| At this stage,the development and utilization of new energy has become the main theme,because excessive use of energy leads to energy shortages,and excessive development also affects the environment.VSC-HVDC technology is used to absorb these new energy sources.It has the characteristics of independent active power control and reactive power control,and there is no problem of commutation failure.These remarkable advantages have led domestic and foreign scholars to conduct enthusiastic research on it.The modular multilevel converter(MMC)is the foundation of this power transmission technology,and all the content is developed around it.Therefore,the paper focuses on the problems involved in the grid connection of photovoltaic power plants via MMC-HVDC.Firstly,analyze and determine the topological structure of the photovoltaic power station connected to the grid via MMC-HVDC and the main circuit structure of the DC/DC converter.On this basis,the working principle of the DC/DC converter and filter circuit is studied;photovoltaic power plant is an important environment for this article.It is necessary to derive the mathematical model of series and parallel photovoltaic arrays and the simulation is carried out to verify that the light intensity and the ambient temperature have a great influence on it.And comprehensive analysis of various photovoltaic array maximum power point tracking methods,determine the use of conductance increment method to achieve photovoltaic array maximum power tracking,through simulation verification that this method can track the maximum power point of photovoltaic array well.Secondly,the core equipment of this power transmission technology,namely MMC,is studied,its topology is analyzed,and the working principle of its internal sub-module(SM)is explained.Because the voltage between the sub-module and the sub-module is not equal,there is Voltage fluctuations and secondary circulation flows between phases.Taking these factors into consideration,derive the mathematical model of MMC;according to the most basic mathematical model,conduct in-depth research on various control strategies of MMC,such as using the sorting algorithm realizes the equalization of the capacitor voltage between the sub-modules and the sub-modules,and so on.Then,it comprehensively analyzes the two modulation strategies of MMC’s nearest level approximation modulation and carrier phase-shifting SPWM,because the traditional SPWM modulation strategy has inherent defects,such as the unequal DC capacitor voltage between the sub-module and the sub-module,and there is voltage the volatility phenomenon.Thus the conventional carrier wave phase shift of the SPWM modulation and optimization strategy innovation modulation strategy employed emerged arm current control sub-module is directly solve the inherent problems and a variety of simulation experiments to prove this analysis the correctness and feasibility of the results.Finally,an improved constant DC voltage/reactive power grid-connected control strategy is studied.The droop control strategy replaces the traditional PI control strategy to further ensure the stability of the DC bus voltage and power balance and improve the power quality.Using MATLAB/SIMULINK software to build a photovoltaic system model based on MMC-HVDC grid connection,by changing the light intensity to verify that the improved constant DC voltage/reactive power control strategy can maintain the stability of the DC bus voltage,verifying the feasibility of the control strategy. |
