Research On Flexible Operation Optimization Of AC/DC Distribution Network

Compared with traditional AC distribution network,DC distribution network with voltage source converter has more advantages in improving power transmission capacity and transmission distance,improving power quality,improving operation economy and power flow controllability.The development of diversified loads and the access of a large number of distributed generators have increased the application of power electronic equipment.Therefore,the application of DC distribution technology in the power supply field of the current distribution network helps to alleviate the pressure faced by the AC distribution system,reduce the intermediate link of DG and DC load accessing to the grid,improve the efficiency of power conversion and power quality.AC/DC distribution network is obviously more suitable for the development requirements of the future urban distribution network.High-penetration renewable energy source accessing to distribution network has brought significantly more randomness,fluctuations and uncertainties to the operation of distribution network.In order to improve the ability of distribution network to cope with system uncertainties,the paper takes AC/DC distribution network as the research object,and carries out the research from three aspects: optimal power flow,flexibility evaluation and flexible operation optimization.Firstly,the paper studies the calculation method of optimal power flow for AC/DC distribution network.Based on AC/DC system with multi-terminal VSC interconnections,the steady-state power flow models of the VSC converter station,the AC network and the DC network are established respectively.Then the determination of initial iteration values of power flow and the method of over-limit correction are discussed.The influence of control modes and voltage control methods of the converter station on power flow calculation is discussed.The alternating iterative power flow algorithm for hybrid AC/DC networks considering the precise loss and control modes of VSC converter stations is deduced and established.Take the network loss as the objective function.Take the controllable devices in the AC distribution network and the operation parameters of converter stations as the decision variables.On the basis of considering the active management equipment of the AC distribution network and the control modes of converter stations,the optimal power flow model of AC distribution network with multi-terminal VSC interconnections is proposed.The alternating iterative power flow algorithm for AC/DC networks and dual particle swarm optimization algorithm are used to establish and solve the model.The DC network with three-terminal VSC is added to the IEEE33 case to form the AC/DC distribution network for simulation analysis.Secondly,the operational flexibility index of AC/DC distribution network is established.The paper summarizes the characteristics of the flexibility of power system and the evaluation methods in the current domestic and foreign research.Then the specific characteristics and process of flexibility balance mechanism are elaborated.The supply-demand balance process of flexibility for the distribution network with high-penetration RES is analyzed.Considering the adjusting range,adjusting speed and adjusting time of flexibility,the flexibility resource from source,network and load of AC/DC distribution network is quantitatively evaluated for steady-state operation.Based on the directional,state-dependence and multi-temporal-spatial characteristics,the evaluation index of operation flexibility of AC/DC distribution network is defined.The computing method of operation flexibility index suitable for AC/DC distribution network is proposed.Finally,the flexible operation optimization method for AC/DC distribution network is proposed.The implementation scheme of hierarchical and partitioned dispatching strategy in AC/DC distribution network is studied.The distribution network is divided into the local dispatching layer,the power flow control layer and the regional dispatching layer.Cooperative dispatching methods of controllable DG,controllable RES,energy storage,reactive power regulation device,electric vehicle and flexible load are analyzed.The operation economy and power quality of distribution network are taken as the objective function.The operation parameters of converter stations and controllable devices in the AC/DC distribution network are taken as decision variables.Then,the flexible economic dispatching model of AC/DC distribution network is established.Based on the analysis of the influence of wind power uncertainty on the flexible demand of the distribution network operation,the constraint transformation of the flexibility demand of the distribution network considering the uncertainty of wind power is realized by the non-parametric kernel density estimation and confidence interval method.The multi-objective flexible operation dispatching method for AC/DC distribution network considering wind power uncertainty is proposed.The modified IEEE33 case with multi-terminal VSC interconnections is simulated and analyzed.