Fault identification method during oscillating based on negative sequence directional element

The conventional percentage differential protection has been widely used in relay protection, but its operating characteristic will be affected badly when internal fault occurrs during power system oscillating, the differential protection will delay dozens or even hundreds of milliseconds action. Regarding the percentage curve as the research starting point, this paper investigate the relationship between differential current and restraint current when fault occurred during power system oscillating, and points out that when internal fault occured during power system oscillating, the percentage restrained characteristic is the cause of delayed action of differential protection. In order to solve this problem, the differential protection studied in this paper is no longer using the percentage restrained characteristic to distinguish internal or external fault, but using the negative sequence directional element to distinguish internal or external fault. Thinking common double lines as an example, through the study on the characteristics of negative sequence current distribution in double lines, this paper put forward a new kind of fault identification method with using both sides of negative sequence current direction to discriminate whether an internal or external fault occurred, relative to conventional negative sequence power direction element, the discriminant performance is improved greatly, with good district internal and external fault identification, and is not affected by the amount of voltage. Through the synthesis of the negative sequence current, the method is effectively extended to various types of differential protection. Finally, through the modeling and kinds of faults simulating, especially oscillating testing when fault occurring at internal and external area. The simulation results show that the method of fault zone identification is good, the negative sequence directional element can significantly improve the action speed of the differential prot- ction when internal fault occurred during power system oscillating, and at same time the differential protection will not act reliably during external fault.