Modeling And Stability Analysis Of AC/DC System With MMC

The voltage source converter(VSC)based high voltage direct current(HVDC)transmission is widely used in renewables' grid-connected and the modular multilevel converter(MMC)has become the mainstream topology used in VSC-HVDC projects due to its modular structure advantages.Compared with two-level VSC,MMC has strong nonlinear and coupling characteristics in its internal electrical quantities,which increases the difficulty in modeling and stability analysis of MMC.As the capacity of the MMC increases,MMC-HVDC is facing the situation of "strong HVDC and weak AC system".Thus,studying the strength of the AC grid that an MMC can be connected to,and the degree of disturbance that the system of an MMC connected to a weak AC grid can tolerate is significant for the reasonable planning and design of the AC/DC system structure and the coordinated development of AC and DC transmission in our country.To this end,this dissertation conducts in-depth research on the power transmission capacity and large-disturbance stability of the MMC-HVDC connected to a weak AC grid from both the MMC modeling method and the MMC-HVDC stability analysis method.First,the MMC electrical model modeling method is studied that can accurately simulate the internal electrical coupling characteristics of the converter and can easily to be established an interactive interface with the controller and the external electrical system.In order to solve the problem that the existing electromechanical transient time-scale MMC model and MMC steady-state model cannot accurately simulate the internal coupling characteristics of MMC,based on the MMC switching function model and MMC dynamic phasor model,a type of MMC reduced-order dynamic phasor model retaining DC dynamics is proposed,which is suitable for electromechanical transient time-scale simulation and large-signal stability analysis of MMC-HVDC;an MMC steady-state phasor model considering internal coupling characteristics is proposed,which is suitable for the studies of steady-state characteristics and static stability of MMC-HVDC.Then,the modeling method of the MMC electrical model is applied to the real-time simulation of the MMC-HVDC system,and,an improved bridge arm simulation model of MMC to improve simulation accuracy and numerical stability is proposed.Thus,two problems are solved that the potential numerical instability caused by a time-step delay in the electromagnetic transient simulation of the controlled source model in the MMC switching function model and the numerical oscillation and voltage waveform distortion in MMC blocking state simulation.Based on the bridge arm model developed by using CBuilder,the simulation of the MMC-HVDC system is established in RTDS,verifying the feasibility and accuracy of the proposed modeling method.Next,based on the proposed MMC steady-state phasor model,the transmission capacity of MMC-HVDC connected to a weak AC grid is studied.An open-loop calculation method for determining the steady-state operation region boundary of an MMC is proposed to study MMC's own working limits,in which the upper boundary is determined by the maximum modulation ratio,and the lower boundary is determined by the singular conditions of the Jacobian of power flow equation,which greatly reduces the calculation amount while maintaining the calculation accuracy.Subsequently,For the grid-following MMC which is commonly used in engineering,an AC/DC interconnection system model of the MMC-HVDC connected to an AC grid via the phase-locked loop(PLL)is established,the practical static stability analysis method of the AC/DC system is developed,and the concepts and algorithms of critical short circuit ratio(SCR)and boundary short circuit ratio are proposed.Thus,the transmission capacity of MMC-HVDC connected to a weak AC grid is quantitatively evaluated.Finally,the ability of MMC-HVDC connected to a weak AC grid to withstand large disturbances is studied.By establishing a large-signal model of the AC/DC system of the MMC connected to an AC grid,based on the stability region theory,the key factors affecting the stability region and its boundary are studied.The stability region estimation method of MMC connected to an AC grid based on the Lyapunov direct method is studied,and a new system strength evaluation index based on critical energy is proposed,which can not only reflect the steady-state strength information contained in the SCR but also reflects MMC dynamic characteristics.