| The emergence of flexible DC distribution technology solves the contradiction between the increasing load demand and the limitation of the traditional power transmission capacity of the AC power grid,and injects new vitality into the development of the distribution network.Flexible DC distribution network has obvious advantages in many aspects,such as easy access to distributed power and energy storage equipment,high power supply quality,large power supply radius and low line cost,which is widely concerned by scholars at home and abroad,and is an important development direction of urban distribution network in the future.Because of the operation mode,the converter structure and grounding mode are different.When the single pole grounding fault occurs in the DC distributed network based on MMC,its fault characteristics are obviously different from the traditional AC distribution network and the DC distributed network based on VSC,so it is necessary to carry out special research on it.Firstly,the distribution model of ±5kV flexible DC distribution network based on MMC is built on the PSCAD/EMTDC simulation platform based on the relevant parameters of the actual project.The transient and steady state characteristics of DC grounding fault in MMC distribution network with DC side clamped large resistance grounding are analyzed by theoretical deduction and simulation verification.When the unipolar grounding fault occurs,the steady state characteristics of all regions are similar,and the area of the straight line unipolar grounding fault can not be identified by the steady voltage and current.However,the change of the transient characteristics is obvious,the change of the positive and negative pole to ground voltage makes the fault current in the DC cable line mainly come from the capacitive current to the ground.Then,based on the transient characteristics of the capacitive current of the line to ground in unipolar grounding fault,a new method based on the generalized S transform is proposed to locate the grounding fault.The generalized S transform is used to calculate the energy difference between the transient current and the frequency at different frequencies.According to the principle of maximum short window energy of transient current difference in fault area under transient characteristic frequency,the fault region is determined.A large number of simulations show that the proposed method is not affected by the fault area,the transition resistance of the grounding point,and the fault distance.Finally,in view of the disadvantage of the current line models commonly used in distribution network fault location,a distributed circuit model of distribution network considering infinitesimal number of high order is used to simulate the urban cable route.Based on this distributed parameter model,a single terminal fault location method for DC cable single pole grounding fault in flexible DC distribution network is proposed.According to the relationship between the voltage and the current of the network with fault modulus,the method estimates the modulus information at the end of the fault port and deduces the modulus current at the fault point.According to the boundary condition of single-pole ground fault,fault locating is realized by finding the minimum value of ranging function by particle swarm optimization algorithm.The method proposed in this paper takes full account of the characteristics of the distribution parameters of the line,and utilizes both the fault transient information and the steady-state information to calculate the fault location.The method takes full account of the characteristics of the distribution parameters of the line,and only needs to sample single-ended information.The method has low application cost.The range precision of this method meets the needs of engineering application and is not affected by the influence of transition resistance and fault distance.By comparing the simulation experiments,the accuracy and applicability of the proposed method are verified. |
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