Stability Analysis And Control Parameter Optimization For MMC-based Grid-connected System

In recent years,modular multilevel converters(MMC)have received extensive attention from researchers due to their high output levels,low switching losses and flexibility in modular design.At present,a number of MMC-based high-voltage flexible DC transmission demonstration projects have been established all over the world.So far MMC has become the mainstream converter topology in this field.However,MMC has sub-synchronous oscillations in the pratical applications such as wind power grid connection and grid interconnection,which seriously affects the output power quality of the converter and even causes the system to shut down.In order to ensure the stable operation of the MMC-based gridconnected system,it is urgent to establish a set of high-precision mathematical models for the MMC-based grid-connected system as the theoretical basis for the system analysis and control paprameters optimization design.At present,there are many documents on MMC modeling and grid-connected characteristic analysis,among which impedance modeling method is the mainstream analysis method.Since capacitors are adopted in each cascaded sub-module of the MMC bridge arm as energy storage elements,there are complex multiharmonic coupling effects inside the bridge arms as well as multiple state variables such as bridge arm currents and capacitor voltages inside the MMC.The charateristics such as multiple frequency,nonlinearity,periodic time-varying,multi-output and multi-output systems of the MMC have brought severe challenges to the establishment of MMC high-precision AC-side port impedance models.The traditional small-signal impedance modeling method is not suitable for multiple-input and multiple-output systems.And it is assumed that the output voltage of the bridge arm the MMC has a constant DC steady-state operating point during the modeling process in the traditional method,ignoring the multi-harmonic coupling effect inside the MMC and resulting in poor modeling accuracy.In recent years,with the introduction of the Harmonic State Space(HSS)theory,the multi-harmonic coupling characteristics of the MMC can be accurately and effectively analyzed.So far researchers have established the high-precision impedance models for the MMC itself under openloop control and AC-side voltage closed-loop control.However,when the converter is connected to the grid with the current closed-loop control,a phase-locked link usually adopted to track the of the grid-side voltage,which has the mirror frequency coupling effect when the small signal analysis is performed.For the MMCbased grid-connected system,the harmonics generated by the phase-locked control link will affect each other due to the internal multi-harmonic coupling effect of the MMC,and ultimately affect the port impedance characteristics of the MMC;further,when the MMC is connected to the weak AC grid,the voltage drop across the grid-side impedance will also affect the port impedance characteristics of the MMC through the PLL.Therefore,finding the way to comprehensively consider the factors above to establish the highprecision MMC port impedance model and guide the optimization for control parameters design based on the impedance-based stability analysis method is very important to improve the stability of the MMC-based grid-connected system.This paper analyzes the stability of the MMC-based grid-connected system,optimizes the control parameters and ensures the stable and reliable operation of the MMC-based grid-connected system.This article firstly adopts the traditional small-signal impedance modeling method to establish the AC-side port sequence impedance model of the MMC,and finds that the accuracy of the model built with the traditional method is insufficient.Secondly,under the assumption that the PLL is ideal,the small signal impedance modeling method based on HSS theory is adopted to establish the port impedance model for the MMC-based grid-connected system with current closed-loop control.And the correctness of the HSS based impedance modeling method is verified by simulation.Then,when MMC is connected to the strong AC grid,the theoretical analysis of the double-frequency image coupling effect produced by the phase-locked link is comducted.The coupling control model is further added to the existing HSS model matrix and the port impedance model of the MMC is established.The theoretical model is verified by simulation and experiment,and the influence of control parameters on the impedance characteristics of the MMC port is also analyzed.What’s more,the multi-harmonic control signals generated by the voltage drop across the grid-side impedance through the phase-locking link when the MMC is connected to the weak AC grid are analyzed.These control signals are further added to the existing HSS model matrix for impedance modeling of the MMC.The theoretical model is verified by simulation and experiment,and the influence of control parameters on the impedance characteristics of the MMC port is analyzed in this condition.Finally,based on the Nyquist criterion,the stability of the MMC-based grid-connected system is analyzed,and the stable and reliable operation of the system is improved by optimizing control parameters.Correctness of the stability analysis and optimization theory is verified by simulation and experiment.