November 23, 2017

Oil-impregnated paper bushing insulation diagnostics

Abstract As reported in CIGRE TB 642, windings, tap changers and bushing related failures are the major contributors to transformer failure. About 17% of transformer...

---

Abstract

As reported in CIGRE TB 642, windings, tap changers and bushing related failures are the major contributors to transformer failure. About 17% of transformer breakdowns are caused by bushing failure which is often followed by explosion, tank rupture, fire and potential human harm. To date, the insulation system in high voltage and extra-high voltage bushings is mainly oil-impregnated paper and resin-impregnated paper; in fewer cases, resin-bonded paper.

Some of the advantages of using oil-impregnated paper bushings, listed in CIGRE TB 445, are the ability to be used at any voltage design level with minimum partial discharge activity, relative low cost, the possibility to perform non-intrusive dielectric diagnosis using dielectric response methods, as well as the extraction of oil samples for dissolved gas analysis. Evidencing these advantages, a combined analysis of the insulation condition of EHV bushings using dielectric frequency response measurements and dissolved gas analysis are carried out at a major North American utility and are described herein.

Keywords: high voltage bushing, oil-impregnated paper insulation, dielectric frequency response, DGA

1. Introduction

The continuous growth in power demand and the need to maintain the highest levels of quality and reliability in the electrical power network enforce asset management teams to examine and pursue testing strategies to minimize the risk of operational down time.

Proper trending of the condition of high voltage (HV) and extra-high voltage (EHV) bushings is a priority. CIGRE TB 642 [1] reports approximately 17% of transformer breakdowns caused by bushing failure are often followed by explosion, tank rupture, fire and potential human harm. It is of paramount importance, therefore, to dedicate sufficient resources to find procedures and methodologies to be applied as part of the asset management strategy.

In this work, specialists investigated different dielectric diagnostic methods capable to provide significant information to warn of potential risk of failure of EHV oil-impregnated paper (OIP) bushings. Data from power factor/dissipation factor (PF/DF), capacitance (C), HV Dielectric Frequency Response (DFR) measurements and Dissolved Gas Analysis (DGA) are considered collectively to establish potential correlation and guidelines for end users.

Subscribe to Transformers Magazine