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TYPICAL FAILURE MODES AND FAILURE CAUSES. WHAT CAN BE LEARNED FROM POST MORTEM INSPECTION

FAILURE STATISTICS. TRANSFORMER AND BUSHINGS DESIGN REVIEW.
TYPICAL FAILURE MODES AND FAILURE CAUSES. WHAT CAN BE LEARNED FROM POST MORTEM INSPECTION

Victor Sokolov

Full Paper (308KB PDF)

Introduction

A failure is usually a "tuning fork" of Life Management procedures.
Failure analysis delivers a key information allowing to determine “what happened” and
“ what to do” in terms of managing network reliability , risk assessment, maintenance
optimization, estimation end of life, and on the other hand improving design and manufacturing
of an equipment.

Global changes in power industry, namely segregation, privatization and followed competition
affected dramatically on reliability related information. For instance in 1998 about 50 Doble
clients contributed to the Technical Questionnaire on large power transformer failures reporting
on 166 events including one client who reported on 35 failures. In 2002 presented information
contained less that 20 failure events. s. Thus, lose of reliability information is becoming a vital
problem.

Failed transformer brings lots of problems associated with direct and indirect costs.
However it may return significant amount of money back if its failure mode and failure cause
was learned properly. It could become a powerful teaching aid, which would save a number of
sister units and suggest an efficient strategical program for life assessment and life extension of
remained population. Proper postmortem inspection may bring multilateral benefit and first of all
that permits performance of full-scale design review.

Design review is a key procedure to answer the questions: What defects and faults can be
expected in particular transformer under real operating conditions? What is the possible path of
defect evolution into the malfunction, and then into failure? What scope of repair would be
reasonable? In many instances modification of design might be necessary in order to ensure
reliable service after repair and refurbishment What’s residual life of cellulose insulation in hot
spot area? How to minimize diagnostic program and select most efficient diagnostic tool?

This paper present “a fresh” information about transformers failures suggesting more typical
failure-modes, discusses some aspects of transformers and bushings design review, and suggest
benefits, which may be obtained on the basis of postmortem inspection.