Numerical Modelling of Transients in Electrical Systems

Abstract: This thesis deals with the numerical modelling of the response of electrical systems to transient surge voltages and currents caused by lightning. Electrical systems, when subjected to high amplitude transients behave in ways very different from its behaviour under normal operating conditions. Modelling the response of the system in a computationally efficient way, while maintaining sufficient accuracy, is the challenge being addressed in this thesis.While modelling transients in a complex system it is required to reduce the complexity of the system in the model to manageable levels, at the same time retaining all the important characteristics of the system, including the non-linear responses. Transmission line model in time domain is simple and can handle non-linearities easily. Therefore transmission line method is utilised for modelling complex systems in this thesis. Transmission line parameters are deduced from the geometry of the system using field simulation methods. The model is then implemented in a circuit simulation software.To enable computer modelling of transient protection, experiments were conducted on metal-oxide varistors and the results were compared with the predictions of the existing varistor models. From these comparisons the deficiency of existing varistor models are identified and a new improved varistor model is proposed that can overcome these deficiencies. Additionally, using the transmission line method a multiconductor cable model is developed and experimentally verified.Several practical problems of industrial importance are investigated using the transmission line modelling method. The co-ordination between the primary and secondary overvoltage protectors in a low voltage power installation is investigated and it is shown that the existing practice of co-ordinating the primary and secondary protectors based only on the current and voltage ratings do not always work. In another application of the method current and voltage distributions during a lighting strike to the railway signalling and traction system is determined. Lastly, simulation of direct lightning strike to a radio communication tower is performed and the division of current between the cable system and the tower construction is determined. The parameters that influence this current division are also determined.