Research On Fault Characteristics And Protection Schemes Of DC Distribution Network

With the development of power electronics technology,distributed generation resources with DC output characteristics such as photovoltaics and energy storage are connected to the power grid on a large scale.DC loads such as data centers,charging piles,and LED lighting are increasing.DC distribution network has broad application prospects because of its easy connection with renewable energy sources,high efficiency,low loss,large power supply radius,high reliability.At present,DC distribution technologies have been widely used in subways,ships,data centers and other proprietary systems,but technologies of medium-voltage DC distribution network are still being researched and explored.As the key technology for the safe and stable operation of DC distribution network,the protection technology has become the research focus of experts and scholars at home and abroad.This paper takes ± 10 k V DC distribution network as the research object,and develops protection schemes based on the fault analysis of DC distribution network.Firstly,the paper elaborates the development history and background of DC distribution network.Based on the current status of domestic and foreign research on the analysis of fault characteristics,protection equipment and protection principles of the DC distribution network,the problems and challenges facing DC distribution protection technology are proposed.The future research directions of DC distribution network protection are also summarized.Secondly,three typical topological structures of DC distribution network are introduced in detail.Comparative analysis is made from the aspects of topology complexity,reliability,difficulty of control and protection,and application scenarios.Based on the construction of a double-end hand-held DC distribution network model,mathematical modeling and control principle analysis of modular multilevel converter,DC transformer are performed.Next,the paper presents the three typical grounding methods of DC distribution network.The fault characteristics of single-phase short-circuit on the AC side of the converter and DC side unipolar grounding are analyzed separately in three grounding modes.The fault mechanism and characteristics are analyzed by extracting the transient characteristic quantities.The advantages and disadvantages of the three grounding methods are compared in terms of fault recovery,power supply reliability,and economy.Then,aiming at the problems of poor selectivity in single-ended protection,poor quick-action and high data synchronization requirements in multi-end protection,a distributed area protection method based on current differential state for DC distribution network is proposed.The method divides the DC line into several areas by load branch public access points and uses the current differential as the fault transient characteristic.Fault identification and localization is realized by exchanging status words of current differential.The method only needs the exchange of fault current differential status information between neighboring protection devices,thus the strict data synchronization isn’t required.In addition,the protection method is highly applicable,which can be used for double-ended or multi-terminal DC distribution networks.Finally,in order to realize the pole-to-pole DC fault ride-through of the converter,a protection method with the cooperation between fault limiting module and DC circuit breaker is proposed.Aiming at two typical current-limiting topologies of inductive type and resistive type,the selection principles of current-limiting inductance and resistance parameters are given by combining the transient characteristics of DC-side faults and the device parameters of converter IGBT,diode,and DC circuit breaker.The method can make sure that the converter is not locked when pole-to-pole DC fault occurs,thus reliable and stable power supply in non-fault areas is guaranteed.