New concepts in insulation diagnostics have been developed that aid in the prediction of insulation performance and lifetime. For example, present day leakage current monitoring systems on outdoor insulation provide little information when the surface is still hydrophobic. However, partial discharge detection provides a useful tool for discriminating the transition from the early ageing period, which is characterized by corona discharge from water droplets, to discharges resulting from dry band arcing and the development of leakage current. Signatures of the third harmonic of the leakage current provide an excellent diagnostic of insulation design and material ageing. Development of organic materials for the housing of insulators, surge arresters, bushings and terminations are quickly evaluated by infrared laser heating.
Detection of early ageing by partial discharge measurement
The wetting process on outdoor insulation produces changes in the distribution of electric stress along the surface and these changes lead to the development of corona discharge from water droplets. This early ageing period which is characterized by corona discharges from the water droplets is a precursor to the development of leakage current. Measurement of partial discharge reveals trends that provide information on the onset of dry band arcing while the partial discharge signatures can be used to discriminate between insulation designs.
Harmonic components of dry arcing as a signature of insulation design
The usual measurements of leakage current pulses, their peak magnitude, or number in a specified interval, provides no correlation to the ageing of insulation. However, the harmonic components of leakage current can provide information as to ageing, specifically as to erosion damage during dry band arcing and signatures of the third harmonic component provides information as to insulation design, shed diameter, spacing and profile.
Evaluation of insulation materials by infrared laser
A near infrared laser diagnostic technique has been developed to buildup heat in specimens to simulate the action of dry band arcing in the ASTM D2303 or IEC 60587 incline plane test methods. By maintaining a constant energy applied to the surface of specimens, and through measurement of material loss, a direct correspondence to the standard methods been demonstrated. In other words the laser technique has been shown to give the identical performance ranking of materials as the standard methods but at a fraction of the time required and with better reproducibility.
