06_pradhan - IEEE Transactions on Dielectrics and...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 13, No. 1; February 2006 227 Assessment of the Status of Insulation during Thermal Stress Accelerated Experiments on Transformer Prototypes M. K. Pradhan Department of High Voltage Engineering Indian Institute of Science Bangalore-560012, India ABSTRACT The status of insulation in power equipment at any given point in time can be as - sessed by monitoring properties which are sensitive to the amount of aging. This procedure, called the diagnostic or non-destructive testing, has been in vogue for some time now. The diagnosis, as the name suggests, enables one to disseminate the condition of the insulation by making terminal or enclosure-centric measure - ments and as applied to transformers, a minimally invasive test procedure involv - ing the examination of the oil phase, for arriving at information on the degrada - tion of the cellulose insulation on the winding is indicated. An examination of the tracks provided by the by-products of chemical dissociation of cellulose, in solution with oil is recognized as a possible approach to condition monitoring. To this end, carefully controlled laboratory aging experiments have been performed on eight, () nominally identical and scaled-down pro-rated models of single phase trans - former units, incorporating all the essential features of actual power transformers under normal operating electric stress and accelerated thermal stress. Acquisition, analysis and validation of experimental data including dissolved gas analysis DGA , examination of oil phase for the presence of Furans, measurement of the degree of polymerization DP of the paper insulation, electrical and certain chem - ical tests have all been performed to evolve an experimental data-base for assess - ing condition of insulation. Index Terms — Transformer insulation, carbon oxides, DP, furfurals, thermal aging, pro-rated units, accelerated stress. NOMENCLATURE DGA s Dissolved gas analysis DP s Degree of polymerization Ž. OIP s Oil impregnated paper Cellulose ON s Oil natural cooling DTCM s Diagnostic testing condition monitoring GC s Gas chromatograph FTIR s Fourier transform infrared detector HPLC s High performance liquid chromatograph DF s Dissipation factor BDV s Breakdown voltage TCG s Total combustible gas TOT s Top oil temperature HST s Hottest spot temperature BOT s Bottom oil temperature MOT s Temperature of the oil at middle height of the tank Table 1 Manuscript recei ® ed on 6 July 2004, in final form 8 August 2005. Furfurals s 5-Hydroxymethyl- 2-furfural s 2-Furfuryl alcohol s 2-Furaldehyde s 2-Acetylfuran s 5-Methyl-2-Furaldehyde AAF s Accelerated aging factor 1 INTRODUCTION HE onset and progression of aging of insulation in T power transformers is intimately connected with the magnitude and the duration of the thermal stress on the insulation. Mapping the temperature profile, as has been wx done earlier 1, 2 , describes the precursors to rate of insu- lation degradation. This has the premise that, the temper-
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06_pradhan - IEEE Transactions on Dielectrics and...

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