Study On Optimal Methods For Enhancing Flexibility And Resilience Of Distribution Network

With the pressure of energy supply and increasing severity of climate change worldwide,the integration of high penetration distributed generations(DGs)and the frequent and intense natural disasters bring great challenges for the planning and operation of distribution network.The high penetration of DGs raises the volatility and uncertainty of the distribution network,which increases the demand for system flexibility.The frequency and intensity of natural disasters lead to large-scale and longterm blackouts,which highlights the lack of resilience to fight against extreme events in distribution networks.In order to alleviate the pressure of energy supply and ensure energy security,the concept of resilience distribution network is introduced.The menthods for enhancing the resilience of distribution network that include the optimization method of distributed buses hosting capacity,flexibility improvement,the resilience of distribution network improvement is studied.The hosting capacity and flexibility are adapted to the inherent variability and uncertainty of intermittent renewable energy generation.The resilience is against extreme disasters leading to large-scale and long-term blackouts.The contributions of this dissertation can be summarized as follows:(1)A bi-level optimization dispatch method for enhancing hosting capacity(HC)of distribution buses is proposed.Two boundaries of HC for distribution buses are proposed considering the technology and economy.Then,new evaluation metrics that can identify the HC region for the distribution buses are presented.A bi-level optimization dispatch method based on GWO-PSO for hosting capacity of distribution buses enhancement is proposed considering the benefits of both distribution network operators and DG owners.The simulation results show that the proposed bi-level optimization dispatch method can effectively increase the HC margin of distribution buses.(2)An optimization dispatch method for flexibility improvement of distribution network with high penetration of DGs is presented.Considering the characteristics of flexibility,the demand response(DR)model and the DG power prediction model are established.Then,distribution network flexibility evaluation indexes which represent the flexibility adequacy of distribution network capacity and the flexibility adaption of the embedded DGs are proposed.A multi-objective optimal scheduling model is established to improve the flexibility considering distributed energy resource,DR and energy storage system.Simulation results show that the flexibility of the distribution network with high DGs proportion can be improved by optimizing the schedule of flexible resources.(3)A novel combined enhancing resilience method via DG planning and reconfiguration is proposed.According to three abilities of distribution systems during the disruption of extreme disasters,the quantitative evaluation indicators of resilience are put forward from two levels on power sources and loads.Considering resilience evaluation indicators,a planning-attack-reconfiguration model for enhancing the resilience of distribution networks is presented.In the DG planning and reconfiguration stage,the indicators,which include node degree of DG bus for critical loads(NDGCL)and restoration rate of critical loads(RRCL),are taken as optimization objectives respectively.In the attack stage,the worst case of N-k contingencies based on generalized nodes is presented to reduce the computational complexity.The simulation results indicate that all the RRCL can reach about 90% in the multi-point fault scenarios.(4)Considering dynamic restoring electricity price(DREP),a coordinated optimization dispatch model for multi-microgrids is presented to improve the resilience of distribution networks.In the post-disaster recovery process of the distribution network,the trading mechanism model of dynamic restoring electricity price is established.Then,the power headroom index of the microgrid and DREP are introduced,so the distribution network achieves the maximum recovery rate and the minimum purchase cost.The three-stage DREP mechanism guides and encourages the dispatchable resources in the micro-grid to positively respond to the power demand of distribution network.The resilience of distribution network is effectively improved through the proposed optimization dispatch method.