Investigation On Lightning Effects In Lightning Protection Systems Of Structures

As modern structures are higher and higher, they are more vulnerable to lightning strike. When a structure is struck by lightning, the large impulse current, flowing through various conducting branches of its lightning protection system, is injected into the grounding system and finally dissipated into the ground. During this transient process, the resultant electromagnetic fields will radiate to signal and power lines and induce the surge overvoltage on them, which can damage to the electric and electronic devices. Meanwhile, the thermal effects are generated in the conductor branches and cause temperature rises in the branches. They decrease the mechanical strength of the conductor branches. In view of the severity of lightning strike, it is important to design the external and internal lightning protection systems of structures. We also need to pay attention to electromagnetic compatibility and lightning thermal effect on lightning protection design of modern structures. The purpose of this dissertation is to predict and analyse thermal and transient electromagnetic effects under lightning strike and to provide guidelines of lightning protection design for reducing losses of lightning damage.This dissertation proposes a reduced-order model for the large scale circuits representing the multiconductor systems. It is based on the Arnoldi algorithm for calculating congruence transformation matrix. By setting up the state equations in frequency domain for the multiconductor systems, the transfer functions are calculated. Furthermore, the time function can be obtained with the help of the inverse Laplace transform and lightning transient responses are given by using the convolution theorem. According to the algorithm, lightning transient responses of the actual structure are calculated. The calculated results are compared with those results obtained by using equivalent circuit method and finite difference method. Moreover, an investigation is made on the influences of lightning current waveform, coupling parameter, calculation step wideth, discretization scheme, lightning stroke manner on transient responses. An experimental setup is built to measure the transient potentials and currents of a reduced-size steel frame structure model.In terms of the transient current responses along the conducting branches, the temperature rise in cylindrical conductors is calculated with uniform and non-uniform current density distributions taken into account. This dissertation also analyses the influences of lightning current waveform, cross-section and material category of conductors on temperature rise. Futhermore, a temperature measurement test of a cylindrical conductor is carried out in this dissertation.By using the hybrid method, the formulas for calculating electric and magnetic fields are derived directly in time domain and the numerical solutions of electromagnetic fields are obtained by the discretization in time and space. After the electromagnetic field distribution inside the building is determined, the induced voltages and currents on the power lines are calculated numerically by solving the transmission line equations using the equivalent circuit method. According to the induced overvoltage level, an attempt is made for simplifying the circuit structure of the surge protective device installed on AC power supply lines of electronic systems. In order to predict the protective performances of the simplified circuits, an impulse experimental arrangement is built to measure their residual voltages.The current distributions inside the grounding electrodes are calculated by developing a grounding grid transient calculation model, and the earth potential distribution is obtained by using the integral method. The adopted human body model is given, which is composed of11elemental blocks and treated by the diakoptic method, and so the electric parameters of the elemental blocks on human body are calculated. The lumped parameter circuit model of human body is built to quantitatively calculate the transient current and energy flowing through the human body. The influence of different positions, amplitude and waveform of lightning current on step voltage is also considered in this dissertation.