Research On Novel Control Strategy Of Modular Multilevel Converter In HVDC Transmission System

With the rapid development of modern power electronics technology and the iterative update of electronic power devices,the development of high-voltage DC transmission has been greatly promoted in recent years.High-power high-voltage converter is a key component of high-voltage DC transmission system,among which modular multilevel converter as a new generation of voltage-source type converter is widely used in flexible DC transmission projects at home and abroad in recent years because of its modular design,strong expandability,high transmission capacity,high reliability and low harmonic level,and also in new energy grid connection,STATCOM,variable speed motor drive and It has also gained attention in the fields of new energy grid connection,STATCOM,variable speed motor drive and energy storage system,becoming the most promising voltage source converter topology.The paper takes MMC-HVDC system as the research object,based on the analysis of the basic operation principle of MMC,the following research is carried out around the optimal control of MMC under small signal disturbance and fault ride-through control under unbalanced network voltage conditions.Firstly,this paper introduces the topology and working principle of MMC,and deduces the mathematical model of AC-DC side and the coupling relationship between each electrical quantity in detail;it elaborates several modulation strategies of MMC and analyzes their respective advantages and disadvantages and applications;it briefly introduces the MMCHVDC hierarchical control system,which provides a theoretical basis for the subsequent research.Secondly,this paper proposes an adaptive backstepping control strategy based on Legendre polynomials to address the problem that the parameter changes in the dynamic model structure of the modular multilevel converter in flexible DC transmission are caused by small signal disturbances such as severe signal interference or device temperature rise during operation,which makes the stability control of the system ineffective and causes system safety operation accidents.The method establishes a dynamic model in the static abc coordinate system,and uses the Lejeune polynomial to approximate the error value caused by parameter changes in the modular multilevel converter to eliminate the influence on the dynamic model,then uses the backstepping control theory to select the state variables,constructs the Lyapunov function,and finds the variables that meet the Lyapunov stability requirements as the stability control.Finally,this paper proposes a multi-objective recursive model predictive control strategy based on energy balance to address the shortcomings of conventional control methods under unbalanced network voltage,such as complex control and difficult to control the energy inside the converter.The method regulates the MMC power distribution by optimally controlling each current component of each bridge arm,and realizes the cooperative control of AC side threephase current and converter internal energy balance;to simplify the control complexity and reduce the control calculation,this paper optimizes the traditional model prediction control,establishes the corresponding prediction model and objective function,and divides the control objective into internal and external characteristics of hierarchical control,using the optimal solution of external In order to simplify the control complexity and reduce the control computation,this paper optimizes the traditional model predictive control,establishes the corresponding prediction model and objective function,and divides the control objective into internal and external characteristics of hierarchical control.