Phase-controlled Arcing Range of Ring Main Unit Filled with C₄F₇N/CO₂ Mixed Gas Under Slightly Positive Pressure

Perfluoroisobutyronitrile (C₄F₇N) and its mixed gas are being widely studied as one of the most potential gases to replace SF₆. Focusing on the breaking process of ring main unit (RMU) filled with C₄F₇N/CO₂ mixed gas under slightly positive pressure, the phase-controlled technology is used to improve the breaking capacity of the interrupter in this paper. The decomposition and consumption of C₄F₇N caused by arcing can be suppressed. Whole process arc model describing the arcing stage and current zero stage is established based on magnetohydrodynamics (MHD) and black box model (BBM). During the opening process, in which both the current and the opening distance are time-varying, influences of the arcing time on internal and external characteristics of the arc, current zero characteristics and post-arc re-ignition characteristics are theoretically analyzed. Reasonable phase-controlled arcing range (PCAR) is calculated and evaluated. According to IEC 62271-103: 2011 and GB/T 3804-2017 standard, synthesis tests are carried out under different arcing times. Results show evaluated PCAR can improve characteristics of the arcing stage and reduce average arcing time. Probability of successful breaking is increased. Effectiveness of phase-controlled technology and evaluation method of arcing range under slightly positive pressure is further verified.