Research On Flexible Operation Of AC/DC Active Distribution Network Considering Multiple Uncertainties

The increasingly severe climate problem and the gradual shortage of fossil energy have attracted wide attention of human beings,so much attention has been focused on research related to renewable energy.The development of renewable energy technology will increase the proportion of renewable energy in the power system.In the future,urban distribution network will be connected with more and more renewable energy,which is not only a trend,but also a research hotspot and challenge.On the other hand,with the rapid development of DC transmission technology and the increase of DC load in distribution network,the proportion of DC in urban distribution network will be higher and higher.Therefore,the development trend of urban distribution network in the future should be AC/DC hybrid distribution network with renewable energy.When DC and renewable energy are added to urban distribution network,there will be many problems,such as the uncertain treatment of renewable energy,the change of power flow model under AC/DC grid structure and the influence of DC on reconstruction.Renewable energy output has great uncertainty,thus will bring some challenges for the operation of the network control and so on,at the same time,the DC technology and mature power electronics technology make the distribution network has better control performance and how it can be used in the operation optimization of distribution network is a meaningful and urgent challenge.In terms of power flow calculation,the power flow algorithms used to solve the AC/DC hybrid grid are mainly divided into alternating iteration method and unified iteration method,but the existing two kinds of methods do not fully consider the control mode of the converter station.The uncertainty of renewable energy is mainly classified as randomness and fuzziness,which are treated as probabilistic power flow and fuzzy power flow respectively.However,few literatures can consider both of these uncertain characteristics simultaneously.Research on pure AC distribution network reconstruction has been very mature,but the addition of DC and VSC has changed the reconstruction model,and different VSC control modes must be considered in the reconstruction process,but there are few such studies in the existing literature.Therefore,this paper intends to analyze and study the above three problems.Firstly,considering the randomness and fuzziness of wind power,photovoltaic power and load,a random fuzzy power flow calculation model of AC/DC distribution network is established based on the unified iterative method.Based on the double uncertainty characteristics,a random fuzzy model of WT,PV and load was established,and a random fuzzy power flow calculation method based on cumulant method and Gram-Charlier series expansion was developed.And the uncertainties of power and the correlation between the power of renewable energy are analyzed.Then,at present,most converters in AC/DC distribution network are Voltage Source converters(VSC).Based on the working model of VSC,this paper builds a complete model of the Converter station under different control modes,and considers the working loss of VSC.A unified iterative power flow calculation model for AC/DC distribution network is established.The unified iterative power flow model combines the operating equations of VSC in various control modes with the equations of AC and DC buses,so that the number of equilibrium equations in AC and DC networks is equal to the number of variables,and the power flow can be directly solved by using the Newton-Raphson method.The effectiveness of the proposed model is proved by comparing the unified iteration method established in this paper with the traditional alternating iteration method in the AC/DC grid.Finally,based on the flexible and adjustable characteristics of distribution network structure,combined with the control function of VSC,a two-layer reconstruction model of AC/DC hybrid distribution network is proposed.The upper layer judges the operation mode of VSC according to the grid structure,and then transmits the information of the grid structure and VSC control modes to the lower layer.The lower layer is the VSC operation power optimization layer,which is responsible for establishing the optimization model to find the optimal VSC operation parameters,and then transmitting it back to the upper layer.The upper layer calculates the objective function value and updates the grid structure accordingly.The reconstructed grid structure and the corresponding VSC control parameters are output in coordination between upper and lower layers.The example results show that the two-layer reconstruction method can effectively optimize the network operation,deal with the fault recovery,and fill the gap in the reconstruction of AC/DC distribution network.