Diagnostic testing and condition monitoring of transformer bushings

This paper describes a novel method for Diagnostic Testing & Condition Monitoring of transformer bushings. The proposed method is based on the measurement of the impedance of the bushing at two different frequencies. Based on the analytical studies and experimental investigation it has been found that there exists some range of frequencies, where the impedance change is very sensitive to ageing. These frequency ranges depend upon the parameters of the insulation equivalent model or its capacitance and loss in physical terms. The effectiveness of the method is based on the fact that the parameters measured are fundamental properties of insulation. When changes occur in insulation, then one or more of the measurable fundamental characteristics also change. A practical method of measuring the change in network impedance is also described based on the plot of voltage across the bushing and the current through the bushing at the two selected frequencies. Selected results, observations and conclusions are presented.