Research On Optimization Scheme Of Distribution Network Protection With Distributed Power Supply

With the rapid development of industrial production in China,the consumption of coal,oil and other non-renewable resources is increasing day by day.Under this background,people pay more and more attention to distributed power generation with clean fuel as the main energy.Compared with traditional thermal power plants,distributed power generation has the characteristics of clean and environmental protection,low investment cost,high flexibility and a wide range of application scenarios.The promotion and development of related industries is an important part of the strategic layout of new energy in China in the future.At the same time,the improvement of grid-connected capacity of distributed power supply also brings new challenges to the existing power grid protection.On the one hand,due to the access of distributed power supply,the topology structure and power flow operation direction of the power grid have changed,which makes the radial network with simple structure become a complex network with multi-power supply operation.and the output of distributed power supply will also change with the power flow of the power grid,so it is difficult to construct protection by calculating the setting value in advance.On the other hand,considering that the distributed power supply is usually connected in series and then connected to the grid in DC microgrid mode,when the fault occurs in the microgrid,the fault location of the system can only be located at the microgrid level.it is difficult to accurately locate the specific fault location within the microgrid,which affects the fast fault removal.Therefore,it is necessary to study the fault location and protection scheme of distribution network with distributed power supply.The main research contents and contributions of this paper are as follows:The main contents are as follows:(1)the output characteristics of inverter distributed power supply under different working conditions and the fault characteristics after grid connection are studied.The control strategy of the inverter distributed power supply determines its output characteristics.Considering that the constant power control strategy is suitable for the grid-connected operation control of the distributed power supply,firstly,the power outer loop control and current inner loop control flow under this strategy are analyzed.In order to ensure that the distributed power supply has a certain low-voltage traversing capability,the output current reference value is set according to the existing distributed power supply low-voltage traversing capability standard,and the output characteristics of the distributed power supply under the control strategy are studied.Then the influence of the gridconnected position of the distributed power supply on the existing current protection is theoretically analyzed,and the simulation model of the distributed power supply connected to the grid is built based on the PSCAD/EMTDC simulation software,and the output characteristics and grid-connected fault characteristics of the distributed power supply are simulated and analyzed.The simulation results show that the distributed power supply can output according to its operating environment and the control strategy set in advance,and verify the negative impact of the distributed power supply on the upstream and downstream feeders and adjacent branches in the case of fault.(2)an optimization method of current protection for distribution network based on multipoint information is proposed.First of all,the hierarchical relationship,equipment specific functions,interactive information and other contents between the master and slave computers configured in all parts of the system are analyzed.Through the collection of the slave information in the system by the host,combined with the principle of graph theory,the node branch correlation matrix and the slave position correlation matrix which can represent the topological structure of the system are formed.Then,according to the element relationship in the matrix,the head and end node information of each branch is obtained,and the Dijkstra algorithm is introduced to analyze the association matrix to get the optimal path between the related nodes.The setting calculation of the short-circuit current is carried out based on the optimal path,and each short-circuit current is superimposed by the superposition theorem to form the optimized current protection scheme.At the same time,according to the fault information obtained by the joint action of all power sources in the fault state,a node branch correlation matrix with different element values is formed,and the element relationship in the matrix can be used to verify the fault position.this method is used as the backup protection of the above current protection scheme,and the two cooperate to form a distribution network current protection optimization method based on multi-point information.Finally,simulation experiments are carried out by PSCAD/EMTDC simulation software to verify the reliability of the above scheme.(3)A DC microgrid fault location method based on control and protection cooperation is proposed.First of all,the DC microgrid system and the voltage source converter are introduced,and the control strategy of some components of the DC microgrid in the fault state is adjusted based on the cooperative idea of control and protection,and the current in the DC microgrid after the fault is transformed into a half-cycle single-phase sinusoidal alternating current.Then the difference feature of the current phase is obtained by using DTU to determine whether the fault location is in the area.Finally,simulation experiments are carried out by PSCAD/EMTDC simulation software to verify the reliability of the above scheme.