|Title:||Analysis of lightning protection systems for radio base stations using the PEEC method|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Building Services Engineering|
|Pages:||xii, 186 pages : color illustrations|
|Abstract:||Lightning, as one of the most severe meteorological hazards, not only causes unpredictable power interruptions and serious damages to structures, but also imposes a threat to vulnerable low-voltage electronic equipment. As the rapid development of modern power networks, more and more electronic devices, such as antennas, microchips, sensors, etc., are used in the equipment. These electronic devices are easily destroyed by the inrushing surges along the supporting structures and connecting cables. Consequently, lightning-generated accidents on the electronic equipment have been reported increasingly around the world. To provide effective protection for vulnerable equipment, it is necessary to carry out transient current analysis in external and internal protection systems for electrical and electronic equipment, where there is still lack of appropriate modelling approaches. In the past decades, various methods have been developed to model lightning protection systems, such as transmission line method (TL), finite element method (FEM), finite difference time domain (FDTD), method of moment (MoM), etc. While, these methods focused on the characteristics and coupling effects among conductors. Nonlinear protective devices could not be included in the analysis using these methods. In other work, both conductors and protective devices were modelled using a circuit modelling approach. This approach could not consider the frequency-dependent characteristic of conductors and the coupling effects among them. In summary, practical conductors and protective devices were studied separately in most cases because of lacking a method to integrate frequency-dependent conductors with nonlinear devices. Therefore, the exact performance of the system consisting of the supporting structures, cables, electronic equipment and protective devices was poorly investigated. In order to develop effective lightning protection systems, it is important to provide an efficient modelling procedure for the systems under the lightning strikes. A practical lightning protection system consists of supporting steels, interconnection signal/power cables, nonlinear surge protective devices, and various electronic devices. Modelling such a complex systems is always a necessary but tricky task. In my work, a comprehensive procedure is presented to model the lightning protection system for transient current analysis. The partial element equivalent circuit (PEEC) method, which transforms the EM problems into equivalent circuits, is employed as the basic theory for modelling conductors. After modelling the conductors, a vector fitting (VF) technique is adopted on the parameters obtained by the PEEC to implement a time domain solution. Meanwhile, various protective devices, which are represented using circuit model, are constructed using parameter extraction from the impulse test. The system is finally evaluated by combining every model of various components together.|
The main contribution of the work is summarized as follows: (1) A discretization PEEC (DPEEC) is proposed to efficiently capture the skin and proximity effects among conductors and cables. A skin-based non-uniform discretization scheme on the cross section of the conductor is proposed. This novel discretization scheme dramatically reduces the number of cells in the discretization. The DPEEC is verified numerically and the comparison results prove the efficiency and accuracy of the method. (2) A hybrid DPEEC and artificial neural network (ANN) approach is proposed to further improve the efficiency of the algorithm for computing the conductor impedances considering skin and proximity effects. This method trains the ANN using the impedances at low frequencies and then predicts that at high frequencies. Thus, only the calculation of impedances at the low frequency range is needed. As the DPEEC at high frequencies leads to a larger number of cells, the method can significantly improve the calculation efficiency both in computation time and memory consumption. (3) Unlike copper-made conductors and cables, structural steels are the ferromagnetic material, which need to be specifically treated in computer simulation. In this work, the behaviour of the steels under the lightning impulse is investigated using an experimental approach. The results show that these steels are statured when they carry the impulse current directly. Therefore, the linear magnetic PEEC formulation is adopted to model such steels. The formulation is finally transformed into an equivalent impedance using matrix operation. (4) In order to construct the model of nonlinear protective circuits, a novel parasitic parameter extraction technique is proposed. The technique obtains the parasitic components of the device directly from the measured V-I curve. Then, the corresponding circuit models are built automatically using an optimization method. Circuit models for various kinds of protective devices are presented. Their behaviours under a lightning impulse are well investigated. (5) System-level experiments of a simplified radio base station are carried in the laboratory. Corresponding simulation is also provided and compared with the experimental measurement. The simulation is implemented by integrating the models of individual components together. Good agreements between experiments and simulations are observed. The comparison proves that the proposed method is accurate for system-level simulation. After the comparison, lightning transients in a practical radio base station system which includes the tower, grounding grid and connection cables terminated with protective devices are simulated as an application of the proposed procedure.
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