Three-Phase short circuit through an electric arc in the 6–10 kV unbalance switchgear

In electric power systems the most severe types of faults are three-phase short-circuits the identification of which is a significant problem. It's especially important for short circuits that are close to the relay protection installation. The paper discusses the features of the three-phase short circuits. Under the real situation, the arc resistances in all three phases are not equal in different phases, and this creates the asymmetrical appearance. The presence of asymmetry in the phase currents gives rise to current and voltage negative sequences. It may be a new information sign of the three-phase short circuit. The article describes the estimation of the impact of the electric arc on the asymmetrical appearance for three-phase short circuits in the 6-10 kV electrical switchgear. It establishes limit relations for current and voltage parameters in the phases and for the symmetrical components belonging to positive and negative sequences taking into account the electric arc resistance and equivalent system. The results of estimation of the three-phase short-circuit current types can be used in the high-speed electrical relay protection of the switchgear 6-10 kV. Additional information demonstrating the arc three-phase short-circuit is the presence of the current of the negative sequence up to 10% from the value of the current positive sequence. This approach may be useful in the study of the relay protection function in electric networks with an effectively earthed neutral.