Surface tracking along the interphase barrier of a large transformer

Partial Discharge (PD) measurements are an important tool for assessment of the condition of the insulation systems of power equipment such as high voltage transformers. One of the most dangerous failure modes in a large autotransformer is due to surface discharge along the interphase barrier board. Previous work has shown electrical failure of barrier boards even if the detected PD level was very low at the time of measurement. Identification of PD patterns due to surface discharging on the barrier board surface is of great importance in terms of diagnosing potential problems within a large autotransformer. An experiment has been designed at the Tony Davies High Voltage Laboratory, University of Southampton, with the aim of recreating this particular failure mode whilst capturing condition monitoring data (e.g. pd measurements). Experiments are being undertaken using three different applied fields. The first is measurement of Phase Resolved Partial Discharge, PRPD, patterns of surface tracking using a local PD source only. Experiments are then repeated with the PD source and additional electric field provided from a single phase source. Finally experiments have been undertaken with a PD source and barrier board within a two phase field (e.g. conditions representative of those experienced by an interphase barrier board in a large energised transformer). Obtained results and discussion highlight differences between the data that may indicate; firstly, the effect of two phase energisation on surface charge behaviors and secondly identify features in the data that are consistent with surface discharging on a barrier board. Chronological PD signals are presented that reflect the PD activities in the inter-phase barrier insulation system over a time interval of more than 5 hours. The results assist in determining the characteristic features of this form of degradation and will help to improve the diagnostics of field data captured in realtime.