Research On Protection Method Of Multi-terminal Ring Flexible DC Distribution Line

The large-scale development of distributed energy,making wide access to renewable energy,local consumption has become the main direction of distribution network development in the future.Compared with the traditional AC energy transmission network,the DC system has obvious advantages in reducing the line loss and the variable current energy consumption of the distributed power access process.In recent years,with the leap-forward development of voltage source converter(VSC)technology,closed-loop DC network operation has become a reality.In addition,the improvement of power electronic control technology and the reduction of converter manufacturing costs further promote the technical innovation of DC distribution network.More and more countries regard the DC distribution system as an important part of the smart grid in the future.However,as a new research hotspot,to realize the large-scale actual operation of DC distribution system,some key problems still need to be solved.Among them,the DC distribution network protection,as the DC system,and even associated with the exchange network reliable and stable operation of the key,is the DC distribution network research top priority.Therefore,in this paper,with the actual topology,aiming at the particularity of fault and protection of DC distribution system,the configuration of DC distribution system protection scheme was studied as follows.Firstly,based on analysis of topology in existing DC distribution field,comparation of advantages and disadvatages of three DC distribution system topology,this research chosed three-terminal ring topology structure as basic topology in DC distribution system simulation model.In design of converters,this research chosed constant DC voltage-reactive power and constant active-reactive power control strategies to control two-level voltage source converter of AC/DC converter;adopt constant current control strategy to control bidirection BUCK-BOOST converter,and built DC distribution system electromagnetic transient simulation model in MATLAB/Simulink.Based on system model in this research,the DC distribution system fault characteristics had been analyzed.Through the theoretical derivation to analysis the DC side pole-to-pole short circuit fault and positive cable grouding fault characteristics of AC/DC converter in DC distribution system and the fault characteristics of AC side fault.Simulated all conditions above in MATLAB/Simulink,and extracted related fault characteristics for protection configuration.Secondly,in this paper,according to the actual topology,aiming at the particularity of fault and protection of DC distribution system,two fast DC protection schemes based on local information were proposed for the multi-terminal ring DC distribution line fault.One of them,the detection of DC line overcurrent was used to achieve the protection of start;The coefficient of variance between the positive bipolar transient voltage and normal steady-state voltage and the coefficient of variance between the negative bipolar transient voltage and normal steady-state voltage were calculated to identify the faulted line.At the same time,the time domain analytical relationship of each electrical quantity of internal fault were derived respectively to calculate the fault distance with the differential equation of internal faults.The result was used to realize the identification of internal and external faults fast and reliably.Another of them,through analyzing the frequency characteristics of DC distribution line,the high frequency current signal was selected according to the resonant frequency at the end of the line,and the high frequency current was used as the protection criterion;The ratio of the absolute value of the positive current and the negative current was used as the criterion of fault polarity.Based on the MATLAB/Simulink simulation,it was concluded that it could quickly protect each fault in the internal DC area,and did not act in the external faults,which verified the effectiveness and feasibility of the protection scheme.Finally,the characteristics of the abrupt current direction at both ends of the DC distribution line were used to solve the problems such as the poor reliability in the existing high-impedance faults and the difficulty of setting the threshold of the DC line longitudinal protection.The slope direction was introduced into the direction of current sudden change detection,the use of low-frequency fault current signal polarity relationship achieved accurate operation of the DC distribution line back-up protection.Based on the MATLAB/Simulink simulation,it verified that the protection method can identify the internal and external faults and meet the requirements of DC distribution network for backup protection selectivity and reliability.