Research On Evaluation And Suppression Methods Of Commutation Failure Risk In LCC-HVDC Systems Considering Probability Characteristics

To promote the large-scale development and efficient utilization of clean energy,and achieve a optimization of resource allocation,the Line Commutated Converter-based High Voltage Direct Current(LCC-HVDC)technology has been widely used in cross-regional power exchange projects such as “West-East Power Transmission” and “North-South Power Transmission” in China due to its advantages of large transmission capacity,low economic cost,and asynchronous interconnection of regional AC power grids.With the continuous expansion of the AC/DC system,the complexity and vulnerability of the AC/DC hybrid power grid are increasing,and the features of “DC systems are stronger than AC systems and multi-HVDC systems feed-in” will be more and more apparent.This may lead to more frequent and complex commutation failure,further expanding the scope of their impact and seriously threatening the safe and stable operation of the power grid.In addition,in actual power grids,asymmetric faults occur more frequently than symmetrical faults,and commutation failure caused by asymmetric faults have significant probability characteristics.Therefore,considering the commutation failure probability characteristics,it is of important research value to quickly and accurately evaluate and prevent the commutation failure risk in the LCC-HVDC system.The research work of this paper is supported by the Joint Fund Program of the National Nature Science Fund Program of China,“Research on Commutation Failure Evaluation Theory and Flexible Control Technology of Multi-infeed DC System(Project number: U1766213)” and the National Science Foundation of China,“System Attributes of Fault and HVDC Protection Strategy Optimization Based on Modal Switching Mechanism in AC/DC Hybrid(Project number: 51677073)”.This paper focuses on the evaluation and suppression methods of commutation failure risk in LCC-HVDC systems.And evaluation indicators that can reflect the probability characteristics of commutation failure,identification methods of commutation failure risk areas,and schemes to prevent commutation failure risk are proposed.It provides theoretical guidance for the planning,safe and stable operation of large-scale AC/DC systems.The main research work is as follows:1)To accurately and comprehensively evaluate the commutation failure level of LCC-HVDC systems,an AC/DC decoupling method is used to decouple the multiple influencing factors of evaluation indicators of commutation failure.Based on an equivalent model,a commutation failure evaluation method is proposed considering the probability characteristics of commutation failure.This method can rapidly calculate evaluation indicators of commutation failure for four types of faults.The results show that evaluation indicators of commutation failure are directly proportional to ESCR and that phase-to-phase fault is the most serious fault type that affects commutation failure,providing a theoretical basis for selecting relevant fault sets in LCC-HVDC systems.The effectiveness of the commutation failure evaluation method is verified by electromagnetic transient simulation results.2)To accurately identify the commutation failure risk area caused by asymmetric faults in a single-infeed LCC-HVDC system,a hierarchical identification method for commutation failure risk areas is proposed based on fault position and AC-DC decoupling methods.The relationship between fault position and extinction angle is established.Based on the criterion of minimum extinction angle,the method can quickly identify three commutation failure risk areas of non-,possible-and inevitable-occurrence,which can provide theoretical guidance for the formulation of actual power grid safety and stable operation strategies.The proposed method’s effectiveness is demonstrated by the automated identification technology of commutation failure risk areas developed using PSCAD/EMTDC.3)To accurately and efficiently identify the concurrent commutation failure risk areas in multi-infeed LCC-HVDC systems,a method for identifying the concurrent commutation failure risk areas is proposed comparing the AC/DC voltage asymmetry factor and the critical AC/DC voltage asymmetry factor.This method can quickly divide the concurrent commutation failure risk areas into non-,possible-,and inevitable-concurrent commutation failure risk areas and can quickly calculate the probability of concurrent commutation failure.The accuracy and effectiveness of the proposed identification method are verified by comparison with electromagnetic transient simulation results.This method can be widely used in the planning,security,and stability prediction of AC/DC hybrid power systems.4)To reasonably configure reactive power compensation devices to reduce the risk of commutation failure in multi-infeed LCC-HVDC systems,a commutation failure risk index is defined.This index considers the type,position,and occurrence time of short-circuit faults and the influence of commutation failure on power fluctuation of the AC power grid.A synchronous condenser optimization configuration model is established to reduce the commutation failure risk index and consider economic factors.Using genetic algorithms,the synchronous condenser optimal allocation,sizing,and number are determined.This method is helpful for promoting the configuration of the synchronous condenser in multi-infeed LCC-HVDC systems with large-scale AC power grids.In summary,this paper provides a comprehensive study of the evaluation and suppression methods of commutation failure risk in LCC-HVDC systems considering the probability characteristics of commutation failure.Some research results have already provided theoretical guidance for the practical work of power grid planning and maintenance departments and achieved good technical and economic benefits,which validate the correctness and effectiveness of the work of this paper.