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.