Powerflow Convergence Characteristic And Control Of Active Power Loops In Power System With UPFC

With its power setting decided by the power system conditions and basic purpose,the unifined power flow controller(UPFC)changes the direction of power flow of the transmission branches,possibly yielding or intensifying the active power loop flows,affecting the safe and economic operation of the system,thus it is necessary to quantify the controllable range of the active power setting of the UPFC to avoid the active power loop flows.At the same time,as the UPFC is more and more widely used in wind power integration,the active power loop flows may also be affected by the output of the wind farm,and its output uncertainty will further affect the controllable range of the active power setting of the UPFC.Accurate power flow model is the preconditition for(UPFC)to control and participate in the stability and economy of the power system.To improve the convergence of power flow,it is neccesarry to select propriate initial values of the UPFC variables and power settings.To solve above problems,based on the unified power flow model for UPFC,the main contents are as follows:(1)A two-stage power flow algorithm based on initial value estimation is newly proposed to avoid the vulnerability of power flow convergence to the intial values of the UPFC variables.The Jocabian matrix is extended to derive the sensitivity model between the series control variables of the UPFC to its power flow controllable range,and the power flow constraints is introduced to determine the range of power setting,thus avoid the divergence or the violation of the series voltage constraint of the UPFC due to its impropriate power setting.(2)The sensitivity between the active power setting of the UPFC and the active power flow of the line is newly proposed,whose monotonicity is validated with the 2~ndd order derivatives.The sensitivity is iteratively modified with Newton method to find the marginal power setting yielding zero active power flow of the critical line,whose power flow direction is sensitive to the active power setting of the UPFC.Based on the direction of controllable power flow,the controllable range of the active power setting is decided.Considering the impacts of the shunt converter and multiple UPFCs on the active power loop flows,the coordinated model between the series and shunt converters and the coordinated model of multiple active power settins of the UPFCs are proposed.The Floyd-Warshall algorithm is improved to find the active power loop flows and the critical lines,and its computational effort is compared with the exsiting algorithm.(3)An interval power flow model based on the unified power flow algorithm for the active power loop flows is proposed to consider the impact of the wind farm output fluctuations on the controllable range of the active power setting of the UPFC.The interval number is applied to describe the fluctuation range of output.A coordinated model between the active power output of the wind farm and the active power setting of the UPFC is proposed.The output intervals which affect the active power settings are selected from the given output range.Affine Krawcyzk method is used to solve the interval power flow,and the initial interval of the variable is linearized at the center point,so the limit interval of the active power setting is obtained by iterations.