| Small current grounding method is widely used in distribution network.Safety and reliability of distribution network directly determine users’ manufacture and life.Single phase fault,which occurs most frequently and is characterized by its low fault current,makes its detection,position and isolation very challenging to operators,especially in resonant grounding system.Among all protection schemes,active protection has a significant effect in resonant grounding system by consciously changing the operation state to obtain ideal mutation characteristic.Arc extinction,fault line selection and location can be realized simultaneously.Actually,the essence of single phase fault precisely results in structural variation of fault line,which makes line structure parameters a strong candidate for fault line selection and location.The development of national intelligent power grid and the applicat ion of advanced power technology,computer technology and measurement technology also create motivation for such schemes.On the basis of structure parameters,this paper draws inspiration from the fact that the asymmetry degree of three phase-to-earth parameters is obvious in fault line,while in healthy line it’s close to zero.Furthermore,a fault line selection method based on admittance asymmetry parameters is proposed.Theoretical analysis demonstrates this parameter remains immune to fault resistance,which can be integreted into other methods.To a great extent it can be a supplement to other methods which are vulnerable to fault resistance.Secondly,with reference to active protection,the mutation of admittance asymmetry parameter is studied in the process of switching the medium resistance.The value of the medium resistance is discussed briefly.A new line selection scheme is proposed and its effect and feasibility is analyzed.It is validated to be effective in resonant grounding system as well.Finally,based on the same active protection principle,the variation of zero-sequence current and admittance before and after switching medium resistance is studied.A parameter describing the variation amplitude of zero-sequence admittance is defined.The simulation model of 35 k V system is established in PSCAD software to verify the feasibility of the line selection and location scheme in the condition with different transition resistances and fault locations. |
