Optimal Power Flow And Loss Analysis Of AC/DC Hybrid System With VSC

In recent years,the rapid development of power electronics technology,coupled with the use of PWM control technology,IGBT-based fully-controlled power electronic converters occupy the dominant position of current converters,of which IGBT-based voltage source converter VSC is obtained.The rapid development has enabled the VSC-HVDC and multi-terminal flexible DC transmission VSC-MTDC technologies at both ends to be realized.The VSC-MTDC system can achieve multi-terminal power supply,which is more secure and reliable than the VSC-HVDC system,more flexible in operation mode and better distributed power consumption.Therefore,studying the optimal power flow and loss of the VSC-MTDC AC/DC hybrid system can provide a strong basis for the safe operation of the power system,system planning and construction expansion plan,and has important value and significance.The power flow calculation method of the AC-DC hybrid system with VSC-MTDC is different from the traditional pure AC system calculation,and its calculation is more complicated.In the thesis,when the alternating current iteration method is used to calculate the power flow in the VSC-MTDC AC/DC hybrid system,the Jacobian matrix elements generated by Newton-Raphson need to be recalculated after each iteration,which affects the convergence speed to some extent,and the converter station loss.Its capacity constraints have an important impact on its calculations.Based on this,this thesis proposes an improved algorithm of alternating iterative method that combines the power and voltage bias of the VSC AC side in the Jacobian matrix and the commutation station loss calculation formula,and fully considers the capacity constraints of the converter station.In the AC-DC hybrid system with VSC network topology,each VSC power flows in both directions,and its reference quantity has a greater impact on power flow and loss than the commonly used radiating topology.The thesis proposes to combine the Newton-Raphson method with the improved genetic algorithm,and to optimize the VSC reference voltage and reference power by using the curve-fitting theory to accurately calculate the converter station loss and DC voltage offset as the objective function of the optimal power flow algorithm.Reasonable distribution of power flow,thereby improving the efficiency of commutation and reducing the loss of the converter station.By programming on the MATLAB platform,the results show the feasibility and effectiveness of the proposed method.In view of the optimal power flow problem of multi-region interconnection withVSC AC-DC hybrid system,the thesis considers the market economy and loss allocation problem of each region after interconnection by VSC-MTDC system.Taking the maximum social welfare and the minimum loss distribution as the objective function,considering the corresponding power flow constraints,the NSGA-II algorithm is used to solve the problem,and the Pareto optimal solution set and the Pareto optimal solution set considering N-1 security are obtained respectively.Under the interconnected power market mechanism formed by the interconnection of multiple regions,the advantages of complementary power surpluses will be greatly exerted,and social welfare and loss sharing will change with the trend.Based on this,the thesis further designs the cross-regional power transmission scenario with the three-region interconnection case containing VSC-MTDC.Combined with the economic Pareto optimal theory,the thesis studies the Pareto-induced optimal power flow change in different regions.Front,the loss of each node and the change in market clearing price.Programming and calculation on the MATLAB platform,the results show the effectiveness and rationality of the proposed method.