Fault Characteristic Analysis And Protection Principle Research Of Multi-terminal Ring Flexible Dc Distribution Line

With the rapid development of new energy and the sharp increase in DC loads,the flexibility of traditional AC distribution networks has gradually revealed its shortcomings in application,resulting in a significant decrease in efficiency;In recent years,the development of power electronics has promoted the development of DC distribution technology.Its characteristics of low power loss,strong absorption capacity,and few commutation links have made flexible DC distribution networks a research hotspot.The protection measures to ensure the stable and reliable operation of flexible DC distribution networks with multi terminal ring topology are still in the initial stage,and the practicality of applying protection of DC transmission systems to DC distribution networks still needs to be studied.Therefore,the paper focuses on the complex multi terminal circular flexible DC distribution network,and combines the analysis of fault characteristics of DC lines to propose new methods for single pole grounding short circuit fault protection and dual pole short circuit fault protection,respectively.The two protection schemes ensure the stable operation of the system and provide corresponding theoretical support for the development of DC line protection technology.Firstly,the topology structure of a multi terminal circular flexible DC distribution network,the working principles and control strategies of each port,and the coordinated control operation of the entire system were introduced.A complex flexible DC distribution system model with a seven terminal circular topology was built,and fault characteristics were analyzed.Subsequently,the correctness of fault theory analysis was verified in the system.Secondly,in response to the problem that transient overvoltage caused by the fault of the healthy pole can damage the insulation of the line when a single pole short circuit fault occurs in the system,a single pole grounding protection method based on the Kendall correlation coefficient of the voltage of the positive and negative limit current reactor on the same side is proposed.Analyzed the correlation differences between positive and negative limiting current reactor voltages on the same side under normal conditions,bipolar short circuits,and single pole grounding short circuits inside and outside the region for multi terminal circular flexible DC lines.Based on this,identified and selected faults within and outside the region.Finally,the established protection model is applied to PSCAD to simulate the proposed protection scheme,and the impact of transition resistance,noise and other factors on protection performance is comprehensively considered to further verify the correctness and practicality of the proposed scheme.Finally,a single terminal protection method for bipolar short circuit faults suitable for multi terminal circular flexible DC lines is proposed.Analyzed the variation law of the voltage of the current reactor during the fault time limit,and further analyzed the characteristics of metal faults inside and outside the area,as well as the difference between high resistance faults inside the area and metal faults outside the area.Based on the above transient characteristics,a flexible DC distribution line protection method based on the accumulation of negative voltage change rate of current limiting reactors is proposed.This protection method consists of a starting criterion and a fault identification criterion;Finally,the simulation results on PSCAD show the reliability,quick action and selectivity of the method,considering the influence of transition resistance,noise and power transmission on the protection performance.