Research On Assessment Of Commutation Failure Risk In Multi-Infeed HVDC System

Line-commutated converter based high voltage direct current(LCC-HVDC)technology has been widely used in long-distance and bulk power transmission projects because of its advantages such as non-synchronous connection,rapid power regulation and low loss.However,because the core component of LCC-HVDC adopts semi-controlled thyristor,the commutation voltage drop caused by AC fault may cause LCC-HVDC commutation failure(CF),power transmission interruption and other consequences.In Multi-Infeed HVDC(MIDC)system,CF may occur at the same time in multi LCC-HVDC,resulting in a large loss of power in the receiving end,which brings severe challenges to the safe and stable operation of the power grid.Therefore,if the risk degree of CF of each LCC-HVDC in MIDC system under AC fault is evaluated in advance,the fault severity and fault location of CF of each LCC-HVDC could be comprehensively analyzed to form tme risk area of commutation failure,which can provide theoretical and technical support for ensuring the safe and stable operation of MIDC system.First of all,in order to judge the influence of three-phase faults in AC bus and AC line on CF of MIDC system,the voltage interaction factors(VIF)are derived by taking different positions of AC bus and line as generalized nodes,and the calculation method of VIF suitable for different fault locations of AC bus and line is given by modifying power flow matrix.It which provides a theoretical support for judging commutation failure risk later.Secondly,under the transient condition before CF occurs after three-phase fault,the approximate analytical method of dynamic characteristics of DC current is obtained by decomposing the components of transient DC current during the fault.Based on this,the critical voltage calculation method of CF is improved,and the boundary conditions of CF of MIDC system are determined.Finally,a judgement method of CF risk considering the dynamic characteristics of DC current is proposed.It effectively improves the judgement accuracy of CF risk when three-phase failure occurs in different locations of MIDC system,and reduces the risk of misjudgment.Finally,to dealing with the needs of quantitative evaluation of CF risk degree of MIDC system and rapid identification of risk area,by analyzing the key influencing factors of CF of MIDC system,a quantitative evaluation index of CF risk degree is proposed by using normalization idea.The critical value of CF risk does not change with the change of operation state of AC/DC system and DC system parameters,so it has good versatility;at the same time,the positive and negative of this index can be used to judge whether CF would occur,and the amplitude can reflect the voltage drop/lift amplitude from the current state to the occurrence/absence of CF.Based on the quantitative evaluation index,a fast identification method of CF risk area under different fault severity is proposed.The simulation results show that the proposed index can be used to quantitatively evaluate the CF risk of MIDC system,and can accurately identify the CFRA of MIDC system under different fault severity.