When attempting to detect faults in grounding grids, the electromagnetic induction method is hampered by the failure of fault diagnosis based on magnetic flux density detection. An electromagnetic induction impedance method is proposed to diagnose corrosion faults by detecting the induced impedance. A non-destructive testing prototype was developed. The sensor of the prototype consists of biaxial sensing coils, a frequency selection module, and a dual-channel synchronous demodulator. The biaxial sensing coils are configured as four identical coils forming a cross. This symmetrical structure realizes the detection of unknown topological structures in grounding grids. The frequency selection module improves the signal-to-noise ratio of the system. The digital demodulator synchronously extracts the phase information in two orthogonal directions. The key technical indicators of the prototype, such as the frequency characteristics, the amplitude accuracy, the phase accuracy, the interference resistance, and the amplitude consistency between channels are tested. Experiments were carried out on a real-size physical model, corroded samples, and an actual operating substation. The results confirm the feasibility of the method and the prototype for the detection of topological structures, disconnections, and faults in grounding grids.

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Current ramp-up is an important stage for a railgun electromagnetic launch system. In lots of launching experiments, a big chunk of deposit aluminum can be found on the rail surface at the start-up position of the armature-rail contact area, sometimes, erosion is even emerged. The transition at current ramp-up is mainly related to local degradation of armature. The larger value of maximum current densities at the rail/armature (A/R) interface would lead to armature local temperature increasing rapidly, which deteriorates the material property of armature, degrades the static electrical contact performance, and reduces the railgun lifetime significantly. In this paper, for restraining start-up transition, several influencing factors of armature degradation and melt are studied based on current concentration. Rail resistivity, rail dimensions, armature resistivity, and the shape of current supply are considered and simulated. At current startup, skin effect and induced current are analyzed, which are considered as the direct factors of armature's current distribution. Armature current distribution and the maximum current density are obtained and compared for each factor. Finally, launching experiments are carried out as the same condition with simulation. On the rail surface at the start-up position of the A/R interface, the depositions reflect the current distribution on armature with different factors influence at the current ramp-up. The results of experiments are in reasonable agreement with simulation description.Index Terms-Armature current distribution, current ramp-up, influencing factors, the maximum current density.

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In the process of electromagnetic launch, the nonuniform contact pressure distribution on the armature-rail interface kept changing with the armature moving forward. At first, the contact pressure was mainly provided by the deformation of the trailing arm, from which it could keep a good metal-metal contact at the armature-rail interface. After the pulse current was constructed, the magnetic pressure would dominate the contact pressure because it was much larger than the mechanical interference pressure. In this paper, the 3-D modeling of contact pressure distribution was carried out with the finite-element method. In the simulation of 3-D contact calculations, the current entered the armature from the trailing edge. The electromagnetic force perpendicular to the armature trailing arm was equivalent to the uniform load on the inner surface of the trailing arm. A typical contour pattern of nonuniform contact pressure distribution was acquired. Some nonuniform coefficients of contact pressure distribution were proposed and analyzed to evaluate the contact performances between the armature and rails. A series of simulations was conducted at different magnitudes of magnetic pressure, and with different armature structure parameters. The characteristics of the nonuniform coefficients were investigated and discussed. The dependences of the nonuniform coefficients on the equivalent uniform load and armature structure parameters were obtained.Index Terms-Contact pressure distribution, C-shaped armatures, equivalent uniform load, nonuniform coefficients, railgun.