Research On Evaluation Methods Of Voltage Stability And Commutation Failure In Multi-infeed DC Systems

Situations are arising where multiple line commutated converter based high voltage direct current(LCC-HVDC)links feed into a same receiving end AC grid.This network configuration is called the multi-infeed DC(MIDC)system.Complicated AC/DC and DC/DC interactions in MIDC systems make issues of voltage stability and commutation failure particularly prominent,which then pose a great threat to the safe and stable operation of the entire AC/DC system.To alleviate the issues of voltage stability and commutation failure in MIDC systems,the integration of high-temperature superconducting fault current limiter(HTS-FCL)and voltage source converter based HVDC(VSC-HVDC)into MIDC systems has become a development trend.The accurate evaluation of voltage stability and commutation failure in MIDC systems,and the analysis of the impact of HTS-FCL and VSC-HVDC on commutation failure have important theoretical and engineering significances.This thesis focuses on “evaluation methods of voltage stability and commutation failure in multi-infeed DC systems”.The main research works are as follows:Firstly,the evaluation index for static voltage stability analysis and fast evaluation method of commutation failure in single-infeed LCC-HVDC systems are proposed.Based on the analysis of static voltage stability mechanism of single-infeed LCC-HVDC systems,an integrated short-circuit ratio(ISCR)strength index is proposed,which can accurately evaluate the static voltage stability.The analytical relationship between the commutation failure immunity index and ISCR strength index is established.This enables a fast evaluation method of commutation failure in single-infeed LCC-HVDC systems to be proposed.Case studies validate the ISCR strength index and evaluation method of commutation failure.The proposed evaluation methods for static voltage stability and commutation failure in single-infeed LCC-HVDC systems lend a theoretical basis for subsequent studies on the evaluation methods for static voltage stability and commutation failure in MIDC systems.Secondly,the evaluation method of static voltage stability in MIDC systems is proposed using the developed multi-infeed ISCR(MISCR)strength index.Based on the power-flow analysis model of MIDC systems and multi-infeed interaction factor index,concept of equivalent coupling admittance is defined.The equivalent coupling admittance enables the assessment of the influence of other DC links in MIDC systems on the studied DC link quantitatively.Based on the analysis of static voltage stability mechanism of MIDC systems,the MISCR strength index is proposed using the developed single-port equivalent model of MIDC systems.Case studies validate the MISCR strength index.Then,the fast evaluation method of concurrent commutation failure in MIDC systems is proposed considering both the fundamental-wave and harmonic interactions between inverters.The mechanism of the influence of harmonics on commutation failure in LCC-HVDC is revealed.The influences of fundamental-wave and harmonic interactions between inverters in MIDC systems on concurrent commutation failure are analyzed.A harmonic multi-infeed interaction factor index is defined to evaluate the degree of the harmonic interaction between inverters in MIDC systems.A practical criterion is then developed for the identication of harmonic interaction induced concurrent commutation failure in MIDC systems.This enables a fast evaluation method of concurrent commutation failure in MIDC systems to be proposed considering both the fundamental-wave and harmonic interactions between inverters.Case studies validate the evaluation method.Moreover,the evaluation index for the impact of HTS-FCL on commutation failure in MIDC systems is proposed.The comprehensive evaluation method of commutation failure in MIDC systems is established by a combination of the evaluation methods of fundamental-wave and harmonic interactions induced concurrent commutation failure,and fault recovery characteristic.Three evaluation indices for HTS-FCL's impact on commutation failure in MIDC systems are then proposed to characterize the commutation failure suppression effect and improvement of fault recovery characteristic.Case studies validate the evaluation indices.Finally,the fast evaluation method for the impact of VSC-HVDC on local commutation failure in MIDC systems is proposed.A single-port equivalent model of MIDC systems including both VSC-HVDC and LCC-HVDC is established,which enables an equivalent voltage stability factor(EVSF)index referred at the LCC-HVDC commutating bus to be defined.The analytical calculation method of the EVSF index is proposed with the power-voltage and phase-voltage characteristics of VSC-HVDC and LCC-HVDC analyzed.The analytical relationship between the commutation failure immunity index and EVSF index is established.This enables a fast evaluation method of the impact of VSC-HVDC on local commutation failure in MIDC systems to be proposed.Case studies validate the evaluation method.