Research On Protection Technology Of Flexible DC Distribution System

The research of the flexible dc distribution network(FDCDN)has been just started.Because of lacking of key technology,operating experience and relevant standards,the application of the FDCDN has been limited.This thesis takes the FDCDN for urban power supply as the research object.By means of theoretical analysis and simulation with PSCAD/EMTDC software,the system architecture and control strategy are analyzed systematicly,the grounding mode and fault characteristics are studied,a new protection scheme is proposed at last.First,the models of key equipments such as rectifier,inverter,dc transformer,dc circuit breaker,as well as distrituted generations like photovoltaic cell and wind power generation were analyzed.A new start control method of the FDCDS was proposed to protect the power electronic equipments from the overcurrent when the system startup.A coordination strategy of protection system and control system was proposed,which can make sure the non-fault area work normally after the fault area was isolated.Second,the theory about the recovery of the unbalanced voltage was introduced,and based on which the effect of different grounding modes to the fault characteristics of the converter was studied in terms of ac-side asymmetric fault and DC-side single-pole-to-ground fault.Unlike analytic methods used before,to improve the reliability of power supply in FDCDS,the block of the IGBT was not applied when analyzing the fault characteristics.How to make sure the IGBT works in un-blocked condition was discussed subsequently.The results show that the grounding mode of the system has a great influence on fault characteristics and the procedure of restoration.It also supplies reference information for selecting grounding modes and deciding the protection configuration.A grounding scheme was selected for the rest research of this paper.Third,under the specific grounding scheme and on the premise that the IGBT is unlocked,the fault feature of pole-to-pole fault and single pole-to-ground fault in medium voltage dc system were theoretically analyzed,as well as its effects to other sections and the impact of the grounding resistance value on the fault characteristic;Fourth,a new protection scheme was proposed.The pole-to-pole fault and single pole-to-ground fault with low fault resistance,which can result in overcurrent and is harmful to power electronic devices,were identified by the main protection by means of comparing the line current state value of each relay with the adjacent one.The single-pole-to-ground fault with high fault resistance was identified by the back-up protection.The back-up protection adopted the unbalanced voltage as the startup criterion,it found and isolated the fault area through comparing each relay’s unbalanced current state value with either of the adjacent one.A first level remote backup protection and a seconed level remote backup protection were designed in case of refusal-operation of dc circuit breakers or failure of communication devices,all the potecton schemes could avoid acting simultaneously through the coodination of time interval.Finally,the main research contents and research results were summerized.