Research On Transient Models Of MMC And Fault Behaviors Of HVDC Grid Under DC-Side Pole-to-Ground Fault

The modular multilevel converter(MMC)based high-voltage direct current(HVDC)grid is an important developing direction of future power system.However,during dc short-circuit faults,different kinds of transient stresses will occur in the HVDC grid,which may seriously damage the system.Thus,dc fault protection is an important technology for HVDC grid,and comprehensive analysis of the transient behaviors is the basic way to realize the dc fault protection.Both MMC with dc fault handling capacity and dc circuit breaker(DCCB)are typtical apparatuses in HVDC grid.After dc fault happens,the MMC will go through the fault detecting,ride through and recovery stages,while the DCCB will go through the fault detecting,dc line deionization and power flow recovery stages.The control structures of both apparatuses will be switched sequentially with the above stages.Considering that the dc apparatus has the sequential switching control structure,and it also contains many non-liner links,the fault transient behavior of the dc grid will be discontinuous and nonlinear.These features bring challenges for the dc fault analysis.The existing study,at the macroscopic view,is lack of systematic understanding for the dc fault transient issues.At the specific research level,focuse on the discontinuous challenge,the modelling of MMC generally varies with the dc fault stages,which results in the deficiency in the universality.Aiming at the nonlinear challenge,the analysis of fault behaviors usually adopts the numerical simulation method,which leads to relatively weak understanding for the fault mechanism.Concentrate on the above deficiencies,this dissertation firstly forms an overall understanding for the dc fault transient issues.Then,since the pole-to-ground(PTG)fault is the most common dc fault.Meanwhile,extreme fault stresses and power ouage problem respectively exist in the fault detecting and riding through stages,which need to be urgently addressed.This dissertation,focusing on these two stages,establishes transient models for MMC under a unfied concept of “exciting-responsing”.Finally,based on the established transient models,the fault behaviors of dc grid are analytically described,which reveals the fault mechanism underlying the fault behavior and offers a preliminary support for the dc fault proection.Details are as follows:1.The overall understanding for the dc fault transient issues in HVDC grid.Take the MMCs with dc fault handling capacity and hybrid dc circuit breaker(DCCB)as examples,the universality of the sequential switching control structure in dc apparatus is elaborated.From the perspective of exciting-responsing relation between dc apparatus and ac/dc network,the physical processes of HVDC grid during different fault stages are analyzed.Based on the mathematical description of HVDC grid at different fault stages,the nonlinear chanllenge of dc fault analysis is identified.Finally,a preliminary framework and methodology is presented for the study of dc fault transient issues in HVDC grid.2.Transient modelling of MMC under PTG fault detecting and fault riding through(FRT)stages.During the detecting stage,a fault detecting model is established for MMC under a concept of “exciting-responsing”.Compared with the average RLC model,the established model not only improves the calculation accuracy but also deepens the understanding of fault mechanism from the SM swiching level.During FRT stage,the input-output relation of MMC is summarized from the same concept of “exciting-responsing”.With numerical calculation and Hilbert-Huang transformation based analysis,it is found that the path from dc current excitation to dc internal voltage response is weak nonlinear.Hence,an FRT model is established for MMC.Both the transient models of MMC are verified by PSCAD/EMTDC simulation.3.PTG fault behavior analysis for HVDC grid under the detecting stage.Focusing on the imbalance and coupling issues brought by the PTG fault,a common-and differential-mode(CDM)transformation based analysis method is proposed.By transforming the HVDC grid into balanced and decoupled CDM networks,analytical expressions of dc fault current and transient voltage are approximately derived.The proposed analysis method is verified by PSCAD/EMTDC simulation.For the setting of equipment parameters that is based on the maximum transient stress,the obtained analytical expression can provide sufficient theoretical guidance.4.PTG fault behavior analysis for HVDC grid under the FRT stage.Based on the FRT model of MMC,analytical expressions of dc fault currents are approximately derived for the scenarios of MMC based simple system and arbitrary dc grid.With the analytical expressions,the impact factors and influence regularities of dc fault current are revealed.Further,based on the MMC's dc and circulating current loops,the influence regularities are fully explained.The correctness of analytical expressions and influence regularities are verified by PSCAD/EMTDC simulation.For the setting of dc grid FRT duration,the obtained analytical expression can provide effective support.