Lightning protection studies of substations and power systems require the knowledge of the dynamic behaviour of large grounding grids during
electromagnetic transients.

Three different analytical concepts are used in order to simulate the transient behavior of grounding arrangements: circuit approach, transmission line approach and electromagnetic field approach.

The electromagnetic field approach describes the problem in frequency domain rigorously applying the full set of Maxwell’s equations with the
minimum possible neglects. An estimation of over voltages in power systems during fault conditions can effectively performed in time domain.

In a stand-alone simulation this can be achieved by inverse Fourier-transform. Modelling of grounding systems as part of the electrical network, i.e. direct
feed-back to the network is mandatory, requires methods to incorporate the grounding system in transients programs.

In order to achieve both full electromagnetic coupling and the interface to the EMTP the electromagnetic field approach is chosen and connected to the
EMTP. Several connections of the live parts to the grounding system can be simulated as mutual coupling between the feeding points is taken
into account.

This is performed by defining the grounding impedance related to the feeding point considered and the mutual impedance between the feeding points in
form of a frequency-dependent transfer function.

The main advantage of this method is that the transfer function is independent of the kind of excitation and depends merely on geometrical arrangement and
soil parameters. The transfer functions are incorporated into the EMTP by means of rational approximation. This strategy permits both the
transformation in time domain and the interfacing with the EMTP using inherent algorithms.

The procedure modelling the coupling between the different feeding points is applicable to an arbitrary number of feeding points. Additionally, the transient
ground potential rise at any point of the grounding structure can be simulated within the EMTP.