Study On Cooperative And Sequential Reconfiguration For Distribution Networks Considering Uncertainty Of Distributed Generations

Under the trend of fossil resources becoming shortage and environment undergoing deterioration, distributed generators (DGs) with clean, efficient, economical and flexible advantages are emerging as required. They are playing a significant role in relieving the energy crisis and environmental problems. DGs, scattered over the distribution network, are able to provide power supply to important load in an emergency as well as participate in network power flow optimization under normal circumstances. Due to the substantial penetration of DGs, distribution networks have transformed from passive networks to active electric networks, thus traditional distribution system operation will be difficult to be fully applied. It is required that the distribution network not only strengthens cooperation with the upper transmission network to achieve cross-regional power resources optimal allocation, but also coordinates with DGs to ensure reliable power supply. Distribution network reconfiguration (DNR) is an effective measure to solve the problem above. It aims at improving the operation performance, e.g., power loss minimization, load balancing, and voltage improvement, by changing the open/close status of sectionalizing switches and tie switches, which eventually improves power supply reliability and power distribution flexibility. The main works on DNR are divided into three parts.1) The application of DGs brings complicated effect on distribution networks, only considering economic efficiency can no longer meet the requirements. Research on multi-objective optimization of DNR seems more reasonable. In this part, three single-objective optimization models are established, in regard to power loss reduction, load balancing and minimization of voltage deviation, in which two of them are chosen to formulate three multi-objective optimization models. It is supposed to contrast reconfiguration results under different goals and objectives, analyze the effect of different objective combination on configuration results and explore the inner link and external cooperation of different objectives. The reference target method is used to deal with the relationship of different objectives. An improved gravitational search algorithm (IGSA) is proposed to optimize network structure and DG injection power. Through the preliminary exploration of three kinds of goals, it is supposed that how to choose or combine objectives would provide decision-making support in DNR which may help obtain expected solution efficiently.2) DGs depending on renewable energy resources, with environmental sustainable features, will be applied in large scale. However intermittent generation of this kind of DGs brings negative impact on distribution networks. In addition, load data for DNR also present fluctuation. Obviously the result of DNR may not be reliable. Therefore this paper proposes a multi-objective model of stochastic DNR problem considering DG output and load forecast uncertainties for systems with distributed solar power generations. The reference target method is used to deal with the two objective functions:total active power loss and load balancing degree. The proposed IGSA is employed to determine optimal configuration. The index of static insecurity probability is introduced for security evaluation of reconfigured networks. The simulation results verify the effectiveness of the proposed stochastic model for DNR which contributes a certain guiding significance to optimal operation of distribution networks.3) The huge penetration of DGs enables distribution networks to strengthen active management and power flow optimization. To accommodate DGs in a much more optimal way, cooperation between transmission networks and distribution networks, even distribution networks and distribution networks, should be further strengthened. A cooperative sequential reconfiguration method is proposed in this paper, which is designed to achieve the optimization of multiple distribution networks in a coordinated way rather than in an isolated way. The global power flow is calculated to reflect interactions among various networks. A sequencing index is introduced for guiding distribution networks to be reconfigured sequentially, which is based on the absolute value of voltage variation at boundary nodes. Reconfiguration for each distribution network is implemented by using the IGSA. The effectiveness of the proposed method is verified by the simulation results with total active power loss reduction and voltage quality improvement.