Fault Protection Based on the Ratio of Zero-sequence Current Projection Components for Low-resistance Grounding System

The electrical characteristics of a single-phase grounding fault in a low-resistance grounding system was aualyzed, and a fault protection scheme was proposed for low-resistance grounding systems according to the ratio of the zero-sequence current projection components. The primary line selection was performed according to the zero-sequence current projection method and then the phase difference value was used for the secondary judgment. The proposed protection scheme was verified by Matlab/Simulink. The simulation results show that the scheme has a certain ability to withstand transition resistance, which effectively improves the sensitivity of the ground fault protection

Magnetic Signal Characteristics in Critical Yield State of Steel Box Girder Based on Metal Magnetic Memory Inspection

Metal magnetic memory testing (MMMT) is a nondestructive testing technique that can detect early signs of damage in components. Many scholars have studied the effect of uniaxial stress on the self-magnetic-leakage field (SMLF)’s strength. Nevertheless, there is still insufficient research on the combined action of bending and shear. We studied the law of distribution of the magnetic signal, ΔHSF(y), at different stress parts of a steel box girder and the quantitative relationship between the magnetic characteristic parameters and the external load. The results showed that the MMMT could accurately detect the early stress concentration zone (SCZ) and predict the final buckling zone of steel box girders. It could be judged that the corresponding parts of the steel box girder had entered the elastic-plastic working stage by the reverse change of the  ΔHSF(y)-F and |HSF(y)|a -F curve trends, this feature could be used as an early warning sign before the steel box girder was deformed or destroyed. The fitted |HSF(y)|ave -F linear expression could be used as the expression between the magnetic signal and the shear capacity. All the evaluation methods were expected to provide a basis for effectively evaluating the stress state of steel box girders with the MMMT method