UV Events Counting
The question maintenance engineers often raise about the threshold level of counting that requires their intervention originates in the thermovision inspection methodology.
With thermography: temperature difference is the main criteria to determine the severity of a fault. When the temperature of an observed object exceeds a certain value above the ambient temperature, it calls for interference or at least for further investigation of a suspicious problem.
With corona inspection, the situation is different: since there are no yet accepted standards and directives, as in IR, interpretation of corona data requires knowledge and experience. Maintenance engineers must be trained to recognize the urgent cases that can cause trouble if not attended on time. Thus, with the exception of corona on NCIs, many of the corona incidents found on hardware do not need immediate attention as long as there are no complaints of Radio or TV interference or Audio noise.
It is important to realize that, despite the available counting option, the camera is not a measuring instrument but a tool to locate the positions of corona and arcing.
With corona discharge the location is as important as the strength
Example 1: On polymer insulators there should be no corona, especially not on the end fitting. In a 220 kV substation, one of newly installed polymer insulators had no corona ring since the construction crew forgot to install it. As a result a corona appeared on the end fitting (fig1). The level of counting will change with the humidity but the important fact is that there is a corona discharge located on the end fitting and it may degrade the insulator if left unattended
Example 2: An 11 kV feeder was failing when raining. The fault was located with the Daycor camera on a punctured pin insulator (fig2). The exact count was not important; the importance was the fact that an anomaly was located and by further checking the fault that caused the failures was found.
Example 3: In a 400 kV substation corona is frequently observed on hardware and this is usually tolerated. On two insulators arcing rings were missing and this was indicated by an additional corona that could be easily located while inspecting with the DayCor camera. Here again the inspector did not bother about count. The important fact is the finding of missing arcing rings on these insulators.
what is the counter for?
- The counter is important for comparative purposes and for reporting on relative strength of the various corona discharges. The counter provides the inspectors an easily measured quantity to describe intensity of corona
- When inspecting a line or substation, coronas at different locations caused by similar faults can be compared. In case that a series of coronas of similar type are observed, handling the problem at different locations can be prioritized according to the counting level, if time and resources are limited
- The counter is useful for "trending". By comparing counts of inspections during different times, engineers can determine if the situation deteriorates or remains stable, provided the inspections were carried out under similar conditions. Maintenance tasks can be prioritized at various locations according to the results of the trend tests
- In testing laboratories counting can be an important tool for acceptance of new equipment. The acceptance standards will be however determined by the experience of the testingengineer. The testing engineer working in the laboratory has the advantage that he operates in a controllable environment and can easily reproduce desirable working conditions
Despite the lack of standards that interpret counting results, these are useful to compare intensity of coronas, to observe trends of faults creating corona, and to prioritize maintenance work.