Small Current Grounding System Single-phase Selection Line And Positioning Technology

Power systems with unearthed or a compensated neutral are popular in China, which can improve the reliability of power supply. When single-phase to earth fault occurs, it doesn' t form a short circuit. The fault component is small, phase-to-phase voltage keeps symmetrical, so it needn' t trip immediately and keep operating for one or two hours. Long-time earthing operation, however, will destroy the systematic safe operation as the result of two-point, more seriously, multipoint earthing as well as the systematic over voltage caused by arc light. Thus how to detect the fault line quickly and accurately is an important subject in power system protection, which has not been solved completely.At first, this paper presents the characteristics of single phase- to -earth fault in small current neutral grounding system and reviews the developing history of the relay about the selection of single phase- to -earth fault line in small current neutral grounding system all over the world, as well as traits and restrictions of various selecting criteria are analyzed in detail. However , theoretical analysis and operating experiences indicate that each criterion has limits and shortages, no one can select the fault line correctly in all kinds of conditions.Then, it introduces a new type of principle about the selection of fault line and the location of fault point in this system —the special signal injection method. When single-phase to earth fault occurs, a special signal is injected to the fault line through the PT by the mainframe of the equipment. Then the detector of circuit signal will incept the special circuit signal and find the fault line and the location of fault point.At last, protects and carries on the introduction with the principle to TY-01 type, TY-02 type. The equipment has some extrusive characters such as being universal , simple connection and location.