| Networks with ungrounded neutral.are widely used in power distribution system with 3~6kv voltage in China, and the single phase to earth fault happens most frequently in distribution, thus how to detect the fault line quickly and accurately is an important subject in electric power system protective relaying.Firstly, this dissertation reviews the development history of the methods of fault line selection of single phase to earth fault in networks with ungrounded neutral in the world, and summarizes all the existing theories on the fault line selection systematically. Then, considering the practical network architecture of power utility of Chongqing downtown.This dissertation studies fault line selection for single phase to earth fault based on zero-sequence transient power direction in power network with ungrounde . A novel method based on zero-sequence transient direction are presented, which are used as the criteria to detect single phase to earth fault. New principle for feeder grounding fault protection based on zero-sequence transient direction improves the performance of traditional grounding fault relays in difficult implementation and poor reliability.Simulation results, which are obtained under high-speed sampling circuit and high-performance DSP operation controller, show that this protection scheme is correct, effective and robust.The following conclusion can be achieved:(1) No matter what kind of compensation type, such as over or under compensation, zero-sequence transient current of fault line have a distinguishing change with that of non-fault line at failure time.(2) At the time of failure, no matter what kind of start state of voltage phase, the zero-sequence transient current has the characteristic of inverse phase.(3) From the view of simulation results, it should be worth to pay more attention that the compensation type has something to do with failure location.
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