Research On Transient Protection Of Transmission Lines In High Voltage AC/DC Hybrid System

To achieve the optimal allocation of energy and load,and consume as much clean energy as possible,China has built the world's largest high-voltage AC/DC hybrid system with the highest voltage level.And ensuring its safe operation has become a major and urgent national demand.The probability of transmission line failure accounts for a relatively high proportion of the hybrid system,especially the AC line fault on the inverter side can easily cause commutation failure of the LCC inverter station,which makes the hybrid system with more complicated fault characteristics,and it directly impacts the protection configuration of AC and DC transmission lines.Therefore,studying the high-reliability and ultra-high-speed protection methods for transmission lines,based on the characteristics of hybrid system,will have important theoretical and practical significance for reducing the hidden dangers of the hybrid system.First,this paper analyzes the mechanism and characteristics of commutation failure through the transient model of the typical HVAC/DC hybrid system,and draws the response requirements for commutation failure from the perspective of relay protection,that is,constructing the ultra-high-speed protection method.Also,the demand for transient signal extraction of the hybrid system is analyzed,and the deficiencies of most existing signal processing methods in terms of time-frequency resolution and weak amplitude signal extraction are pointed out.And then this paper proposes the application of synchrosqueezing S transform in transient signal extraction of hybrid systems.Secondly,for AC transmission lines near the LCC-HVDC inverter side,the frequency characteristics of the AC bus at the inverter side are analyzed,and the unsuitability of the protection principle that only depends on the traveling voltage is pointed out.Furthermore,the propagation mechanism of the transient components of the fault current is analyzed,and a pilot directional protection method based on the forward and reverse fault current transient energy ratio is formed.Thirdly,the above protection idea based on transient energy ratio is extended to the DC transmission line protection,and a fault pole selection method based on positive and negative voltage transient energy ratio is proposed.Besides,for HVDC transmission lines,the composition and transmission characteristics of the line boundary are analyzed.The ratio of the tuning voltage to the low-frequency voltage is used to form the non-unit transient based protection algorithm,and the ratio of positive and negative pole voltage micro-increments is used for fault pole selection.Finally,extensive simulations of the PSCAD model based on Yunnan-Guangzhou±800kV HVAC/DC hybrid system and some field data were used to verify the proposed methods.The results show that the above protection methods meet the action requirements of ultra-high-speed protection,and are not affected by commutation failure,and with high reliability.The application of synchrosqueezing S-transform makes the realization of the protection algorithm more convenient and efficient.The two HVDC protection methods each has their strengths and can be considered for comprehensive use.