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EVALUATION AND IDENTIFICATION OF TYPICAL DEFECTS AND FAILURE-MODES OF 110-750 KV BUSHINGS

Victor Sokolov, Boris Vanin

Full Paper (DPF)

ABSTRACT

ZTZ-Service Co.This paper discusses the typical defects and failure-modes of HV transformer bushings on the base of service experience of a huge bushing population and failures studies.
Failure developing considerations, determination of defective condition and images of defects are presented. Specific failure-mode caused by deterioration and appearance of discharges across the inner part of the transformer-end porcelain is discussed as well as effect of transformer itself on bushing behavior.
Some diagnostic technique, particularly, methods of evaluation of the oil condition and inner porcelain contamination are presented with complementary typical examples and cases history.

INTRODUCTION

There is a huge aged population of HV transformer bushings and problem of their actual Life Span is a vital one.
At 1996 CIGRE SC-12 discussion meeting 1 an opinion was expressed that failure rate of bushing is small and nearly negligible relative to general population of bushings.
However, transformer failure analysis shows that in many cases bushings were involved as initially faulty component. Failure of HV bushing is often followed with catastrophic consequences as explosion, tank rupture, fire, etc. 2
As follows from analysis of Doble Clients Replies to annual Technical Questionnaires in average about of 10% of transformer failures cause by the bushings damage. This percent is essentially more for Large Transformers.
Some information about relative failure rate of bushings collected from 13,14,15 shows that irrespective of geography of transformer installation and difference in design, HV bushings remain as one of the weakest components and may cause from the fifth to the third part of general large transformer failures.
Thanks to developed preventive maintenance number of defective bushings which annually removes from service more than in ten times as many as failed number. As follows from Doble Clients Replies on Technical Questionnaires in 1992-1996 during 5 years over 1200 transformer bushings have been replaced basically due to negative test results.
Apparently many failures were prevented due to scheduled removing the bushings from service which design was recognized as unreliable.3… 9
One can suppose that condition of replaced bushings was different and advising more reliable methods of identification of defects could significantly reduce maintenance costs.
On the other hand, in some cases bushings have been exploded without any significant sign of faults prior to failure. The typical unexpected failure-mode involves flashover along the internal surface of the lower porcelain, 11, 12.
In June 1996 an expert of ZTZ-Service Co. took part in investigation of GSU 360 MVA/161 kV failure at Dairy Land Power Cooperation, which caused by explosion of the 160 kV GE type “U” bushing, catalogue # 11B424, manufactured in 1977.
The main cause of the fault was deterioration of oil, deposit deterioration products on the inner surface of the lower porcelain what resulted in formation partial discharge along the inner surface of low porcelain.
There was no chance to recognize the incipient fault in oil by means of used traditional methods.
This type of bushing fault has been introduced in several papers 2, 9, 10, 12, however, mechanism of failure initiation and developing is still not clearly understood.
The goal of this paper is an attempt to summarize the typical defects and failure-modes of HV bushings and to discuss some methods to improve Off-Line and On-Line diagnostic techniques.