Research On The Improvement Method Of AC Microgrid Inverse-time Low-impedance Protection Counting The Influence Of DG

Microgrid is widely accepted as a system for effective networking of distributed generation and load,with efficient utilization of distributed generation.However,with the massive penetration of distributed generation,the microgrid structure and current distribution have changed,and the traditional microgrid protection can not meet the requirements of protection selectivity and reliability.In this paper,the research on AC microgrid inverse-time protection method under distributed generation access is carried out,and the following work is mainly accomplished.Firstly,for the constant power control model of grid-connected distributed generation,the original grid-connected control method is improved according to the low voltage ride through requirements.The distributed generation equivalent impedance magnitude-phase expression before and after the fault are derived,and the magnitude-phase model is established according to its change law to lay the foundation for the subsequent research on the protection method for distributed generation access.Secondly,the impedance correction-based inverse-time protection scheme is improved.According to the traditional inverse-time protection rectification,the causes of protection selectivity mismatch due to distributed generation are analyzed.By analyzing the fault characteristics,an impedance acceleration factor is introduced to reclose the rectification of the fit between the upper and lower levels of protection.The simulation verifies that the protection scheme has good selectivity when the fault location changes,but it can not meet the protection reliability requirements under the fluctuation of power output from distributed power generation,which provides a research direction for the subsequent improvement.Finally,an inverse-time protection scheme based on the power output of distributed generation is proposed.The impedance-corrected inverse-time protection proposed in the previous paper can not meet the protection requirements under the frequent fluctuation of power output of distributed generation.The inverse-time characteristic curve is selected by the power output of distributed generation,so as to speed up the protection operation time to the maximum extent and ensure the cooperation relationship between upper and lower levels.A microgrid simulation platform is built on Matlab/Simulink platform to verify the proposed scheme’s quickness and reliability.By comparing the proposed protection scheme with the traditional protection method,it is verified that the proposed protection scheme can cut off the fault in terms of additional impedance compensation reliability,transition electrical impedance disturbance and resistance to distributed generation output fluctuation.At the same time,the hardware simulation test platform of the protection scheme is designed on DSP,which proves that the proposed protection scheme can cut off the fault quickly and reliably.