Research On Fault Isolation Technology Of Low Voltage DC Microgrid

As the key technology to restrict the safe and reliable operation of low-voltage DC microgrid,relay protection technology has become the current research hotspot.In the relay protection scheme,fault isolation technology as its physical means of implementation,its importance is self-evident.However,the realization of fault isolation of traditional low-voltage DC power grid mostly depends on the fast breaking ability of DC circuit breaker.When DC system fails,the fault current is very large,and there is no natural zero crossing,which leads to the difficulty of arc extinguishing at the fault point.Therefore,DC circuit breaker is still one of the technical difficulties hindering the development and application of DC system.Since most devices in the low-voltage DC microgrid system are indirectly connected via power electronic converters(VSC,bidirectional DC/DC),the converter itself has a certain fault isolation capability.Therefore,the cooperation between the fault isolation characteristics of converter and the system fault isolation scheme can be studied.Control and protection cooperation,that is,after the system fault,through changing the control strategy of the inherent or external components in the converter,actively control the fault characteristics of the system,and then lay the foundation for fault location and fault isolation.According to the sequence of execution time,the protection scheme can be divided into three stages: fault control,fault location and fault isolation.This paper focuses on two stages of fault control and fault isolation.First of all,this paper studies the fault characteristics and fault isolation characteristics of traditional VSC and bidirectional DC/DC converters.On this basis,based on the idea of control and protection cooperation,the converter is modified,and its fault isolation characteristics are re analyzed.From the perspective of relay protection,a bidirectional DC/DC converter is selected for DC microgrid.Finally,the load side is modified to have fault isolation capability,which provides application basis for the subsequent configuration of low-voltage DC micro network fault isolation scheme.Secondly,based on the idea of control and protection cooperation,this paper proposes a fault isolation scheme based on the action of AC circuit breaker and the action of quick disconnector.In the fault isolation scheme based on the action of AC circuit breaker,in the fault control stage,the system is under active control,and each bridge arm of VSC is modified by using thyristor to change the fault current of VSC output into AC;in the fault isolation stage,the AC circuit breaker can be used to realize fast fault isolation.Thirdly,in the fault isolation scheme based on the action of quick disconnector,this paper proposes two fault isolation schemes based on the action of quick disconnector.In the fault control stage,the thyristor is used to reconstruct the bridge arms of VSC;in the fault isolation stage,the VSC is controlled to reduce the fault current to zero,and then the fast isolation switch can be used to realize the fast fault isolation.Finally,a model is built on PSCAD/EMTDC simulation platform to verify the effectiveness of the fault isolation scheme proposed in this paper,and the feasibility of the project is demonstrated,which provides a reference for the subsequent practical engineering application.