Single Line-to-ground Fault Analysis For A Low-resistance Grounding Distribution System With Distributed Generations

Distributed generations(DG)have been widely used around the word.The grounding of DG(which is determined by both the winding connection of interfacing transformer and the neutral grounding of DG)will have an great effect on the single line-to-ground fault(SLG)currents.DG can adopt LRNG,direct grounding or isolated neutral grounding in a low-resistance neutral grounding(LRNG)distribution network.Different grounding type of IIDG will differently affect the Thevenin equivalent zero-sequence impedance at the fault point and change the SLG current characteristics,then lead to a series of problems for the existing ground relays in an existing distribution networks.Firstly,a fault analysis approach for a LRNG distribution network with a rotary DG or an inverter-interfaced DG(IIDG)is proposed.Based on the proposed approach,the point-of-fault ground-fault current,positive-,negative-and zero-sequence currents in lines,phase fault currents and their relationships are analyzed respectively when DG adopts various neutral treatments(isolated,low resistance and direct).Then there is a further analysis to fault point current,the positive-,negative-and zero-sequence current and the three-phase fault current for a LRNG distribution network with multi-DGs.And the impacts of DG on the existing ground relays and power supply reliability are summarized briefly.The result shows that when DG is ungrounded,the impact of DG on ground-fault currents can be nearly ignored.When DG adopts LRNG or direct grounding,point-of-fault ground-fault current increases greatly and zero-sequence currents in lines change obviously.Besides,there is several-hundred amperes fault currents flowing in sound phases of the fault lines with direct or LRNG DG or in three phases of sound lines with direct or LRNG DG,and the distribution of ground-fault currents is significantly different before and after the integration of DG.All of these differences will cause the false tripping or the blinding of existing ground relays and may also improve the sensitivity of existing ground relays at times.As a whole these will decrease the power supply reliability.Besides,the gronuding selection in active distribution network should take the grounding change,line-to-ground characteristics and overvoltage into consideration.Lastly,analysis conclusions are verified with Matlab/Simulink.The thorough analysis of the SLG fault characteristics in a LRNG distribution network with different grounding type of IIDG is meaningful to perfect the fault analysis and can provide theoretical basis for the grounding selection of IIDG,ground-relay configurations and the grounding optimization in active distribution network.