Reaserch On Power Flow For Hybrid System In Coordination With VSC-Multi-Terminal Direct Current Transmission

In recent years, multi-terminal direct current transmission system (MTDC) has been continuously applied and has received more attentions and researches. With the continuous development of high-voltage direct current transmission (HVDC) technology, the number of the ports of MTDC system is increased, and the network topology is increasingly complex, and the traditional load flow calculation method for alternating and direct current system cannot totally adapt to the current trend. Based on State Grid sicience project, this article takes the voltage source multi-terminal direct current transmission system (VSC-MTDC) technology as the core, and develops deep research to the load flow calculation including VSC-MTDC alternating and direct interconnected power transmission system on the basis of the research of the operating principle and the control mode, which provides effective theoretical foundation to the algorithm development of engineering practical scheduling software.Firstly, this article analyzes and researches the principles and operating characteristics of traditional direct current transmission (LCC-HVDC) and VSC-HVDC, and compares the advantages and disadvantages between them, and researches the operating principle and control mode of VSC-MTDC on this basis, and then establishes the steady-state model used for load flow calculation; it also discusses in details and establishes the mathematic model for VSC-MTDC load flow calculation, which lays the theoretical foundation for VSC-MTDC load flow calculation.Next, this article achieves the application of the sequential method and the simultaneous solution method in VSC-MTDC system load flow calculation. The two-terminal direct current transmission system load flow calculation used for the traditional converter can be divided into two categories:the sequential method and the simultaneous solution method; the article analyzes in details the principles and the solution procedures of the two algorithms, and gives the handling methods for Jacobian matrix under various control plans aiming at the mathematic model and solving procedures of the simultaneous solution method; it establishes the interface equations under different control plans aiming at the alternating iterative method, and achieves the interfacing at the alternating current and the direct current sides during the power supply converter load flow calculation, and proposes an improved alternating iterative load flow algorithm.And then, it achieves and conducts verification and comparison to the traditional algorithm and the improved algorithm on examples by MATLAB programming; the simultaneous solution has better computational accuracy, but it has difficulty on the interface with the existing alternating current load flow program, and there will be hidden danger of convergence in large system calculation; it is easy for the traditional sequential method to interface with the existing alternating current load flow algorithm, and it is easy to achieve the algorithm by program, but with poor convergence; the improved alternating iterative method utilizes the advantages of the simultaneous method and improves the convergence disadvantage of the traditional alternating method, but the calculation performance under the conditions of multi-terminal and interconnection of large power grids is not ideal.Therefore, this article proposes the distributed load flow calculation method including VSC-MTDC hybrid system based on the asynchronous iterative method. This method depends on the load flow model of the convertor station, and divides the hybrid system, and achieves the global convergence of the whole system by coordinating the boundary variable. This method overcomes the disadvantages of the existing VSC-MTDC load flow methods, such as slow operating rate, poor convergence and even no convergence, under the conditions of large number of convertor stations and enormous number of alternating current network nodes, and takes the multiple control modes of the convertor station into consideration. At last, the performance of the algorithm is verified by cases.