| Smart distribution network(SDN)technology plays an important role in the development of smart grid,and it can enhance the grid's compatibility with distributed generators effectively.By achieving the ability of coordinated operation of multiple energy sources,the reliability,stability,and economy of power grid are improved.According to relevant statistical results,more than eighty percent of power outages in China are caused by faults in power supply lines.Therefore,studying the SDN cooperative self-healing control technologies,which can improve the power supply ability of SDN to load when the distribution network is faulty,can enhance the power supply reliability of the distribution network effectively.At the same time,due to the differences in the power output characteristics of distributed generators,SDN has a higher requirements on control and operation strategies.Thus,SDN cooperative self-healing control technologies are studied to improve the power supply ability of the distribution network under fault conditions,and ensure the operational stability,safety,and economy of SDN in the self-healing operation and control stragety.The traditional key nodes identification methods adopt link interruption or cascading failures to study the impact on the network integrity and connectivity of node failure in the network.They usually ignore that the SDN with multiple distributed generators always contains different types of node units and the connectivity path is relatively fixed,different types of nodes may have dependencies that span the intermediate path nodes.Thus,the heterogeneous interdependent network(HI Net)is proposed to identify the key nodes in SDN.By analyzing the heterogeneity of node dependence and network degradation mechanism,the node coupling coefficient is proposed to describe the effect of node on its neighbor nodes in different fault scenarios.Furthermore,the node coupling dynamic propagation algorithm is designed to analyze the state coupling between the nodes and the network,and finally realize the identification of key nodes.Taking the SDN structure decline and state decline as experimental scenes respectively,the validity of the proposed key node identification method is verified by calculating the degree of the influence of different node failure or state stability oscillation on the network structure and operation status.Taking the distribution network with non-reliable generators as the research object,the distribution network island partition strategy considering the protection of key nodes under the main power supply fault scenario of distribution network is proposed.Considering the characteristics of power fluctuations and intermittency of unreliable power generators,an objective function for maximizing islanding allocation of SDN's power supply capacity is established.The key node identification method in the grid state decline mode is adopted to further restore the important load in the key nodes,reduce the influence of the transient oscillation during the power supply recovery process on the stability of the power grid,and prevent secondary fault diffusion of the distribution network.A mutually exclusive firefly optimization algorithm which considers territorial competition is proposed to improve the search ability of the optimal solution for island partition.At the same time,for the 0-1 planning problem of dividing uncontrollable generators into islands,a binary firefly optimization algorithm is constructed.Taking the PG&E69 node system as the test object,it is verified that the proposed island partition strategy can effectively guarantee the reliability of power supply for important loads and key nodes,and reduce the transient oscillation level of the SDN during power restoration effectively.In order to prevent a second time failure caused by the transient oscillation of island switching during the self-healing stage of SDN,the causes for the transient oscillation of the island switching and the suppression methods are studied.Firstly,the state of island switch process is decomposed to analyze the main incentives which restrains the transient stability ability of the master inverter.Then,the power flow tracking system and the master inverter control signal tracking system are designed separately to form an island switching double-layer control system and its control strategy,to reduce transients oscillation caused by supply and demand imbalances and sudden changes in the master inverter control signal during island switching.Taking the microgrid with a low proportion of master inverter as the simulation object,the experimental results show that the proposed island switching system can effectively switch the microgrid operaiton mode from grid connected to island smoothly and guarantee the stability of the power supply in the autonomous regions when their operating model changes.For the multi-microgrid group cooperative operation control in SDN,the load fluctuation of regional power grids is firstly analyzed,and the grey correlation method is used to extract objective characteristics that have strong correlation with load changes,and an adaptive random enhancement random network is established.A layered pyramid model structure,which uses roll-out mechanism to improve the self-adaptability of the prediction model to changes in the load characteristics,is proposed to improve the accuracy and stability of the load forecasting.Considering the problem of power allocation during multi-island coordinated operation,the multi-island cooperative operation mechanism and energy management strategy are established to realize the power supply-demand balance and operation economy after island interconnection.To prevent the spread of faults due to transient oscillations during island interconnection,a multi-island pre-synchronization control system was constructed.Finally,the simulation experiment verifies that the proposed energy management strategy and multi-island cooperative operation control strategy can guarantee the operating economy and stability during the multi-island coordinated operation stage.Taking a radial distributed distribution network with multiple distributed generators as the test object,the effectiveness of the proposed cooperative self-healing control process technologies in improving the fault response capability of the intelligent distribution network are verified.In self-healing stage,the key nodes are identified according to the parameters such as system load,distributed power distribution and power conditions,and line impedance.Then,combing with the data of key nodes and load important level,SDN is divided into serval island to supply power to important load and key nodes.In addition,the islanding smooth switching strategy proposed in this paper,which can prevent secondary failures caused by transient oscillations of the operation mode switching,is adopted to the islanding switching of series islands and parallel islands to improve the power supply quality.The islands' energy optimization plans are formulated with the objective of operating economy.Assuming that there is a risk of supply and demand imbalance in one isolated island,the interconnection operation and synchronization control are implemented to the islands which will interconnect to achieve coordinated operation among multiple islands to enhance the reliability and stability of SDN in self-healing operation stage. |
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