Research And Application Of Thevenin Equivalent Method Based On Measurement Data

With the continuous expansion of the interconnected power network scale,the rapid growth of power demand and the integration of new energy such as solar energy and wind power,the power system has gradually entered an era of comprehensive power network,ultra-high voltage and large generating units.With the interconnection of power network,the structure of power system becomes increasingly complex,the data sharing of power network becomes more difficult,and the operation state of power system is getting closer to the limit level.At the same time,with the rapid development of the wide area measurement system and its wide application in the power system,the scholars all over the world have paid more attention to the on-line voltage stability analysis and control based on the measurement datas and the Thevenin equivalent.The Thevenin equivalent algorithm is one of the core algorithms for the on-line analysis of power network voltage stability.It is the basis for realizing the online safety and stability assessment of power grid voltage to use Thevenin equivalent model to simplify complex networks and quickly and accurately identify Thevenin equivalent parameters.This paper takes the marine micro-grids and the land transmission network as the research object,considering the common characteristics of the marine micro-grids and the multi-regional interconnected power systerm and the influence of the special environment of the ocean on the marine micro-grids,modeling based on the Thevenin equivalent,the equivalent parameters are accurately identified,so as to realize the on-line analysis and monitoring of voltage stability of power system and ensure the safe and stable operation of power grid.The specific research contents of this paper are as follows.(1)In the first part,the research background and significance of Thevenin equivalent are introduced,and its applications in the actual power system are described.Moreover,this paper summarizes the research status of equivalent parameters identification at home and abroad,and the main tasks of this subject.(2)The problems existing in the current Thevenin equivalent parameters identification are analyzed,and the three common methods of the Thevenin equivalent parameters identification are described and discussed in detail.The initial value selection theory is used to solve the problem of initial value dependence,and the stability and accuracy of the algorithm are improved.(3)Based on the basic structure and particularity of the marine micro-grids,considering its difference with traditional terrestrial power grid and the requirements for the comprehensive modeling of the marine power systems,the model of the marine micro-grids is simplified using the Thevenin equivalent;It is assumed that the marine micro-grids has little changes during the sampling time,and its equivalent impedance and equivalent power supply are derived theoretically based on the covariance theory,Finally,the equivalent parameters are obtained based on the power flow measurement datas at the coupling point of the main micro-grids.In order to verify the method presented in this paper,a real marine power system model is selected,parameters identification and power flow calculation examples are compiled,and compared with the traditional method.(4)Aiming at the influence of gross error,bad leverage measurement data on parameter identification and real-time tracking in parameters,a cost function of Thevenin equivalent parameters based on variable forgetting factor and projection statistics algorithm is proposed,and the function is solved by recursive least square method.This algorithm solves the inverse problem of ill-conditioned matrix in function solution by regularization technology,and uses Huber M estimation and projection statistics to suppress the influence of outliers and bad leverage measurements on parameter estimation,The variable forgetting factor is used to realize real-time tracking of the equivalent system.Standard IEEE30 and IEEE39 node systems are selected for testing,and the accuracy and effectiveness of the proposed method are verified by comparing with traditional and common methods.The theoretical analysis and simulation results show that the comprehensive modeling of the marine micro-grids group by Thevenin equivalent can effectively simplify the power network and realize the plug-and-play function.The equivalent system impedance and power supply obtained based on the covariance theory can accurately reflect the response characteristics of the marine micro-grids.In terms of improving the identification accuracy of equivalent parameters,The Huber M equivalent parameter identification method based on variable forgetting factor and regularization can eliminate the influence of measurement outliers and bad leverage measurements,and shows stronger robustness and real-time tracking ability.