Development and Calibration of an Impulse Magnetic Field Measurement System for Lightning Electromagnetic Pulse Investigation

In order to evaluate the impulse magnetic field distribution inside buildings truck by lightning or by nearby stokes, a set of measuring system composed of self-made magnetic detection coil and fiber transmission system was developed. The sensitivity coefficient of the optical fiber transmission system was calibrated by sine wave generator. Connecting the output of a portable 8/20μs current generator with a circular coil whose diameter is 0.3 m to compose the magnetic field generator, the detection coil was fixed in the center of the circular coil for calibration. The B/V calibrated coefficient error of the system is less than 3%. A demonstrating application of the developed measurement system is in the investigation of the electromagnetic environment in buildings under direct and nearby lightning strokes. Experimental verification of such application has been carried out on metal frame structure models of building in laboratory. Two scaled metal frame structure models are set up in the laboratory. The measured transient magnetic field distributions inside the building frame structures are verified by comparison of the testing results with the calculated results using the circuit method simulation. In the circuit method, the lightning channel and the building’s metal cage are modeled by a multi-conductor system. Every branch conductor is divided into several suitable segments. The length of each segment is less than one tenth of the wavelength corresponding to the maximum frequency in the equivalent spectrum of the lightning transient current. Each segment is modeled by a coupled π -type lumped circuit. The test results of the developed measurement system coincide with the calculated results well. Under the situation of nearby lightning strokes, the developed measurement system is also used for the investigation of the shielding effectiveness of the building frame structure. The test results of the shielding effectiveness on the metal frame structure model shows that the effectiveness coefficient increases with the decrease of grid width of the frame. While the width of the grid changes from 0.5m to 0.25m, there is an increase of shielding effectiveness of approximately 10.2%. While the width of grid changes from 0.25 m to 0.125 m, the shielding effectiveness increases about 5%. To simulate the situation when metal plate wall exists in the building, 0.5 mm steel plate was attached on the frame structure model. Under such situation, the shielding effectiveness increases to 4.9. The application experiments of the developed measurement system show that it can fulfill the requirements of lightning impulse magnetic field measurement. Key-Words: impulse magnetic field, calibration, magnetic field generator, magnetic field measurement, lightning protection, fiber transmission system