Research On The Fault Analysis And Protection Method Of Distribution Networks With DG

The expansion of the power grid scale makes the traditional centralized power supply system can not fully meet the improvement of user’s requirements for the power quality as well as its security and reliability.With the increasingly serious environmental pollution and energy shortage,distributed generation has been widely used in distribution network in recent years because of its high efficiency,flexibility and convenience.The access of DG has changed the structure and the load flow distribution of the distribution network,and with the increasing access capacity of DG,the traditional protection of the distribution network has been unable to meet the requirement of power supply reliability.The new protection method for DG access has become one of the main research directions.This thesis analyzed the fault current output characteristics of rotating DG and inverter interfaced DG,and then analyzed the effect of different fault factors on its output fault current characteristics.On this basis,the effects of DG access on the short circuit current and traditional protection is analyzed.That provide basis for finding new protection method of distribution network with large capacity DG.The phase difference relationship between the current variation and the load current(Characteristic phase difference)of protections at different positions is analyzed when distribution network with rotating DG or inverter interfaced DG fault.Obtained that the absolute characteristic phase difference of the forward current fault line protection is less than 90 degrees,and the absolute characteristic phase difference of the reverse current fault line protection is greater than 90 degrees.The absolute characteristic phase difference of the upstream power forward current line and the downstream reverse current line protection of the fault line is less than 90 degrees,and the absolute characteristic phase difference of the upstream reverse power line and the downstream forward current line protection is greater than 90 degrees.According to the characteristic phase difference in fault line and the downstream non fault line protection and the flow direction,a directional blocking protection method is constructed.The local directional blocking protection can clear the fault line accurately under various fault conditions,but can not recognize the power flow fluctuation.Analyze the current variation phase angle characteristic when large transition resistance(geater than 10Ω)fault and power flow fluctuation occurs.Found that under the same current variation level,the phase angle of the current variation is very small when the short-circuit fault occurs,and the phase angle of the current variation is larger when the power flow fluctuated.The wide area protection method is constructed by using the characteristic phase angle difference,the load current direction and the current variation phase angle in feeder head.Which has simple protection configuration and stronger fault tolerance,can correctly identify the faults and the power flow fluctuation.The distribution network model with DG is built in PSCAD to verify the effectiveness and applicability of the proposed directional blocking protection and wide area protection algorithm.