Wideband Modeling of Large Grounding Systems to Interface With Electromagnetic Transient Solvers

This paper presents a new technique for wideband characterization and modeling of multiport large grounding systems in the form of an accurate Electromagnetic Transients Program-compatible model. This helps to consider the exact behavior of grounding systems in the electromagnetic transient analysis of power systems. The methodology involves three stages. First, a general electromagnetic approach based on the method of moments solution to Maxwell´s equations is utilized to obtain the grounding system admittance matrix over the frequency range of interest. In the next stage, a rational approximation of the grounding system admittance matrix is inferred by making use of a modified matrix pencil method (Modified-MPM) developed in this paper. The Modified-MPM is well suited to the matrix fitting applications with a common set of poles selected for all matrix elements. The obtained rational approximation is finally employed to construct a multiport time-domain model of the grounding system expressed in the form of state-space equations. The proposed modeling approach is applied to the lightning transient analysis of a real subtransmission (63/20 kV) substation considering its grounding system as an integrated part of the electrical system. It is shown that the traditional method of using a simple resistive model for representing the grounding system results in a noticeable underestimation in the calculation of lightning-generated overvoltages in electrical systems.