Research On Break Line And High Impedance Grounding Fault Protection Strategy Of Low-resistance Grounded Distribution Network

Nowadays,the low-resistance grounded system(LRGS)has been widely used for the distribution networks in large and medium-sized cities.The zero-sequence(ZS)overcurrent protection scheme performs well during a grounding fault with low fault impedance.However,the environment of distribution networks is complicated,especially for the overhead line.The break line fault(BLF)and grounding fault caused by lightning or outside force damage happened frequently.When the line breaks and falls down to the ground with imperfect dielectric,the high impedance grounding fault(HIGF)and BLF will happen in the same time(BLHIF).However,the sensitivity of the ZS protection scheme for HIGF is insufficient.Generally,it will even be invalid to the situations with fault impedance approximately over100Ω.The override trip will happen when the ZS protection become invalid and the fault last for long time,which will cause more power failure and influence the system stability.More serious,the step voltage from the fault will be extremely dangerous to the pedestrian.Based on the factors above,this paper has conducted the following research:(1)Considering the characteristic change of the BLF and HIGF,a detection criterion for the LRGS networks is proposed based on the ZS power and ratio of negative sequence current and positive sequence current.The simulations carried out in the PSCAD/EMTDC software proved the criteria is discriminative in the scenes of BLF,HIGF and BLHIF.(2)According to the criteria above,a break line and high impedance grounding fault protection strategy for the LRGS networks is designed which also considering the external factors of the algorithm detection effect.The HIGF protection is aimed to detect and cut the HIGF and BLHIF(source-side grounding).The BL protection is aimed to detect and cut the BLF and BLHIF(load-side grounding).The simulations carried out in the PSCAD/EMTDC software proved that values from the faults all exceed the threshold value and the protection can detect and cut all kinds of the faults above correctly.(3)A further test for the protection is taken based on the real network testing devices,which simulated several kinds of the real faults.The test result proved that the essential factors of the protection(selectivity,reliability,speed and sensitivity)all achieved the expected effect and the protection has high practical value.