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.

Development of CASRock for modeling multi-fracture interactions in rocks under hydro-mechanical conditions

The interaction between multiple fractures is important in the analysis of rock fracture propagation,fracture network evolution and stability and integrity of rocks under hydro-mechanical(HM)coupling conditions.At present,modeling the mechanical behavior of multiple fractures is still challenging.Under the condition of multiple fractures,the opening,closing,sliding,propagation and penetration of fractures become more complicated.In order to simulate the HM coupling behavior of multi-fracture system,the paper presents a novel numerical scheme,including mesh reconstruction and topology generation algorithm,to efficiently and accurately represent fractures and their propagation process,and a potential function-based algorithm to address contact problem.The fracture contact algorithm does not need to set contact pairs and thus is suitable for complex contact situations from small to large deformations induced by HM loading.The topology of fracture interfaces is constructed by the dynamic adding algorithm,which makes the mesh reconstruction more rapid in the modeling of fracturing process,especially in the case of multiple fractures intersections.The numerical scheme is implemented in CASRock,a self-developed numerical code,to simulate the propagation process of rock fractures and the interaction of multiple fractures under the condition of HM coupling.To verify the suitability of the code,a series of tests were performed.The code was then applied to simulate hydraulic fracture propagation and fracture interactions caused by fluid injection.The ability of this method to study fracture propagation,multi-fracture interaction and fracture network evolution under hydro-mechanical coupling conditions is demonstrated.