| There are many excellent performance of AC/DC flexible distribution network,such as flexible regulation properties,strong absorption ability for renewable energy,and good matching performance for AC and DC load.However,because of the interconnection of AC/DC interlinking converter(ILC)and distributed generator(DG),the AC/DC flexible distribution network has prominent power-electronized features,and its fault characteristics are polymorphic,controlled,and weakly fed,which brings many problems and challenges to the operation control,fault analysis and protection for distribution network during faults.According to the structural and operating characteristics of AC/DC distribution network,with the goal of ensuring the safety and reliable operation of ILC and AC/DC distribution network,systematic studies are carried out on fault ride-through(FRT)control,fault characteristics analysis,and configuration schemes and principles of relay protection of ILC and distribution network.This research has important theoretical significance and engineering application value for ensuring the safety of ILC and AC/DC flexible distribution network and improving the reliability of power supply.ILC is the key equipment of AC/DC flexible distribution network,its FRT control is an important basis for reducing the influence range of grid faults and assisting the voltage recovery after faults,which also directly affects the fault characteristics of AC/DC flexible distribution network.Considering the influencing factors such as fault type,power flow,AC/DC coupling effect,etc.,the AC-side FRT control strategies of ILC are proposed for different control modes of constant power,constant DC voltage and constant AC voltage.According to the structural characteristics of different types of ILCs,from the aspects of startup scheme,current limiting control,and automatic recovery,the DC-side FRT control strategies of ILC are proposed.The simulation results show that the proposed FRT control strategies can not only ensure the safety of ILC,but also enhance the ability of power grid to cope with faults,effectively suppress the coupling effect of AC and DC power grid faults,and speed up power supply recovery after faults.ILC has a high degree of modularity,thus,the fault location and fault-tolerant control of submodule faults are important means to improve the operating reliability of ILC.According to the structural characteristics of the cascaded ILC,considering the influencing factors such as control mode,power flow and load level,the fault characteristics of various open-circuit and short-circuit faults of submodules are theoretically analyzed.On this basis,new methods for submodule fault location are proposed,and their good performance are verified via simulation results.From the perspective of both economy and reliability,a dual submodule common redundant structure and its fault-tolerant control scheme are proposed.Through the flexible switching of the connection mode between the common redundant submodules,the continuous operation of ILC can be realized under any faults of single or dual submodules,for which the operating reliability of ILC is significantly enhanced.There may be overcurrent or overvoltage during internal faults of ILC ports,and a complete protection system is needed to ensure the safety of ILC.Aiming at the problem that the protection of cascaded ILC is inadequate,a systematic theoretical study is carried out around short-circuit and open-circuit faults of the AC and DC ports of cascaded ILC.A comprehensive protection scheme for AC-side short circuit based on phase overcurrent,zero-sequence overcurrent and zero-sequence overvoltage,and a protection principle for DC-side short circuit based on the characteristics of voltage amplitude and current change rate,are proposed,which can realize rapid removal of internal short-circuit faults.Based on the internal open-circuit fault characteristics of ILC,an AC-side open-circuit fault detection method that utilizes the characteristics of current interruption and sequence components,and a DC-side open-circuit protection scheme based on characteristics of voltage deviation,are proposed.Moreover,the AC and DC ports of the ILC are supplemented with overvoltage protection measures to effectively ensure the safety of the ILC.In the AC distribution network interconnected by ILC,the random and intermittent features of DG and the complex fault characteristics of ILC and DG make it difficult for the relay protection based on local information to meet the application requirements.According to the FRT strategy of different power electronic equipment such as ILC,a unified fault equivalent model is established,and the distribution characteristics of current amplitude during faults of AC distribution network are analyzed.On this basis,in order to solve the problem that there are various components in the distribution network and the sampling synchronization between measuring points is difficult,a networked protection configuration scheme without strict sampling synchronization is proposed,and a new networked protection principle based on the amplitude comparison of the fundamental frequency components of phase current and sequence currents is proposed.Compared with the traditional local protections,the networked protection is simple to be set and coordinated,and can realize the rapid removal of the fault of distribution network.Furthermore,it is able to greatly shorten the fault time,so as to effectively avoid the disconnection of DGs,and reduce the impact of voltage drop on the sensitive load.In the DC distribution network interconnected by ILC,affected by the current limiting control of ILC,grounding mode of DC grid,distributed capacitance of feeders,etc.,the pole-to-pole fault presents controlled characteristics,and the pole-to-ground fault has weak characteristics,which brings challenges to fault detection and protection.According to the structural characteristics of different types of ILCs,the fault characteristics of the DC distribution network are theoretically studied,and the directional characteristics of the shortcircuit current are analyzed.Based on this,for the pole-to-pole and pole-to-ground faults,the fault direction judgment principles based on the polarity comparison and integral value comparison of the fault component of current are proposed.The principles do not rely on boundary elements and have good anti-interference and anti-noise performance,and the direction judgment result is accurate.Based on the directional element,the main protection and backup protection configuration scheme of DC distribution network is proposed.The main protection of the medium-voltage distribution network adopts the directional pilot protection,which can realize the rapid removal of the whole network fault.In addition,its communication volume is small,and no sampling synchronization is required.The backup protection and the protection of the low-voltage distribution network are based on the local fault direction information and delay coordination.The backup protection scheme is simple to be set and coordinated,and is convenient for the engineering application.At the end of this dissertation,summary bout the research works and main innovations are given.Moreover,the further research directions are pointed out. |
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