King Saud University

DEANSHIP OF SCIENTIFIC RESEARCH

Research Center – College of Engineering

Final Research Report No. EE-15/25/27

STUDY OF THE EFFECT OF LIVE CONDUCTOR MICRO IRREGULARITIES ON THE BREAKDOWN OF LIQUID INSULANTS

By

Prof. Abdulaziz A. Al-Sulaiman and

Dr. Mohammad I. Qureshi

ABSTRACT

            This experimental investigation presents impact of AC voltage and both polarities of short duration high voltage pulses and the resulting streamers that emerge due to metallic asperities present on stressed electrodes in a high voltage equipment filled with mineral/silicone based insulating oils.  The asperities on live electrodes were simulated with a sharp point-plane electrode configuration.  A short duration pulse generator was designed, fabricated and put into successful operation, while streamers generated in the electrode gap were observed using shadow-graphic optical system.  The compiled results address the role of voltage polarity on AC breakdown, while effects of applied voltage wave shapes on the discharge initiation, propagations and their velocity are also evaluated at lengths.  The breakdown of insulating oil under power frequency voltages occurs under negative polarity with a breakdown probability of ~70% in mineral based insulating oils whereas it is opposite in the silicone oils.  The reasons behind this antagonistic behavior are explained.  Under short duration pulses, three distinct modes of filamentary discharges were found to emanate under positive polarity, whereas under negative polarity thick discharge structures were found.  Positive discharges propagate with sonic to supersonic velocity, whereas negative discharges are mostly subsonic events.  The bush-type positive structures are mostly electronically controlled.  A physical model of bubble dynamics was tested on the growth and collapse of negative discharges and the experimental results confirm that these are formed due to vaporization of liquid film adjacent to the stressed protrusions.