| Modular multilevel converter-based high-voltage direct current(MMC-HVDC)transmission technology has been widely used under the background of China’s "dual carbon" goal due to its advantages of flexible control,easy expansion,no commutation failure,and the ability to supply power to passive networks.However,the nonlinear characteristics of power electronics of the MMC-HVDC transmission system also make profound changes in the resonance characteristics of the power system,the resonance frequency band is greatly expanded,and the phenomenon of medium and high frequency resonance appears,thus the traditional research method will no longer be fully applicable.Therefore,this paper establishes the MMC impedance model,studies the medium-and high-frequency resonance mechanism of MMC-HVDC transmission system,clarifies the resonance risk and proposes improvement measures,which provides theoretical and technical support for the safe and stable operation of MMC-HVDC transmission system,and has important practical significance.The main work is as follows:(1)An MMC full-band impedance model considering the MMC closed-loop control system and internal and external coupling characteristics is proposed,which solves the problem of impedance calculation error caused by incomplete modeling.Firstly,through the analysis of the electrical circuit and MMC control system on the AC and DC side,the frequency coupling characteristics of MMC and the electrical coupling effect of MMC and AC/DC system are studied.Then,by analyzing the phase-locked loop control,outer loop control,positive-and negative-sequence inner loop control and circulation current suppression control in turn,the input-output relationships of MMC control are established.Thus,the influence of each control link on the MMC impedance is revealed.The equivalent impedance models of MMC AC-side and DC-side system are established respectively in combination with the established MMC AC-and DC-side circuit models,which can accurately calculate the MMC equivalent impedance in the whole frequency band.Finally,the correctness of the impedance model is verified by simulation,and the influence of important factors on impedance is analyzed.(2)A reduced-order equivalent impedance models of medium and high frequency bands in the AC and DC sides of MMC based on normalized sensitivity index are proposed,which solves the problem of unreasonable simplification for high-frequency influencing factors and lack of quantitative analysis for influencing factors in the current research.Firstly,to preliminarily simplify the model,the factors that mainly affect the MMC impedance of lower frequency are ignored,and the MMC equivalent models of AC and DC sides for medium and high frequencies are derived.Secondly,a normalized sensitivity index is proposed,and the influence of key factors such as control parameters,control link delay and arm parameters on equivalent impedances of MMC AC and DC sides is quantified,and then the influencing factors are graded according to the influence reflected by the quantified index,and the reduced-order impedance models are proposed for different application scenarios.Finally,the rationality of the classification and the correctness and validity of the reduced-order impedance model are verified by simulation.(3)The risk analysis method of medium-and high-frequency resonance in the MMC-HVDC transmission system considering the influence of MMC parameters and system impedance is proposed,which solves the problem of lack of comprehensive research on resonance risk caused by MMC and AC/DC system parameters.Firstly,by considering the main range of MMC parameters in practical engineering,the phase margins of MMC AC-and DC-side systems under the comprehensive influence of system short-circuit ratio,MMC control and circuit parameters are calculated,and the resonance risks caused by AC and DC systems and MMC are explored.Then,based on Nyquist criterion,the system impedance range and the feasible domains of MMC parameters that need to be met to maintain the stability of the AC-and DC-side systems are analyzed,which provides reference for the selection of system and MMC parameters.Finally,Finally,by establishing a simulation model,the feasible domain verification is carried out.(4)A middle-and high-frequency resonance suppression strategy based on multi-strategy suitable for multiple resonance types is proposed in AC-side system,which solves the lack of research on the effect comparison of resonance suppression strategies by analytical methods.Firstly,according to whether the system has a harmonic source,the resonance is divided into active resonance and passive resonance,and the generation mechanism and influencing factors of the two types of resonance are analyzed and summarized,which provides a more comprehensive view for the resonance analysis.Furthermore,based on the above mechanism analysis,resonance risk evaluation indexes are established to evaluate the suppression effects of different strategies,and the resonance risk indexes under the single suppression strategies and multiple suppression strategies are evaluated and compared,so as to propose comprehensive suppression strategies suitable for different scenarios.Finally,Finally,the correctness and effectiveness of the resonance analysis and the proposed suppression strategy are verified by calculation and simulation. |
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