On the inclusion of switch arc restrikes and lossy ground in EMI analysis for HV substations

A perennial problem in the areas of control and measurement in switchyards is that of electromagnetic interference (EMI) whereby unwanted signals are coupled from the power circuits during the transients into control or measuring circuits, to cause damage to or malfunctions of these. This paper gives a simple nonideal macromodel of switch arc restrikes for analog simulation of HV substations using SPICE. This model takes into account the phenomena that influence the transient behaviour of currents and voltages to perform a more realistic analysis of the electromagnetic environment. Among these critical phenomena there are the finite speed of the mobile contacts and their non-contemporaneous movements, the restriking and the nonlinear internal characteristic of the arc. A section of a 115 kV substation has been considered. For the computation of the field generated by the bus currents, the entire structure is subdivided into segments that carry a uniform time-varying current and can be assimilated to short dipoles that radiate an electromagnetic field into space. At any given point, the field produced by the currents is the superimposition of the contributions due to all the N dipoles that constitute the structure. The calculation of the reflected field from the lossy ground has been developed in the time domain making use of a rational polynomial fit of the Fresnel reflection coefficients and of a convolution integral. The paper gives rules to set the values of the active and passive circuit elements of the macromodel and their dependence on the switch characteristics and operations. A comparison between field values in presence of perfectly conductive (PEC) or lossy ground inside the HV substation is reported and the limit of the PEC approximation is discussed