Optimal Operation And Planning Methods For The Unbalanced Active Distribution Networks With Soft Open Points

Large-scale integration of distributed generations(DGs)and wide application of new power electronic equipments have become the inevitable trend and important characteristics of distribution network development.Soft open point(SOP),a new type of power electronic device,has flexible,fast and accurate power control characteristics and power flow transfer capability.These features make SOP have obvious advantages in promoting DG integration,improving the operation efficiency and enhancing the service restoration.In addition,the distribution networks are usually unbalanced due to unbalanced load and asymmetry line parameters,while the individual phase power control characteristics of SOP and DG converter helps to alleviate the three-phase unbalance of the distribution network.Therefore,the active distribution network based on SOP is gradually receiving attention.The active distribution network based on SOP is significantly different from the traditional distribution network in terms of primary equipment,network structure and control mode,etc.Key technologies such as operation control and planning also need to adapt to the morphological evolution of distribution network.The main contents are as follows: 1)the coupling of active and reactive power of DG and SOP converters makes the active distribution network tend to coordinate the control of active and reactive power,and the three-phase power of the converter can controlled flexibly to alleviate the negative effect of the three-phase unbalance of the distribution network;2)the flexible and changeable control modes of SOP and DG affect the network topology,which aggravates the complexity of service restoration model of active distribution network;3)The active network management of unbalanced distribution network directly affects the results of optimal allocation of DG and SOP,and the uncertainty and diversity of operation scenarios further increase the difficulty.Based on the above background and difficulties,this thesis focuses on the modeling and algorithm of three-phase linear power flow,optimal operation,service restoration and optimal allocation of unbalanced active distribution network based on SOP.The main research work of this thesis is as follows:(1)In order to improve the convergence and efficiency of optimal operation,service restoration and optimal allocation model of unbalanced active distribution network,a three-phase linear power flow model for distribution network is derived.Firstly,the three-phase power control characteristics of SOP under normal operation and restoration are analyzed,and a three-phase power linear model of SOP based on second-order cone relaxation and polyhedron approximation is established.A linear model of three-phase power flow in the form of cartesian coordinates is derived by introducing voltage deviation and approximate elimination into the equation of three-phase power flow.Finally,the accuracy of the proposed three-phase linear power flow model is verified by error analysis.(2)For the distribution network with large-scale DGs,the peak value of DG output may make the node voltage exceed the upper limit,and DG active power has to be curtailed if necessary.SOP can balance feeder power and regulate reactive power,which is helpful to reduce DG active power curtailment.Therefore,an active and reactive power of DG and SOP coordination optimization model is proposed.Firstly,a multi-objective optimization model is established to minimize DG active power curtailment,network loss and voltage unbalance.Through active control of DG and SOP three-phase power,DG consumption is promoted,the operation efficiency of distribution network is improved and voltage profile is mitigated.Then,the non-convex nonlinear optimization model is transformed into a linear optimization model based on three-phase linear power flow,which improves the convergence and efficiency of the optimization model.Finally,a case study is given to verify the feasibility and effectiveness of the proposed model.(3)The service restoration of distribution network is very important to ensure the reliability of power supply.SOP can realize the power flow transfer between feeders and indirectly provide voltage and frequency support for the power outage area,which is helpful to improve the service restoration ability of distribution network.The service restoration model of unbalanced active distribution networks based on SOP is studied in this thesis.Firstly,the service restoration principle based on DG,SOP and switch device coordinated control is analyzed,and the internal relationship between DG and SOP control mode and network topology is revealed,and a modified spanning tree model is proposed to describe the radial topology considering DG and SOP control modes;Then,a deterministic model of service restoration is established,which aims to maximize the restoration load and minimize the voltage unbalance.According to the three-phase linear power flow model,the service restoration model is transformed into a mixed integer linear programming model.Considering the uncertainty of the fluctuating DG output and load demand in the long-term restoration state,a data-driven stochastic service restoration model is proposed.Finally,the test systems with different nodes and fault levels verify the effects of SOP on service restoration and the effectiveness of the proposed model.(4)The flexibility of SOP is helpful for DG integration to distribution network,but the location and capacity of SOP will affect the DG allocation results.Therefore,DG and SOP should be planned together,and the operation mode of unbalanced distribution network will also affect the optimal allocation results of DG and SOP.Therefore,this study proposes a coordinated planning model of DG and SOP considering the active management of unbalanced distribution network.Firstly,a bi-level programming model is established for the optimal allocation of DG and SOP.For the upper level problem,the location and capacity of DG and SOP are determined to minimize the total investment and operation cost of the distribution network.For the lower level problem,the minimum loss and voltage unbalance of the distribution network are taken as the objective results,and the optimal control schemes of DG and SOP in different scenarios are determined.In order to solve the bi-level programming problem,the non-convex nonlinear programming problem of the lower level is transformed into a linear programming problem according to the three-phase linear power flow model,and the dual problem of the lower level problem is derived according to Karush-Kuhn-Tucker condition,so as to establish the mathematical programming with equilibrium constraints.Finally,the influence of DG and SOP three-phase flexible control on allocation results is analyzed through different scale test systems,and the efficiency and convergence of the optimization model are also verified.