Study On An Improved AC/DC Power Flow Algorithm Considering Converters' Loss

With the development of renewable energy such as photovoltaic,wind energy,solar energy and so on,as well as the continuous research on power grid technology,High Voltage Direct Current(HVDC)has become one of the main power transmission ways,the new transmission technology not only has power ability of long distance and large capacity transmission,but also provides technical support to the network structures like regional power grid interconnection,multi-point power supply and multi-point load.However,with the development of HVDC technology,the power flow calculation of power system is facing a new challenge,how to use the power flow calculation to obtain the operation status of the power system is the key to achieve the power system planning,power system static safety analysis,relay protection device settings.In terms of AC / DC system,the gird with hybrid structure contains complex network topology and two kinds of converter including many control methods,but the current AC / DC power flow algorithm is incomplete for this study.Therefore,the goal of this paper is to study the current AC / DC algorithm deeply,and find the power flow algorithm for the AC / DC interconnected system by theoretical deduction and simulation comparison.The main work and achievements are as follows:(1)In this paper,the principle and control method of voltage source converter and Line commutated converter are analyzed theoretically by treating hybrid high voltage direct current(Hybrid HVDC)as a research network.Also,the model including converter's loss of the equations and analysis of the two control methods are achieved.(2)The AC / DC power flow equations and their solving methods are introduced for two types of converters.The corresponding mismatch equations for different converters under different control methods are given,and the Jacobi matrix elements are modified as well.According to united AC/DC power flow algorithm,Bi-direction iteration algorithm and decoupled AC/DC algorithm,an improved AC / DC power flow algorithm is proposed,which inherits the structure characteristics of AC / DC decoupling method,and enables the data exchange between AC and DC systems,and an improved way focused on setup of initial condition and bad convergence at the early stage of calculation is achieved.The algorithm uses the decoupling conditions to acquire initial value accurately,and the convergence precision in the process of AC / DC power flow is continuous modifying,so as to improve the computation speed and reduces unnecessary calculation of the grid.(3)By using a study case,decoupled AC/DC power flow algorithm(excluding converters' loss),the unified solution algorithm,the Bi-direction iteration algorithm and the improved algorithm are compared.Under the converter model with or without consideration of converters' loss,it is concluded that decoupled AC/DC power flow algorithm has a large error on the virtual nodes of certain converter due to the neglect of the converter loss.The improved method avoids the case that the bidirectional iterative method does not converge due to the decoupling condition.(4)The control strategy of AC-DC system including Hybrid-HVDC is summarized.Based on the simple and regional interconnected network structure,the four methods are simulated and compared by software MATLAB,and the improved AC / DC decoupling method has better performance,and is easy implemented to all kind network structures.For the computing speed,the proposed algorithm shows quicker compared to previous algorithms,for the calculation accuracy,the improved method fulfills the requirement of convergence,while further verify the feasibility of the proposed method.In this paper,the converter is the main reason for the poor performance of the current AC / DC power flow algorithms.Through the study of the model and control method of the two types of converter,it is found that the AC / DC decoupling method effectively solves disadvantages like the large computational complexity,Poor convergence,but the calculation error is relatively large.In addition to preserving the advantages of the original method,the improved method further enhances the computational accuracy of the algorithm and increases the practicability of the algorithm.It provides a new idea for the existing power flow algorithm and has a positive effect on the implementation of the AC/DC power grid.