Research On Active Protection And Fault Location Of The Small Current Grounding System

Small current grounding system is widely used in 6kV-35kV voltage level for power distribution network. However, with the increasingly complex of power grid, when single-phase grounding fault occurs in a small current system, the arc and the damage to personal safety is getting more and more serious. Thus improving the reliability of the small current grounding system is very urgent, and the line selection and positioning of the small current grounding system are key problems.Firstly, the small current grounding system is briefly introduced in this paper. Then according to the research on the change of steady state and transient state, line selection methods are designed for different electric parameters. Based on the basic principle of active protection method, active protection method for single-phase grounding fault happens in small current grounding system is put forward. This method selects the fault line according to the difference of high frequency component amplitude of zero sequence current before and after the fault and polarity of zero sequence current after the bus initiatively grounds. Finally in order to prove the validity and effectiveness of the proposed method, simulation model is built in PSCAD/EMTDC with the overhead line using the Bergeron model. The output current of different lines is then decomposed by the wavelet transform.After selecting, the fault line through the line selection method stated above, it still needs to determine the fault location to further improve the reliability of protection action. In this thesis, the connection mode of system is changed by setting fast switch in the neutral position and the switch acts when positioning. The change of mode makes it convenient to use easier fault location method of large current grounding system, such an impedance method. When data collection is over, the fast switch is taken off. Finally, the practicability of this method is investigated with 14 nodes system model in PSCAD/EMTDC. At the same time, the influence of transition resistance, load fluctuation and fault distance on the accuracy of the proposed positioning method is studied.