Research On Fault Characteristic Analysis And Protection Technology For MMC Based Power Electronic Transformer

This paper mainly focuses on the study of fault characteristic analysis and protection technology for MMC based power electronic transformer(PET).The PET is crucial to the realization of interconnection and energy exchange among HV/LV,AC/DC power grids and loads.The faults of the PET have serious impacts on the reliability of power supply and the safe and stable operation of the power grids.Therefore,it is of great significance to study the fault characteristics and configure the corresponding protection scheme for PET.Therefore,the following work was carried out in this paper:Firstly,the theoretical analysis and simulation verification of the possible fault types in PET were carried out,including power switch component fault,AC input side fault,MV DC side fault and LV DC side fault,etc.The overvoltage and overcurrent level of PET under various faults were classified and tallied.The fault types which have greater impact on PET in distribution network were pointed out.Secondly,the influence of different kind of grounding modes on the main grounding fault of PET was studied in this paper,including single-phase grounding fault in AC input/output side and single-pole grounding fault in MV/LV DC side.Based on existing grounding schemes of MMC-UPFC,MMC-HVDC and LV AC distribution network,different kinds of grounding modes suitable for PET were analyzed in this paper,including AC input side zig-zag transformer grounding,AC input side large inductance grounding,MV DC side large resistance grounding,LV DC side capacitor midpoint grounding and AC output side zig-zag transformer grounding.The characteristics of above different grounding faults in different kinds of grounding modes were compared and analyzed.The grounding modes and fault characteristics were verified with PSCAD simulation.Combined with analysis results of fault characteristics and current manufacturing technology,the conclusion was derived that AC input and output side zig-zag transformer grounding is more advantageous in PET.Finally,the corresponding protection schemes were configured for the problems with special attention in the protection of PET.The fault ride-through strategy for MMC based PET was proposed,and a modular redundancy protection scheme was designed.Based on the analysis of the influence of the AC grounding resistance on the AC single-phase to ground fault and the DC single-pole to ground fault,an operation protection strategy was put forward,which is putting a small or big resistor to the grounding device under the normal situation and the single-pole to ground fault,respectively.To suppress the overcurrent under the DC pole-to-pole short circuit fault,an improved scheme was proposed for the existing PET topology where the clamp double sub-module was selected to replace the half bridge sub-module in input stage and the half bridge module was selected to be paralleled with capacitor of primary side in dual active bridge,and the restarting ability of the improved topology was also analyzed.The correctness and effectiveness of the above protection methods were proved by PSCAD simulation.