Power Flow Modeling And Algorithm Research On LCC-VSC Multi-Terminal Hybrid AC/DC System Incorporating Large-Scale Renewable Energy Resources

With the scale of power system increasing,the grid structure becoming gradully complex,the rapid development of DC transmission technology and the large-scale integration of renewable energy resouces through DC technology,China will build large-scale complex hybrid AC/DC system incorporating high proportion of renewable energy resources in the near future.However,power flow calculation is the core foudamental research of hybrid AC/DC system,it provides essential basis for analyzing the potential risks of system operation,operationtional failure analysis,stability analysis,economic optimization schedualing,reliability analysis,probabilistic power flow analysis,relay protection configuration and planning or designing for hybrid AC/DC system.This paper conducts deeper study on the shortcomings and deficiencies in the current deterministic power flow calculation for hybrid AC/DC system.Moreover,in order to reveal the operation impact of hybrid AC/DC system caused by fluctuating and random renewable energy resources connecting into the grid.The paper conducts a probabilistic power flow analysis on the complex hybrid AC/DC system,which provides a more abundant and comprehensive decision-making information to ensure the stable operation of the interconnected large power grid.The main contributions of this paper are summarized as follows:The power flow calculation method for multi-terminal hybrid AC/DC system considering the losses of VSC(Voltage Source Converter)is studied in this paper.Firstly,the steady-state power flow model and the accurate power loss model of the VSC converter are proposed,and also the operation styles and control modes of HVDC system are analyzed.Furthermore,the power flow calculation algorithm for VSC multi-terminal hybrid AC/DC system is estabilished based on the unified iterative method and the alternating iterative method.The validity and universality of the proposed model are verified in the modified IEEE-57 bus system.Due to the different control characteristics of LCC(Line Current Converter)converter station and VSC converter station,the current research on the coordinated control of hybrid LCC-VSC HVDC system is insufficient.Therefore,this paper established a unified expression for coordinated control of the hybrid LCC-VSC HVDC system,based on the control strategy,a unified iterative power flow algorithm for LCC-VSC hybrid AC/DC system is firstly proposed considering accurate losses model of LCC and VSC converter stations;in addition,the over-limit check and control mode switching strategy are proposed subsequently,the proposed model enriched the contents of power flow calculation for hybrid AC/DC system.Finally,the effectiveness of the model is verified on the modified IEEE-9 bus system.As the penetration rate of renewable energy resources increased continually,the impact of fluctuating renewable energy resources on the frequency of power system can not be neglected.Moreover,the power transmitted by DC system is very large,once the blockage or failure occurs,it will cause the frequency instability of power system,in order to evaluate the frequency stability of hybrid AC/DC system incorporating high proportion of renewable energy resources,this paper proposed an unified iterative power flow algorithm for hybrid AC/DC system taking the frequency characteristics of the generator,load and VSC converter station into account,what’s more,the LMTR(Levenberg Marquart Method Combined with Trust Region Technique)algorithm is introduced to solve the model.Finally,the validity and effectiveness of the model is verified in the modified IEEE-39 bus system.The uncertainty of renewable energy resources will pose a huge challenge to the stable operation of hybrid AC/DC system.In order to reveal the impact of renewable energy resources with strong uncertainty on the hybrid AC/DC power grid,this paper proposed a probabilistic power flow algorithm composed by an improved NATAF transformation and LHS(Latin Hypercube Sampling)method to analyze the complex hybrid containing a two-teminal LCC-HVDC system,a multi-terminal VSC-MTDC(Voltage Source Converter-Multiple Terminal Direct Current)system and a hybrid LCC-VSC MTDC system considering high uncertainty of renewable energy resources for the first time.Finally,the impact of uncertain renewable energy resources on the actual operation control and probabilistic information for hybrid AC/DC power grid are analyzed and discussed in the modified IEEE-118 bus system.