Lightning-induced impulse magnetic fields in high-rise buildings

Abstract

With the development of lightning protection system (LPS) installed in high-rise buildings, the probability of breakage on building due to lightning strokes has decreased to a very low level. However, the application of LPS imports another hazard to the electronic sensitive equipment in the high-rise buildings, i.e. lightning-induced impulse magnetic field (LIMF). This is because the working principle of a LPS is to attract lightning at their air terminals. An extremely large lightning current will flow through the downconductors of LPS into the earth. This current will generate an intense transient magnetic field around the downconductors inside the building. This transient magnetic field will bring serious electromagnetic interference to sensitive equipment inside the buildings, such as the malfunction or even destruction of sensitive electronic equipment, the over-voltage inside power and communication cables, the pace voltage and touching voltage to threaten human lives and so on. With the increasing application of electronic equipment and network in high-rise buildings in the past decade, such as computers network and telecommunication equipment, the hazard of LIMF becomes an important problem and proper measures should be taken to protect those properties and occupants against it. Before seeking proper measures for the protection against LEMI, the lightning-induced electromagnetic environment (LIEE) inside buildings should be studied in advance. The previous studies on the LIEE inside the buildings protected by LPS focused on the conventional LPS which uses the internal metal structure of buildings as downconductors. Recently, the early emission streamer (ESE) LPS which uses isolated downconductors (e.g. coaxial cables) instead of the building structure is widely applied in modern high-rise buildings. Unlike the conventional LPS, large lightning currents are introduced into the buildings directly by the isolated downconductors. Then harsher LIEE will be induced around the buildings and greater threat will be imposed on properties and occupants. The issue of LIMF inside the building protected by the ESE LPS has been little addressed. In the previous studies, the building metal structures made of reinforcing bars were considered as a wire-grid structure. Recently, metal plates, e.g. metal decking, are widely adopted in the construction of high-rise buildings. The issues of modeling the building structure with metal plates and characterization of LIMF around the structure have been little addressed. In this thesis, the equivalent circuit (EC) modeling approach for a high-rise building installed with the ESE LPS is proposed. The corresponding solution procedure in the time domain is developed by using Electromagnetic Transient Program (EMTP). With the modeling approach and corresponding solution procedure, LIMF inside a typical building in Hong Kong was evaluated and characterized. Furthermore, empirical formulas for evaluating the shielding effect of a gridlike building structure with an isolated downconductor were derived by the curve fitting technique. With these formulas, the shielding effect against the structure dimension, the grid width and the distance between downconductor and structure, could be evaluated quickly. For metal plates, a partial element equivalent circuit (PEEC) modeling approach was proposed and the corresponding solution procedure in the frequency domain was developed in this thesis. The PEEC modeling approach of metal plates was integrated with the EC modeling approach for the wire-grid structure. A novel modeling technique, named as the hybrid analytical-numerical equivalent circuit (HANEC) modeling technique, was proposed. With the HANCE modeling technique, LIMF inside a scaled wire-plate structure were characterized. In order to validate the proposed EC modeling approach, the PEEC modeling approach and the HANEC modeling technique, a series of experiments conducted in laboratories were presented in this thesis. From the characterization results of a typical building in Hong Kong, it is found that LIMF around the isolated downconductor is significantly high and the metal building structure can attenuate LIMF obviously. Some recommendations on the installation of a LPS and the placement of electrical and electronic equipment are provided and some measures to protect the sensitive systems and equipment in critical areas are also proposed. From the characterization results of a scaled wire-plate structure, it is found the building structure with metal plates has much better shielding effect than that without metal plates. From the validation experiments, it is found that the measured results agreed with the simulation results quite well. With the proposed modeling approach and the corresponding solution procedure in this thesis, researchers can study LIMF as well as related issues in high-rise buildings, such as protection measures of critical equipment, induced-voltage in power and telecommunication cables. With the characterization results and empirical formulas, practicing engineers can assess the threat of LIMF to critical equipment in a high-rise building. So proper measures can be taken to protect the critical equipment, e.g. placing critical equipment in safety areas and enclose critical equipment with proper shielding. Furthermore, under the guide of the work in this thesis, the architecture designer can optimize the design of a building structure to minimize LIMF radically.