Application Of Holomorphic Embedding Theory In Power Flow Calculation Of New-Pattern Power Grids

Under the Target of Carbon Neutrality,China is building a new power system with renewable energy as the major player at an unprecedented rate.The large-scale integration of renewable energy has profoundly affected the network structure and configuration throughout all levels of the traditional power system.Consequently,new paradigms and patterns of power grids have emerged,such as the AC/DC hybrid transmission grid,the integrated transmission and distribution system,and the islanded microgrid,which have posed new challenges to the power flow calculation which is the basis of power system analysis.On the one hand,the conventional power flow algorithms can hardly adapt to the various control and operating characteristics of the new-pattern power grids.On the other hand,since the power system is becoming heavily loaded due to the growing electricity demand,it is difficult for the conventional iterative methods to converge.In view of this,there is an urgent need for a novel power flow algorithm that can not only adapt to the newpattern power grids,but also achieve reliable convergence,providing an accurate and robust power flow analysis tool for the planning and operation of the new-pattern power grids.This paper carries out a study on the application of a novel non-iterative algorithm-the holomorphic embedding method in the power flow calculation of new-pattern power grids,and develops a set of embedding techniques suitable for solving power flow equations of new-pattern power grids.The main research contents are as follows:(1)At the transmission level,in view of the problem that traditional iterative methods for solving power flow of the AC/DC transmission grid depend on initial estimates and may suffer from the convergence issue,and considering the control and operating characteristics of the voltage source converter-based high voltage direct current(VSC-HVDC),a holomorphic embedding power flow approach is proposed for the AC/DC hybrid transmission grid incorporating VSCs.The proposed method can preserve the advantage of the canonical embedding in deriving reliable power flow solutions,and flexibly accommodate different VSC configurations,control modes,and system operating conditions in the hybrid transmission grid.(2)At the integrated transmission and distribution level,with the growing number of distributed generations and loops in the distribution grid,the convergence of the iterative master-slave splitting method(MSSM)may deteriorate when solving the power flow of the integrated transmission and distribution system.Therefore,by utilizing the constructive property of the holomorphic embedding,a distributed holomorphic embedding power flow algorithm that can accurately describe the transmission-distribution interaction is proposed for the integrated transmission and distribution system.The proposed non-iterative algorithm can effectively circumvent the convergence issue faced by the MSSM when handling distribution grids with distributed generations and loops.In addition,the computational efficiency of the power flow calculation for the integrated transmission and distribution system can be enhanced as well as the convergence reliability.(3)At the microgrid level,aiming at the problem that traditional iterative power flow algorithms for the islanded AC/DC microgrid are sensitive to the initial condition,and considering the droop modes of distributed generations and interconnected converters(IC)as well as the steady-state characteristics of frequency,load,and nodal impedance matrix in the islanded hybrid microgrid,a holomorphic embedding power flow algorithm is proposed for the islanded AC/DC hybrid microgrid with droop control.Through a proper embedding technique,the proposed algorithm can reliably converge to the operable solution without dependency on initial guesses.Moreover,the proposed method can effectively capture the influence of the enforcement of IC capacity limit,the droop control parameter,and the IC outage on the power flow distribution,indicating the good applicability of the proposed method.(4)On the basis of the droop control of the microgrid,the steady-state operational characteristics of the microgrid under the hierarchical control are analyzed.Based on the constructive nature of the holomorphic embedding,a holomorphic embedding power flow algorithm is developed for the islanded microgrid considering the hierarchical control.Simulation results reveal that the proposed algorithm can accurately reflect the hierarchical control effect of the islanded microgrid,and flexibly adapt to different load types,topology structures,and node scales.Besides,the proposed method shows good overall performance especially in terms of robustness and applicability.