Experience with on-line monitoring of capacitance and tangent delta of condensive bushings

Capacitance and tangent delta are acknowledged as some of the main parameters for bushing insulation condition diagnosis, because they are directly affected by the deterioration of the insulation. This article intends to display a technique used in on-line monitoring of capacitance and tangent delta of bushings, in addition to an actual field deployment experience in monitoring 550 kV and 245 kV bushings on auto-transformers and power reactors. Results shown were obtained during the course of approximately one year, including on-line detection of insulation deterioration for a 550 kV bushing, avoiding a possible failure with explosion, later proven by off-line measurements of capacitance and tangent delta and by gas-chromatography analysis of oil sample from the bushing. Additionally, some practical consideration about practical issues related to installing online bushing monitoring systems on transformers equipped with bushing potential devices (BPD), where the tap of the bushing is already occupied, as well as on equipment on which it was possible to make the connection directly to the tap of the bushing.