Propagation of combined longitudinal and torsional stress waves in a functionally graded thin-walled tube

An analytical model for the propagation of combined stress waves in a functionally graded thin-walled tube subjected to combined longitudinal and torsional impact loading is established.The material properties of the tube are assumed to be continuously graded along the length according to a power law function with respect to the volume fractions of the constituents.The generalized characteristic theory is used to analyze the main features of the characteristic wave speeds and simple wave solutions in the functionally graded thin-walled tube.The finite difference method is used to discretize the governing equations.Two types of typical solutions are obtained for the functionally graded tube and the homogeneous tube subjected to combined longitudinal and torsional step loading.The numerical results reveal some abnormal phenomena in the stress path and wave process of the functionally graded thin-walled tube.

Analysis and suppression of voltage oscillation at DC circuit breaker terminal in DC fault transient process

High-frequency voltage oscillation occurs at the initial stage of DC short-circuit fault in the modular multilevel converter(MMC)-based high-voltage direct-current systems,which directly threatens the reliable operation of the secondary equipment in DC circuit breakers(DCCBs).Therefore,this paper focusses on the voltage oscillation characteristic at DCCB terminals,considered the stray parameters of connecting lines,the parasitic parameters of main equipment and the impedance of grounding grid,established equivalent circuit models of MMC-HVDC converter station at the initial stage of pole-topole and pole-to-ground fault,and obtained the analytical formula for the voltage oscillation frequency.Then,the comparison with the DC short-circuit test data of�200 kV MMC-HVDC converter station verified the validity of the models and revealed the mechanism of voltage oscillation.Finally,the influence of the parameters of the converter station on the oscillation characteristics was discussed.It was proposed to install the DCCB at the MMC valve's terminal near the DC side,and the oscillation frequency is about 10–60 kHz,which is more than 5 times lower than that at the DC reactor's terminal near the DC pole bus,and the disturbance of oscillation to DCCB was alleviated.