Research On Transient Stability Control Strategy For Multi-machine Power System Based On EEAC Theory

The interconnection of regional power grid is the inevitable trend of power system development.The interconnection of regional power grid brings great benefits to the social and economic development.However,because of the existence of transient stability problem,it makes the structure complex and huge power grid has serious security risk.The existing methods and control strategies of transient stability are the way to generate the policy table offline,or the way to refresh the policy table online.However,the two kinds of policy tables could not include all fault conditions,inevitable flaw in the plan.And in the process of control,if the system can not query the corresponding strategy,it will follow the most serious situation to implement the control strategy,which can easily lead to excessive control.Secondly,the algorithm based on network structure has a large amount of offline computation,and it must be recalculated when the grid structure changes.Therefore,the transient stability identification and control method based on extended equal area criterion(EEAC)theory can effectively solve the existing problems.EEAC theory is one of the most important methods to judge the transient stability of power system.However,EEAC theory also has its own shortcomings.First,the correctness of the identification of the critical unit directly affects the results of stability identification.Second,there is a certain error in fitting the power-angle curve by least square method.Third,the error of curve fitting not only affects the accuracy of the stability of the system when the system is stable,but also affects the amount of control when the system is unstable.These problems will affect the final effect,when using EEAC theory to determine the stability of the system and the implementation of control.Based on the above problems,the following aspects of the problem are analyzed and compared:Firstly,The existing critical unit identification methods are analyzed and compared,and the critical unit identification method based on residual power discrimination is proposed.The above contents are simulated and analyzed.It is pointed out that it is not easy to identify the critical unit in a short time in the early stage of the fault because of the influence of the inertia of the generator rotor.At the same time,it is pointed out that when using the compound angle to identify the critical units,it is necessary to select the reasonable parameters to identify the critical units in different situations.Based on the above analysis,a method of using residual power to identify critical units is proposed.The superiority of using the residual power to identify the critical unit is verified by simulation.Secondly,The error of the curve fitting using the least squares method is analyzed.The formulas of power-power-angle curve fitting to trigonometric function models are introduced,and the errors of different function models are compared and analyzed.It is proved that the error of curve fitting using the 1-order Fourier function is the smallest.Based on the error analysis,the transient stability identification method of EEAC is verified by simulation,and the transient stability of EEAC is improved.Finally,based on the results of analysis and simulation for error of fitting power-angle curve,the corresponding transient stability control strategy is formulated,and the cutting capacity of the system under the condition of losing stability is modified.The simulation results show that the control strategy can improve the transient stability control effect of multi-machine power system to some extent.