Joint Limiting Control Strategy Based on Virtual Impedance Shaping for Suppressing DC Fault Current and Arm Current in MMC-HVDC Systems

This paper proposes a joint limiting control strategy for suppressing DC fault current and arm current in modular multilevel converter-based high-voltage direct current(MMC-HVDC) systems, which includes two target-oriented current limiting controls. To limit the DC fault current in the early fault stage, an equivalent modular multilevel converter(MMC) impedance is obtained, and its high-frequency part is reshaped by introducing virtual impedance, which is realized by adjusting the inserted submodules adaptively. Following the analysis of MMC control characteristics, the arm current limiting strategy is investigated, with results showing that the inner-loop control has significant effects on arm current and that a simple low-pass filter can reduce the arm current in the fault period. Finally, by combining the virtual impedance shaping and innerloop control, the fault currents of DC lines and MMC arms can be suppressed simultaneously, which can not only alleviate the interrupting pressure of the DC circuit breaker, but also prevent the MMC from being blocked by the arm overcurrent. Theoretical analysis conclusions and the proposed strategy are verified offline by a digital time-domain simulation on Power Systems Computer Aided Design/Electromagnetic Transients including DC platform, and experiment on a real-time digital simulator platform.

Fluorescence detection of Escherichia coli on mannose modified ZnTe quantum dots

Rapid detection and identification of Escherichia coli(E.coli)is essential to prevent its quickly spread.In this study,a novel fluorescence probe based on ZnTe quantum dots(QDs)modified by mannose(MAN)had been prepared for the determination of E.coli.The results showed that the obtained QDs showed excellent selectivity toward E.coli,and presented a good linearity in range of 1.0×10~5~1.0×10~8 CFU/mL.The optimum fluorescence intensity for detecting E.coli was found to be at pH 7.0 with a temperature of25℃and incubation time of 20 min.Under these optimum conditions,the detection limit of E.coli was4.6×10~4 CFU/mL.The quenching was discussed to be a static quenching procedure,which was proved by the quenching efficiency of QDs decreased with the temperature increasing.

Anti-biofouling NH_(3) gas sensor based on reentrant thorny ZnO/graphene hybrid nanowalls

Since toxic gas leakage may cause ecological environmental problems and even life-threatening damage,effective monitoring of toxic gas is of great importance and subject to increasing demand.However,complicated environmental factors,as well as various coexisting interferences can easily affect the sensitivity and selectivity of gas sensors,hindering their performance.Recent reports have successfully demonstrated the development of hierarchical nanostructures with desirable self-cleaning properties,yet gas sensors that can resist contamination have rarely been realized.Here,we developed a reentrant thorny ZnO/graphene hybrid nanowall structure that simultaneously repels liquid contamination and possesses NH_(3) gas sensing properties.The unique reentrant and hierarchical structure,featuring an interconnected vertical graphene nanowall framework with numerous ZnO nanospikes branched on the top nanowall,is highly repellent to liquids,even biofluids with low surface tension.The hierarchical structure consisting of gas sensing graphene and ZnO can be successfully applied as an NH_(3) gas sensor at room temperature,exhibiting not only excellent sensitivity,selectivity,and repeatability,but also outstanding stability even after bacterial contamination.This study provides a versatile method for fabricating reentrant and hierarchical structures with excellent liquid repellency,and offers a promising method for designing reliable gas sensors with anti-biofouling properties.

Dual innervation of spinal ganglion neurons on perineum and bladder

Experiments were performed on 13 Wister rats. Fastblus (FB) was injected into subcutaneous tissue in perineum area and nuclear yellow (NY) was injected into subserous lamina of the bladder wall, respectively. FB, NY and FB+NY were mostly found in the spinal ganglions of L6, S1 ans S2 segments. The numbers of FB,NY and FB+NY labeled cells were 146, 186 and 81, in a total of 463 labeled cells and their proportion was 40%, 51% and 9%, respectively. The result indicates that the spinal ganglion neurons dually innervate both the somatic and visceral tissues, and the convergence of somato_visceral sensory pathways might occur in the spinal ganglion cells.

Impact of Grid Topology on Pole-to-ground Fault Current in Bipolar DC Grids:Mechanism and Evaluation

The fault current level analysis is important for bipolar direct current(DC)grids,which determines the operation and protection requirements.The DC grid topology significantly impacts the current path and then the fault current level of the grid,which makes it possible to limit the fault current by optimizing the grid topology.However,the corresponding discussion in the literature is indigent.Aiming at this point,the impact of grid topology,i.e.,the connecting scheme of converters,on the pole-to-ground fault current in bipolar DC grids,is investigated in this paper,and the ground-return-based and metallic-return-based grounding schemes are considered,respectively.Firstly,the decoupled equivalent model in frequency domain for fault current analysis is obtained.Then,the impacts of converters with different distances to the fault point on the fault current can be analyzed according to the high-frequency impedance characteristics.Based on the analysis results,a simplified fault current index(SFCI)is proposed to realize the fast evaluation of impact of grid topology on the fault current level.The SFCI is then applied to evaluate the relative fault current level.Finally,the simulation results validate the model,the analysis method,and the SFCI,which can effectively evaluate the relative fault current level in a direct and fast manner.