Calculation of creepage discharge safety factors against the tangential component of electric elds in the insulation structure of power transformers

Creepage discharges are a main cause of insulation damage in power transformers during repeated tests or during operation. In this paper, a hazardous region in the insulation structure of power transformers (the interface between solid and liquid insulation at the top edge of a high-voltage electrode) is considered, and the impact of a very important factor of the surface discharge initiation and propagation (the tangential component of the electric eld) on the strength of the insulation structure is investigated in power transformers. Insulation system reliability against this tangential component is presented as a safety factor. Two methods based on cumulative stress characteristics and breakdown probability are proposed for safety factor calculation and for the recognition of weak insulation regions. These methods provide essential information on insulation system design and optimization for reducing insulation deteriorations. A case study is performed on the insulation structure of a power transformer that failed during a test at the Iran Transfo Corporation. The redesigned transformer has shown no test failures in the past 10 years.

PDF

___

[1] Hekmati A. Proposed method of partial discharge allocation with acoustic emission sensors within power trans- formers. Appl Acoust 2015; 100: 26-33.
[2] Nelson JK. An assessment of the physical basis for the application of design criteria for dielectric structures. IEEE T Electr Insul 1989; 24: 835-847.
[3] Tschudi DJ. AC Insulation Design: Paper-Oil Insulation Systems. In: Proceedings of the WICOR Insulation Conference; September 1996; Rapperswil, Switzerland. pp. 1-9.
[4] Kim YH, Seok BY, Lee YJ, Koo JY. Experimental investigation of the effect of the design parameters of pressboard in mineral oil on creepage discharge propagation. IET Sci Meas Technol 2013; 7: 23-31.
[5] Li J, Si W, Yao X, Li Y. Partial discharge characteristics over differently aged oil/pressboard interfaces. IEEE T Dielect El In 2009; 16: 1640-1647.
[6] Liu Q, Wang ZD. Streamer characteristic and breakdown in synthetic and natural ester transformer liquids with pressboard interface under lightning impulse voltage. IEEE T Dielect El In 2011; 18: 1908-1917.
[7] Yi X, Wang Z. Creepage discharge on pressboards in synthetic and natural ester transformer liquids under ac stress. IET Electr Power App 2013; 7: 191-198.
[8] Saker A, Atten P. Properties of streamers in transformer oil. IEEE T Dielect El In 1996; 3: 784-791.
[9] Okubo H, Okamura K, Ikeda M, Yanabu S. Creepage ashover characteristics of oil/pressboard interfaces and their scale effects. IEEE T Power Deliver 1987; 2: 126-132.
[10] Nakao Y. In uence of insulating barrier on the creepage discharge in transformer oil. IEEE T Dielect El In 1997; 4: 775-779.
[11] Moser HP. Transformer Board. 1st ed. Rapperswil, Switzerland: Scientia Electrica; 1979.
[12] Nelson JK. Some steps toward the automation of the design of composite dielectric structures. IEEE T Dielect El In 1994; 1: 663-671.
[13] IEC. IEC Standard 60076-3. Power Transformers-Part 3: Insulation Levels, Dielectric Tests and External Clearances in Air. 2nd ed. Geneva, Switzerland: IEC Publications; 2000.
[14] Siodla K, Ziomek W, Kuffel E. The volume and area effect in transformer oil. In: International Symposium on Electrical Insulation; 7{10 April 2002; Boston, MA, USA. New York, NY, USA: IEEE. pp. 359-362.
[15] Kurita H, Hasegawa T, Kimura K. Dielectric breakdown characteristics of clean oil. In: International Symposium on Electrical Insulation; 7{10 June 1992; Baltimore, MD, USA. New York, NY, USA: IEEE. pp. 433-436.
[16] Gerhold J, Hubman M, Telser E. Breakdown probability and size effect in liquid helium. IEEE T Dielect El In 1998; 5: 321-333.