Research On Optimal Enhancement Method For Improving Static Voltage Stability

With the rapid development of power industry and the gradual promotion of power market,the operating conditions of modern power systems are usually close to the static stability limit,which places strict requirements for the safe and stable operation of power systems.In recent decades,several major blackout accidents caused by voltage instability have occurred worldwide,resulting in huge economic losses and adverse social impacts.Therefore,it is of great practical significance to adopt appropriate control measures to improve static voltage stability.As the most direct and efficient controls to improve static voltage stability,generator redispatch and line switching have been widely used.In view point of the practical applications,operators prefer to use as few operations as possible to meet the control requirements,but in the previous optimization models and algorithms,the constraints of the number of operations of control measures have been less explored.Hence,this thesis investigates the mathematical model and algorithm of the two control measures of generator redispatch and line switching with the goal of the minimum number of operations,the main work includes:First,there are many control variables in the power system optimization model,the direct solution calculation is inefficient and easy to fall into the " dimensional disaster".In this thesis,the sensitivity method is used to screen the control variables to reduce the search space of control variables.The sensitivity formulas of load margin to generator active output and line parameters are derived.The sensitivities are used as the pre-screening index of the control variables,and the effectiveness of the screening method based on the sensitivity is verified by simulations.Secondly,a minimum generator optimal redispatch model considering static voltage stability constraints is established in this thesis,which is a mixed integer nonlinear programming problem including discrete variables and continuous variables.In order to quickly obtain high-quality feasible solutions,a staged solution method,consisting of fast filtering and model transformation is proposed in this thesis.The sensitivity of the load margin to the active output of the generator is used to filter the candidate generators by proposed method.In the model conversion stage,based on the feasibility pump algorithm,the original problem is transformed into a mixed integer linear programming sub-problem and a nonlinear programming sub-problem to be solved alternately,and the generators to be adjusted and their adjusted load margins are solved.The simulation results show that the proposed optimal model and solution method can effectively improve the load margin,as well as reduce the number of adjusting generators.Finally,the minimum number of line switching problem is studied.Compared with the optimal generator redispatch,the optimization problem of line switching has a higher degree of nonlinearity.The admittance parameters of the transmission line lie in the nonlinear term of the power flow equations,while the active output of the generator lie in the linear term.In addition,because the line parameters cannot be continuously adjusted like the active output of the generator,they have obvious discrete characteristics.In addition,there are a large number of transmission lines,which creates difficulties to solve the proposed line switching problem.To avoid the direct multiplication of discrete variables with the line admittance parameters,the branch addition method is adopted to establish the line switching model in this thesis,and the optimal line switching model that meets the static voltage stability requirements is proposed,and the above mentioned staged solution method is used to solve the model.The screening stage is based on the sensitivity values of load margin to the line parameters to screen out the candidate lines and the invalid lines are eliminated from the candidates;the model transformation stage transforms the original model into an alternating solution of two sub-problems.The simulation results show that the proposed optimization model and solution method can effectively improve the static voltage stability with a less quantity number of line switching.