AC/DC Power System Voltage Stability Analysis Based On Generalized Short Circuit Ratio

The increasing use of line commutated converter-based high voltage direct current(LCC-HVDC)brings technical and economic advantages for long-distance and high-capacity transmission.Particularly,in China,LCC-HVDC transmission projects have led to the configu-rations called multi-infeed LCC-HVDC(MIDC)systems jointly with ultra-high-voltage ac transmissions,which will bring the total installed capacity of HVDCs exceeding 700GW by 2020.On the other hand,when LCC-HVDCs are connected to weak ac system,it may cause potential stability issues such as temporary overvoltage,concurrent commutation failure and the maximum available power(MAP)that is less than the rated transmission power.The short circuit ratio(SCR)has been extensively used to assess the strength of single-infeed LCC-HVDC(SIDC)systems with exact boundaries.However,the straight-forward application of the concept of SCR for SIDC systems to MIDC systems has caused certain ambiguity since the impact of the interactions among HVDC inverters cannot be reflected at a referred inverter ac bus.And the current multi-infeed short circuit ratio(MISCR)literature falls short in assessing strength of MIDCs clearly and exactly as SCR.The main contributions of this dissertation are as follows.1.The relationship between the SCR and static voltage stability is analyzed in this paper for the MIDCs.A new concept named generalized short circuit ratio(gSCR)has been proposed to exactly depict the strength and voltage stability of AC-DC systems according to the eigenvalue decomposition method,which can overcome the unclear physical mechanism of the traditional MISCR and unify the concept of SCR in single and multi-infeed DC system physically and mathematically.The gSCR is general since it is defined under weaker assumptions.Furthermore,some properties on the gSCR are given,including the unique existence of the gSCR and that the gSCR is not less than the minimum MISCR,and it is shown that the MISCR is a special case of gSCR under the condition of strong additional assumptions.Numerical simulations illustrate that for a MIDC,the strength of the AC system can be described better by the defined gSCR than the MISCR.2.The influence of the operation points of HVDCs to gSCR is analyzed in this paper for the MIDCs.A new concept named generalized operation short circuit ratio(gOSCR)has been proposed considering the operation conditions of HVDCs.It can be used to describe the voltage stability of MIDCs better.The gOSCR can be easily used to estimate that whether the MIDC can operate stably at any operation point.And,not only the static voltage margin,the fastest instability direction but also the power margin with a specific way to the operation point can be given by gOSCR.Theoretical analysis and numerical simulations illustrate that,for a MIDC in which the HVDCs are operating at different operation points,the characteristics of MIDC can be assessed better by gOSCR.3.Under several assumptions,gSCR can be used to measure the connected AC strength from the voltage stability point of view.This dissertation discussed the applicability of gSCR under the weakened assumptions.Based on power bifurcation analysis,we consider two HVDC control modes:constant current constant extinction angle(CC-CEA),constant power constant extinction angle(CP-CEA).It can be found that,the derivation of gSCR is same under CP-CEA and CC-CEA,and the critical generalized short circuit ratio(CgSCR)and boundary generalized short circuit ratio(BgSCR)are unified on the two traditional HVDC control modes.Meanwhile,based on gSCR,line impedance angle and tie-line power have been considered.Numerical simulations illustrate that for a MIDC on different HVDC control modes,the strength of the AC system can be described better by the defined gSCR,and the critical and boundary gSCR decrease after considering line impedance angle and tie-line power,but have little change in value.The influence of reactive power compensation capacitor to gSCR is analyzed in this paper for the MIDCs.A new concept named generalized effective short circuit ratio(gESCR)has been proposed based on effective short circuit ratio(ESCR),which can overcome the drawback that the reactive power absorbed by HVDC need to be assumed compensated locally by the compensation capacitor.Meanwhile,based on gESCR,the concept named bidimensional generalized effective short circuit ratio(DgESCR)is proposed for assessing the strength of MIDC,where the line resistance should be considered by impedance angle.Theoretical analysis and numerical simulations illustrate that,for a MIDC in which the reactive power absorbed by HVDC is not compensated locally by the compensation capacitor or the line resistance cannot be neglected,the strength of MIDC can be assessed better by gESCR and DgESCR than gSCR.4.This dissertation discusses the voltage stability of the MIDCs which contain asynchronous motor loads based on gSCR.The Jacobian matrix becomes singular when the voltage stability boundary reaches,and based on this,the mathematical expressions have been derived,which declares the relationship between gSCR and asynchronous motor loads.It can be found that,under appropriate control strategies,the asynchronous motor loads can improve the MIDCs' voltage stability.Several simulations have been conducted based on the developed single-infeed HVDC system benchmark,to analyzed the influence of the asynchronous motor loads and their control strategies to the gSCR.