Studies On Grounding Fault Analysis And Location Methods Of AC/DC Distribution Network

In order to cope with the global coal resource depletion and environmental pollution,the proportion of new energy generation,represented by the wind power and photovoltaic power,is constantly increasing,and the novel flexible power distribution technology has made great progress.In this senario,the traditional AC power distribution network gradually presents the characteristics of AC-DC hybrid and high power electronization.On the one hand,it aggravates the complexity of the original location problem of single-phase-to-ground fault of noneffectively grounded AC distribution network.At the same time,it brings new requirements for fault feature analysis and fault location research of DC distribution network.In this context,many researchers have carried out positive and in-depth studies.Based on the existing distribution network locating methods and research results,the steady-state fault information is not reliable enough in locating the AC single-phase-to-ground faults,and it fails to match the actual requirements of rapid locating for DC faults.Fault location methods based on fault transient information attract more attention,but rely on signal detection devices of high sampling frequency.Moreover,a wide application in large AC distribution networks is highcost,and the data transmission and synchronization pressures will increase when a multiterminal information interaction is conducted.Besides,these methods are susceptible to harmonics or noises in small fault current scenarios.It is worth mentioning that,the protection concept that uses power electronic devices to actively inject characteristic signals or disturbances into the systems has been explored in the fields of fault detection in high-voltage transmission system,circuit breaker reclosing and island detection of microgrid.A full utilization of the powerful signal modulation ability and regulation function of the installed converters in AC/DC hybrid distribution networks may provide new solutions or research ideas for the development of fault location technology of AC/DC distribution networks.The main content of this paper is as follows:Firstly,the operation mechanism of each key device in AC/DC hybrid distribution networks is analyzed,and a transient analysis model of a medium-voltage AC/DC active distribution network containing photovoltaic generations and energy storages is established on the PSCAD simulation platform according to the typical architecture.On this basis,the characteristics of the single-phase-to-ground fault are analyzed in a radiant AC active distribution network grounded by an arc-suppression coil,and the fault characteristic differences of the single-pole-to-ground fault are analyzed in a multi-terminal looped DC distribution network considering different connection modes of the converter transformer.Secondly,aiming at the difficulty of locating the single-phase-to-ground faults in resonant grounding distribution networks,an active fault section location method is proposed by generating an additional three-phase symmetrical voltage of low frequency via the converter in AC/DC distribution networks.Due to the over compensating effect of the arc suppression coil under the additional low-frequency signal,this method highlights the inductive current’s circulating path between the fault point and the arc suppression coil.According to the inherent amplitude ratio of the inductance current to the capacitive current in the low-frequency network,an adaptive criterion is formed,which achieves an effective judgement of the fault section.This method is unaffected by the transition resistances,system operation modes or line lengths,thus has high reliability.Comparative simulation results demonstrate the correctness and superiority of the method.Finally,in order to promote the performance of the RL-model based drouble-end fault location algorithm for the single-polar-to-ground fault in DC distribution networks,a moving data window and minimum distance variance calculation are utilized to reduce the locating errors when the double-end data are nonsynchronized,and a criterion for removing unstable points is constructed to avoid the division operation of small numbers.Meanwhile,aiming at the high resistance grounding fault,a short-time "zero mode" operation of the converter is adopted to inject a small disturbance into the system,and cooperated with the mean filtering algorithm,the ability against fault resistance of the location algorithm is enhanced.Simulation results indicate the effectiveness of the improved strategy.