engleski

Forward and backward fast Fourier transformation in lightning transient study analysis

This paper presents the use of fast Fourier transform in solving a complex problem in the field of power engineering. Lightning transients process appears when lightning strike hits a tall objects, such as buildings, transmission towers, and communication towers. A field theory method has to be used to examine the transient behaviour of lightning process in the tower structure and the associated grounding system. The lightning surge current considered in this paper is defined by a double exponential function. First, the time domain surge current is decomposed into its frequency spectrum by a forward FFT. Let $I(t)$ be the transient lightning current injected into the network. In the frequency-domain approach, $I(t)$ is expressed as the superposition of its harmonics. A finite number of representative frequencies from frequency spectrum are then automatically selected using algorithm developed, to intelligently select a small subset of frequency samples (referred to as ``critical" or ``recommended" frequencies henceforth) to represent the whole spectrum. More frequencies with finer steps are selected in regions where there are rapid changes. At each selected frequency, the electromagnetic fields are computed at various observation points using HIFREQ module in CDEGS ({f C}urrent {f D}istribution {f E}lectromagnetic Interference {f G}rounding and {f S}oil Structure Analysis) software package. The computation frequency can vary from $0$ Hz to several tens of MHz. By means of the Inverse Fourier Transformation, the scalar, vector potential, and electromagnetic field in the time domain are calculated.

lightning transients; surge currents; electromagnetic fields; Fourier transformation; software package

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