Dead Zone Problems Of Two Differential Protections And Corresponding Solutions

The differential protection theories have come out for nearly a century and have played an important role in the development of relay protections. The main characteristics of differential protection are high reliability, high sensitivity and absolute selectivity. Thus,in the Urban Power Grid of Shanghai(UPGS),differential protections are widely applied as the main protections. However,the differential protection has the"dead zone"problem which may cause its misaction. To improve the performance of the differential protection is very important for the security and stability of UPGS. Focusing on above problem, this paper obtains three achievements.(1) Based on practical experiences, the paper summarize and analyze the applications of the differential protections in UPGS,including the longitudinal differential protection of transformers and the transverse differential protection of paralleled mutual-feeding dual lines.(2) According to the differential protection principle, the"dead zone"problems in the transformer tri-point-style differential protection in the inner-bridge-mode 110kV substations and the transverse differential protection of dual paralleled mutual-feeding lines are highlighted.(3) Several measurements are proposed to solve the problems, and the advantages and disadvantages are compared. The practical operational results show that these methods are effective to avoid the"dead zone"problems and improve the security and stability of UPGS.At the end of the dissertation, a systematical summary is given. Further work to be studied is pointed out.