Assessment Methods Of LCC-HVDC System Impact On Short-term Voltage Stability Of The Receiving-end AC System

Being technically and economically advantageous,LCC-HVDC is expected to remain prevalent for bulk power delivery through long distance transmission and asynchronous interconnection in the foreseeable future.When the commutating voltage of LCC-HVDC converters dips due to an AC system fault,a commutation failure will occur at the inverter of LCC-HVDC,which gives rise to transients in the active and reactive power of a LCC-HVDC system,and thus threatens the stability of the receiving-end AC system.With the increasing scale of HVDC,the short-term voltage stability issue,which is caused by AC fault occuring in the receiving-end AC system with multi-infeed HVDC,becomes more and more serious,and thus poses a new challenge to the secure and stable operation of AC/DC system.To solve the problems,this paper devotes to assessment of HVDC commutation failure,assessment of HVDC impact on short-term voltage stability of the receiving-end AC system,assessment of short-term voltage stability of the receiving-end AC system with multi-infeed HVDC systems.The work is summarized as following:(1)An AC/DC system decoupling based assessment method for HVDC commutation failure is proposed.Because HVDC system has no contribution to the short-circuit current and the change in DC current is very small before commutation failure,it is reasonable to apply the AC/DC system decoupling method to assess HVDC commutation failure caused by the AC fault.As the present commutation failure criterion,which only considers the converter commutation voltage magnitude drop,is insufficient,this paper provides a novle criterion taking into account both the magnitude drop and phase shift of the converter commutation voltage.Comparing with the PSCAD/EMTDC simulation results,the accuracy and effectiveness of the proposed criterion are verified.Based on this,a commutation failure assessment method is proposed and is applied in China Southern Power Grid.(2)The mechanism of HVDC impact on short-term voltage stability is revealed.After analyzing HVDC reactive power transients under several common large disturbances,the most serious HVDC reactive power response scenario for the short-term voltage stability is revealed.Then the inherent reactive power transients of HVDC system and impact of HVDC control on the transients are analyzed.Furthermore,the source-load transformation in HVDC reactive power and the most important factor for HVDC to influence short-term voltage stability are revealed.(3)An analytical method is proposed to assess the impact of LCC-HVDC system on STVS of the receiving-end AC system.Compared with the conventional indices based on the concept of SCR,the proposed index takes into account the effects of HVDC reactive power transients,IMs,transmission grid topology and voltage supporting strength of the AC system.A simplified AC/DC power system and a real case in China Southern Power Grid are utilized to investigate the performance of the proposed index.Time-domain simulation results have fully demonstrated the satisfactory performance and superiority of the proposed index over SCR related indices for assessing the impact of LCC-HVDC system on STVS.Based on the positive results,the practical application of the proposed index is promising.(4)Based on critical unstable slip,a method for assessment of short-term voltage stability of the receiving-end AC system with multi-infeed HVDC system is presented.Combined with the physical essentials of short-term voltage unstability,a novel assessment index of short-term voltage stability of the receiving-end AC system is derived,which considers the effects of HVDC reactive power transients,IMs,transmission grid topology and voltage supporting strength of the AC system.In China Southern Power Grid,the short-term voltage stability of the onverter commutation bus is assessed,and the assessment results are very consistent with those according to critical cleaning time obtained by simulation,thus verifying the satisfactory performance of the proposed assessment method.This paper is supported by the National High Technology Research and Development Program(“863” Program)of China(2012AA050209)and the National Science Foundation of China(51377059 and 51407079).Some of the research results have been applied in China Southern Power Grid,and the economic benefits and technical advantages verify the correctness and feasibility of the work.