Application of numerical field simulations for low-voltage switchgear

The increasing performance of computers makes it possible to perform numerical simulations of several field distributions and the force on various parts of switchgear equipment. This paper firstly deals with numerical two- and three-dimensional finite element method calculations of the magnetic field and forces in current-limiting miniature circuit breaker systems. Examples are presented for the transient calculation of forces on the plunger of a magnetic tripping coil under consideration of the influence of eddy-currents in the plunger. Magnetic blast forces caused by current loops and ferromagnetic parts in the circuit breaker are calculated for transient three-dimensional models as well as for different 2D and 3D simplifications. A comparison shows that a combination of a 2D transient analysis for the influence of the attractive force of ferromagnetic parts and a 3D static analysis yielding the force of current loops gives satisfactory results with the advantage of less computing time and modelling effort as compared to the full three-dimensional transient model. The equations of motion for the parts of the breaker involved in the tripping and contact opening process are solved using the results of the transient calculations of the plunger force. The second part of this paper deals with temperature distribution and the current-voltage characteristics of arcs in rectangular channels by the use of finite difference method discretization. Static arc voltages over a wide current range from 3 A to 30 kA are presented, and the dynamic back-commutation out of a deion grid is simulated.