Simulation of switching behavior of free burning electric arcs

Summary form only given. Predicting the switching behavior of free burning arc in high voltage devices poses an engineering challenge. It is very much related to understanding the physics of the arc near current zero. In this paper, we present an advanced simulation technology for the prediction of the switching behavior of the free burning SF₆ arcs, for small rated interruption currents. The arc simulations solve a coupled Navier-Stokes and Maxwell equations based on real SF₆ gas data. For calculating radiative energy balance, SF₆ gas absorption data are used. For specific application of the presented simulation technology, we consider induced current switching by SF₆ gas insulated earthing switches. The presented simulation technology consists of three main steps. First, transient arc simulations are performed with a constant DC current applied between the contacts, until it reaches a steady arc voltage and fully develops arc instabilities. The applied DC current takes the value of the rated AC current. The arc simulations are validated by experiments, by comparing measured and simulated arc voltages. Next, we run transient arc simulation with the rated AC current applied between the arcing contacts, up to current zero. As the current goes to zero, the arc cools off and the arc resistance increases. For small rated currents, we observe that the capacitive nature of the arc is observed to be dominant after current zero. That is, the arc loses conducting path at the point of already developed sausage instability, with high electric field stress applied for this insulating gap. Finally, we run the transient arc simulation after current zero, with the prescribed transient recovery voltage from the load circuit side applied between the arcing contacts. We show that the arc re-ignition after current zero is a result of hot dielectric failure, and this observation from the arc simulations has been validated by measured data fro- - m a number of type tests. The dielectric stress across the arcing zone is determined by the race of the increasing transient recovery voltage and the increasing capacitive gap due to the arc cooling. We show that the maximum value of the dielectric stress across the insulating gap is strongly correlated with how much the arc instability develops. The presented simulation method can be applied to predict the contact distances for the arc extinction.