Steady-state Security Region Based Optimal Power Flow

The security region(SR)methodology is a new methodology developed based on the point-wise method.Nowadays,the contents corresponding to the transmission grid have been deeply studied,and the obtained achievements have been successfully applied to the practical power grid.However,the research of SR in the distribution grid started late.Especially,the steady-state security region(SSSR)to ensure the AC power flow security in distribution grids needs further study.In addition,with the increasing penetration of the distributed(especially the intermittent,fluctuating and uncertain renewable energy)generation,the operation states of distribution grids become more and more complex.The traditional point-wise analysis method for solving secure constraints cannot meet the requirements of the smart grid online operation optimization.Therefore,it is necessary to study the theory,methodology and application of SR in the smart distribution grid based on the AC power flow model.To this end,this dissertation has carried out the corresponding work,and has achieved the following two parts of results:1.In this dissertation,SSSR in node power injection spaces is defined as the intersection set of the steady-state voltage security region(SVSR)corresponding to node voltage constraints,the thermal security region(THSR)corresponding to line current constraints,and the active/reactive power generation capacity constraints of devices(such as distributed generations).Meanwhile,under this definition and several reasonable assumptions,the mathematic proofs are given for the characteristics of SSSR,i.e.,for a given grid topology,the SSSR of the distribution grid is uniquely determined and is irrelevant with the change of operation state;each boundary of SSSR can be approximated by a hyperplane from an engineering perspective.Furthermore,a fast generation method for the hyperplane expressions of the SSSR boundaries is also proposed.2.The hyperplane expressions of the SSSR boundaries have been successfully applied to the optimal power flow problems,including:(1)A two-stage SVSR-based strategy is proposed to solve the problem of voltage/var control.According to the weighted sensitivity coefficient determined by SVSR,this strategy can automatically select the voltage regulation devices.Meanwhile,for each device,it can evaluate its voltage regulation capacity based on SVSR and quickly determine its output,which accelerates the execution speed of the voltage/var control and greatly shortens the voltage violation time.(2)For the coordinated optimization of transmission and distribution grids,the constraints are described by the approximate hyperplane expression of the SSSR boundaries.Then the distributed optimization model and algorithm are established with the objective function of minimizing the entire power supply cost,which can greatly reduce the required optimization time for of each subsystem and the iteration times of boundary conditions,and improve the solution speed of the whole optimization process by several orders of magnitude.(3)The SSSR methodology is applied to the optimal power flow considering the uncertainties of node power injection,and the linear representation of each chance-constrained condition is constructed.Then the quadratic optimization model is established with the node power injection regulation as variable and the objective function of minimizing the grid loss expectation.As a result,the solution speed of the chance-constrained optimization problem can be greatly improved.