A novel approach for bushing fault diagnosis: Power grid india experience

Dissipation factor (Tan δ) test at 50Hz has been used for condition assessment of the bushings since last two decades in POWERGRID. Many bushings have been removed from service based on detection of abnormality during periodic testing. In spite of that about 40% failure of EHV Transformer and Reactor in POWERGRID were attributed to random failure of OIP bushings at an early stage of operation. In all these failures, condition of the bushings deteriorated to such a level that it led to unexpected failure before it could be detected in schedule testing. For avoiding such random failure of bushing, supplementary diagnostic tests like capacitance and dissipation factor (Tan δ) measurement in variable frequency as well as in variable temperature and dissolved gas in oil analysis (DGA) of bushing are being performed and these techniques have proved to be effective for better diagnosis of the bushings as well as in reduction in random failure.

Several brochures, publications and research have been made over last two decades on frequency domain-spectroscopy( FDS) and polarisation depolarisation current(PDC) measurement for charecterising solid insulation-of Transformer as well as bushings. However, information on performance of FDS for diagnosis of EHV bushing at site is limited. Effect of moisture and ionic contamination on the dielectric properties of the insulation system is more pronounced at low frequencies (i.e in mHz). Hence, FDS measurement is preferred for measurement in the frequency range of 0.1 mHz to 1000Hz [1]. To avoid voltage dependent effects low voltage levels are recommended for FDS measurements [1]. However, as experienced, chances of disturbances in FDS response performed at 140 volt rms is more as compared to FDS measurement carried out at 1400 volt rms. As experienced, sensitivity of dissipation factor in problematic bushing is high at low frequency and the problem could be either high moisture content in the paper insulation or bridging of grading insulation with normal moisture content in the insulation. Hence, for problematic bushing, corelation of dissipation factor at low frequency has been well established. However, sensitivity of dissipation factor at high frequency is yet to be known.The bushing capacitance is a sensitive quantity for the detection of partial breakdowns between grading foils. Capacitance measurement does not give any certainty during offline measurements in the problematic bushing. It is seen that sensitivity of capacitance of bushing is high at low frequencies (below 1 Hz) incase there is a problem with the bushing. As an supplement to FDS measurement, Variable frequency disipation factor( VFDF) measurement frequency ranging from 15Hz to 400Hz is proven to be effective for diagnosis of the the problematic bushing.

This method has an advantage of getting repeatability and error free response over FDS measurement. However, not much variation in Capacitance is seen at 15Hz also with respect to 50 Hz result if there is any problem with the bushing. As a contribution to the forum, authors of this paper have shared the field exprience on FDS and VFDF as well as Tanδ at variable temperature for effective diagnostics of the problematic bushings with real cases based on service experience.