Research On Single-Ended Distance Measurement Protection Technology Of Ring DC Microgrid Based On Control Cooperation

As an important part of the future flexible intelligent power distribution system,DC microgrid has received extensive attention from scholars.As a key technology to ensure the safe and reliable operation of DC microgrid,relay protection technology has become a hot research topic.When the DC microgrid fails,the fault current has strong impact and rapidity,and the protection requirements are extremely high.In view of the fact that most of the components of the DC microgrid system are connected by power electronic devices with control capabilities,and the existing protection configuration schemes mostly passively receive the fault signals of the original system,it is difficult to meet the requirements of the system for protection,and consider the DC microgrid.The synergy between the self-protection control function of the power electronic converter and the system protection strategy has emerged.Control and security coordination,that is,after the system failure,by changing the control strategy of the inherent or external controllable components in the converter,the active control of the system fault characteristics is realized,which lays a foundation for fault location and fault isolation.In addition,the control and insurance synergy will speed up the fault clearing and improve the system’s fault traversal capability.Therefore,it is of great significance to study the protection scheme based on the control and security coordination to improve the reliability of the DC microgrid.Firstly,based on the model of the toroidal DC microgrid system based on voltage source converter(VSC)operation,this paper takes the system interpole fault as the research object to achieve the uninterrupted power supply and the single-ended quantity protection as the target.According to the different fault isolation methods,two single-ended control and protection schemes are proposed.The general idea is: After the system fails,the active control of each controllable component in the system is first carried out,and then the protection is implemented on this basis.The specific protection scheme can be divided into two stages: fault control and protection implementation.For the first protection scheme,the fault isolation is based on the circuit breaker,and only the VSC outlet capacitor is reserved in the fault control phase to provide the ring network fault feature;in the protection implementation phase,each protection device is based on the traditional RL line model,and only utilizes the local electrical quantity.Accurate fault location can be achieved,and then DC breakers are used to achieve fault isolation;For the second protection scheme,only the fast isolation switch is used for fault isolation.In the fault control phase,the active current of the converter is controlled to control the DC line fault current to zero,which greatly reduces the discharge current of the fault current to the system;In the protection implementation stage,based on the periodicity of the VSC DC-side output voltage and the controllability of the power electronic components after the active control,the unique loop of the VSC and the fault point is constructed,so that there is no need for communication at both ends of the line to achieve single-ended precision ranging.And after the line current continues to zero,the fault isolation is achieved by the fast isolation switch.Finally,through a large number of simulations,the advantages and effectiveness of the two kinds of single-ended ranging protection technology based on control and protection are proposed.