This paper introduces the basic principle of the synchronization of sample time,the synchronization of sample number and the channel time delay computation used in pilot current differential protection using the digital channel,compares the difference of protection performance of adopting different synchronization methods,and deeply researches the influence of channel time delay change on differential protection brought by the switch process of double longitudinal channel in different switching modes.Through the study,this paper makes it clear that the synchronization of pilot current differential protection is incorrect in a period after the channel switching or in the switching process,and the countermeasure of protections must be considered,otherwise,false tripping would occur.

分析了一起具有代表性的失灵保护误动事故,其误动原因是由于光耦开入回路设计的不完善,在发生直流正极接地时,由于电缆存在较大的分布电容,造成光耦开入回路误导通而失灵保护动作出口.研究制定出华北电网的反事故措施,对相关元件的动作参数提出了明确要求.这些措施已经在华北电网得到普遍应用,其合理性和有效性已经通过现场试验和该次事故得到了证明.

This article introduces the type and principle of secondary circuit interference of DC system in substation and power plant.Through field test and laboratory simulation,some serious interference cases,such as DC earthing and AC/DC hybrid,are analyzed in detail and preventive measures are proposed.

The effects of mutual inductance of parallel transmission lines on longitudinal zero-sequence protection was introduced.Through theoretical analysis,simulation in laboratory,and accident analysis of power grid,the operation of parallel transmission lines with high current-high magnet and low current-strong magnet was analyzed in this paper.The preventive measures were also put forward.

According to the characteristics of autotransformer, the fault current of autotransformer is analyzed when the ground fault happens in and outside of the protected zone. The advantages and disadvantages of three kinds of zero-sequence current differential protections of autotransformer are discussed and some suggestions are given. This paper has something important to improve the application and sensitivity of zero-sequence current differential protection of autotransformer.