Research On Distributed Reactive Power Optimization Method For The Integrated Power Transmission And Distribution Systems Considering Static Voltage Security Constraints

Traditionally,transmission and distribution grids independently carry out reactive power optimization.However,in recent years,due to the popularization of hierarchical voltage control,reactive power optimization decision-making of transmission grids is becoming more and more mature,and the period of optimization in the whole system is shortened.And because of the large-scale penetration of renewable energy sources to the grid,the information interaction’between transmission and distribution systems is frequent in the optimization process.In addition,with the development of distributed power generation,active load,distributed energy storage and distribution automation in the distribution systems,the distribution systems not only operate in a more changeable manner,but also have a significant increase in the ability of regulation and control and the influence on the transmission system.It is difficult to solve the problem of power mismatch and voltage mismatch at the boundary nodes of the transmission and distribution grids,independently,by reactive power optimization methods.Reactive power optimization of the power system is a high-order nonlinear programming problem with both continuous and discrete variables.Therefore,reducing the difficulty of solving the optimization model is a problem that needs to be solved emphatically.The objective function of the quadratic programming method is a quadratic function,and the constraint conditions of the quadratic programming method are linear.The quadratic models are simple and the solutions are mature,the convergence property is ideal,and their results have higher accuracy and reliability.Therefore,in this paper,the linear mapping relation between the state quantity(voltage)and the control quantity(excitation voltage and capacitor bank)is constructed according to different control measures of the power transmission and distribution network.And construct a quadratic programming problem with the constraint of the linear inequality with respect to the the control quantity.When the load of the power grid is heavy and the scale of the system is large,the number of iterations for the reactive power optimization,and the calculation time is longer by using the master-slave split method increases.In this paper,a distributed reactive power optimization method based on the Thevenin equivalent parameters is proposed,and different reactive power optimization models are constructed for the power transmission and distribution network,respectively.The information interaction is realized by passing the Thevenin equivalent parameters of the transmission grid to the distribution network,and sending the total power value of the distribution network to the transmission level at the boundary nodes.Thus,the overall problem of the reactive power optimization is decomposed into two sub-problems which are separately optimized,and the solution size of the problem is reduced.With the large-scale penetration of renewable energy such as wind power,the voltage security problems caused by its randomness and volatility need to be paid attention to.The objective function of the conventional reactive power optimization is generally the minimum power loss,and the static voltage stability margin is not taken into account.Thus,the limit value of some individual bus voltages is within the stable range in some occasions,and the voltage quality of some buses decreases.In order to reduce power loss and improve the static voltage stability margin,the static voltage stability margin of the load node in distribution network is calculated based on the Thevenin equivalent parameters.The approximate linear mapping relationship between the static voltage stability margin and the control quantity is established by using the perturbation method,and the new reactive power optimization model is established.Then,the static voltage security margins of load nodes are added to the inequality constraints,so that the static voltage security margin of load nodes are higher than the required threshold values.The MATPOWER simulation tool is used to build the transmission and distribution network model,and the performances of various reactive power optimization methods are compared,and the effectiveness and advantage of the proposed method is verified.