| Grid Interconnection is one of the trends of modern power systems.The interconnected grid helps to optimize energy sources in a wider area,but brings new challenges for the stability control and other issues to the grid.Once the instability accident happens,it is likely to cause more economic losses as the size of the interconnection is increased.Particularly,at the receiving side,the blackouts caused by voltage instability and a wide range of load power outages caused by voltage sag have occurred frequently during the recent years.Thus the stabilization for voltage security has received more and more attention.However,how to make a more adaptively unified stability defense system and the development of control method are still complicated scientific issues.This thesis focuses on voltage stability issues of large-scale grid such as voltage stability preventive control,multi-line defense coordination control,and studies local voltage stability index,the sensitivity analysis method between the arbitrary control parameters and voltage stability index,voltage stability preventive control model,the optimal allocation method of D-STATCOM and the coordinated control method of voltage stability preventive control and emergency control.The related innovative achievements are summarized as follow:1)Impedance modulus margin considering the influence of load characteristics is proposed.A comprehensive nonlinear equivalent system is presented based on the dynamic impedance and according to the non-analytic dynamic analysis method.Combining with the limited transmission criterion,a load impedance modulus margin(LIMM)is proposed.In order to extend LIMM to the comprehensive static load(ZIP),constant impedance load and constant current load are directly converted to the integrated dynamic equivalent impedance and equivalent sources.In this way,the influence of load characteristics on LIMM is considered.This index can describe the nonlinear system features by means of the derivative of the local node voltage to its injection current,which not only accurately assesses the level of the load node voltage stability,but also greatly reduces the amount of network solution.Therefore,it has significantly advantages of online applications.2)An analysis method for the sensitivity between LIMM and the control variables is proposed.Through the first derivative of the control variables with respect to the voltage of weak node and injection current,the sensitivity between them can be obtained.Based on this,further considering the variation of the total load as an input signal for excitation conditions,the second-order partial derivative of the node voltage and injection current to the control variables and the variation of the node load are obtained to correct the results of sensitivity analysis.Thereby,the sensitivity relationship between LIMM and the control variables is presented based on the equation of the dynamic impedance and load impedance of LIMM.The proposed method uses the local real-time information to calculate sensitivity,which avoids solving limit points and left eigenvectors of eigenvalues zero of the Jacobi matrix.Therefore,the computing speed is accelerated and the precision can be guaranteed.That will contribute to make the effectiv e decision of stability control with higher efficiency.3)A preventive control method for voltage stability is proposed based on sequence linear programming(SLP).By use of the advantage of calculation speed of LIMM and its sensitivity with respect to th e control variables,an improved two-stage contingency and participated control set screening method is proposed.Besides,further considering the constraints of the valid space of control measures and predicted load margin,a preventive control method for voltage stability is proposed based on SLP.The area of failures can be greatly reduced and the time of solving the control optimization problem can be saved.Therefore,the algorithm can update preventive control list faster and enhance its adaptive abil ity of the change of operation conditions.Moreover,since power system is a kind of nonlinear system,the control scheme,which is determined by solving a linear programming model,may not be able to achieve the control objectives.So it is necessary to t ake the state after the last control scheme implemented as the next initial state,and to correct calculation process according to the SLP until the control objectives are satisfied.Thereby,the control accuracy is ensured.In addition,the rapid predicti ve value of load margin is designed as a complementary constraint in the optimization model to overcome the poor linearity problem of the local index near by the saddle node bifurcation,thus the effectiveness of control optimization decisions is ensured.4)A new method for identifying instability areas and a method of optimal allocation of D-STATCOM are proposed.At a first,the identification indicator is defined based on the results of LIMM and the weight coefficients determined by their comprehensive characteristics of each district grids.By the sort of the identif ication indicators,the voltage stability weak areas are rapidly identified.Then,taking the weak areas as the study cases,the uncertainties of faults in the weak areas are taken into account,and Monte Carlo method is used to model the risk of voltage sag.Finally,a single objective optimization method is proposed,which takes the risk of voltage sag as the optimal objective.The proposed method directly aims to reduce the loss of voltage sag of voltage stability weak area,so it is more practical.Moreover,this method overcomes the disadvantages of multi-objective optimization methods,which provides a more reasonableway to use FACTS devices to improve the stability,increase the performance of self-healing control,and improve the ability of protection itself.5)A coordinated control method for voltage stability preventive control and emergency control is proposed.In order to consider uncertainty of system operation,risk theory is introduced.The influence of preventive control and emergency control on the economical operation of power system is quantified in the form of voltage risk.The voltage risk consists of the load loss due to voltage violations,voltage collapse and control costs.Then,a comprehensive control model for coordinating voltage stability preventive control and emergency control of power system is proposed.The optimal model takes the minimum voltage risk as the objective.Meanwhile,positive constraint relaxation algorithm(PCRA)is presented to search the optimal coordinating control strategy.In the progress of search,the fault with the smallest severity factor in the set of preventive fault has the priority to be selected as an emergency fault.Under the new condition,the risk index and the set of control measures will be re-determined until the searching termination criterion is unsatisfied.The proposed method is significant not only to achieve the optimum balance between the security and economy of power system,but also to better meet the requirementsof the economy,security and stability of modern power systems. |
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