Abstract
Modelling for very fast transients (VFTs) requires good knowledge of the behaviour of gas insulated substation (GIS) components when subjected to high frequencies. Modelling usually takes the form of circuit-based insulation coordination type studies, in an effort to determine the maximum overvoltages and waveshapes present around the system. At very high frequencies, standard transmission line modelling assumptions may not be valid. Therefore, the approach to modelling of these transients must be re-evaluated. In this work, the high frequency finite element analysis (FEA) was used to enhance circuit-based models, allowing direct computation of parameters from geometric and material characteristics. Equivalent models that replicate a finite element model’s frequency response for bus-spacer and 90° elbow components were incorporated in alternative transients program-electromagnetic transients program (ATP-EMTP) using a pole-residue equivalent circuit derived following rational fitting using the well-established and robust method of vector fitting (VF). A large model order is often required to represent this frequency dependent behaviour through admittance matrices, leading to increased computational burden. Moreover, while highly accurate models can be derived, the data extracted from finite element solutions can be non-passive, leading to instability when included in time domain simulations. A simple method of improved stability for FEA derived responses along with a method for identification of a minimum required model order for stability of transient simulations is proposed.
Highlights
With the exception of certain types of electromagnetic pulse (EMP) phenomena, very fast transients (VFTs) are the highest frequency class of transients that pose a direct threat to the insulation of high voltage equipment
During each breakdown event, following the subsequent collapse of the electric field sustained between the contacts, an EMP is initiated with a rise time of several nanoseconds and propagates throughout the system, reflecting, refracting and attenuating at discontinuities
The superposition of electromagnetic waves at any point within the system can lead to overvoltages, termed very fast transient overvoltages (VFTOs), which have magnitudes up to 2.5 pu, with frequencies extending up to Energies 2020, 13, 698; doi:10.3390/en13030698
Summary
With the exception of certain types of electromagnetic pulse (EMP) phenomena, very fast transients (VFTs) are the highest frequency class of transients that pose a direct threat to the insulation of high voltage equipment. Mode to surface wave propagation in the VFT frequency range, which are subject to lower attenuation This finding could impact the extent of the region under study and allow numerically stable, frequency-dependent line or cable parameters to be produced directly in ATP-EMTP. Following the conversion of S-parameters to admittance matrices, the frequency-dependent behaviour is captured as a rational function approximation using vector fitting (VF) This rational approximation is used to derive multiple order, pole-residue equivalent circuits, capable of replicating a component’s frequency response. For the purpose of this study, the techniques were compared with a circuit-based modelling approach for two complex and important GIS components, the bus-spacer and the 90◦ elbow. S-parameters directly in transient simulations, using a convolution-based technique This allows single or multiple pole-residue equivalents to be validated
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