
- Corona is an indication of contamination, like salt, on insulators. In some cases it can indicate imminent tripping
- After washing of a line or substation the level of corona is an indicator of the effectiveness of the washing procedure
The level of corona discharge may change from time to time according to humidity (increasing at higher humidity) and air temperature. However, corona problems are not problems that sort themselves out on their own. They always become worse with time. The first step to care about corona is to locate it and this is done with minimum effort and cost by making a UV inspection by using a corona camera.
When exposed to a high humidity environment, salt polluted insulators develop a conductive layer, which reduces the insulating properties of the insulator. In extreme cases flash over will occur when the insulator can no longer cope with the voltage of the line. In most cases these line tripping will occur before dawn or early in the morning when humidity is at its peak value. As a preventive step utilities wash the lines, usually with water. Inspection for corona can tell if the washing was effective.
EPRI developed Guides for Corona and Arcing on Overhead Transmission Lines and in substations. The guides mention conditions of porcelain insulators, polymer insulators and conductors that will generate corona.