Analysis Of Single-Phase To Ground Fault Characteristics In Unbalanced Distribution Networks And Research On Novel Protection Principles

Low-resistance grounded systems(LRGSs),whose neutral point is grounded via a low-resistance,are suitable for urban distribution networks consisting of primarily cable lines.Therefore,LRGSs have been widely used in large and medium-sized cities,particularly in South China.In LRGSs,due to the obvious magnitude of fault current,traditional zero-sequence(ZS)overcurrent protection scheme performs well during a single-phase to ground fault(SPGF)with low fault impedance.However,the sensitivity of this protection scheme for SPGFs with high fault impedance is insufficient.Generally,it will be invalid to the situations with fault impedance approximately over 100 ?.Therefore,an effective detection method is urgently needed to decline the negative effects of this type of faults on normal operation of distribution systems and public safety.On the other hand,high-level imbalance is one of the main characters of distribution networks,but little research has concentrated on the effects of imbalance on SPGF characteristics,not to mention the corresponding solutions to eliminating such effects.However,as the fault impedance increases,certain original insignificant influences of system imbalance may be amplified,especially those on ZS components.As a result,this may lead to misidentification of some SPGF detection methods.Based on the above analysis,this paper focuses on unbalance distribution networks,especially LRGSs,and has conducted the following research:(1)Considering unbalanced distribution systems,combining the compensation idea of the methods of sequence coordinate and certain reasonable approximations,a controlled source-based simplified linear calculation model for SPGFs is proposed.Using this calculation model,low-dimension and linear solutions to a network can be obtained rapidly.Finally,the high calculation accuracy of this model,especially for the phase and the ZS components,is verified by simulations.(2)Based on the proposed calculation model,from the perspective of theoretical analysis,the effects of two main sources of imbalance,namely,unbalanced loads and asymmetrical lines,on SPGF characteristics in medium-voltage distribution networks are analyzed in detail.This analysis considers several factors,including the line parameters,the faulty phase and the magnitude of fault impedance.Moreover,two existing SPGF detection methods have been tested and analyzed in an unbalanced distribution system.(3)An SPGF protection scheme based on single-phase instantaneous power theory is proposed for LRGSs in this paper.First,in order to tackle the problem brought by the system imbalance on the ZS components,an approximate-eliminating method based on the(35)-circuit components is devised.Using the(35)-circuit components,with the help of single-phase instantaneous power theory,a comprehensive criterion,subtly considering not only the arcing features but also the inherent characteristics brought by the unique grounding mode,is involved in the proposed protection scheme.The validity of this scheme of accurately and reliably identifying various SPGFs,including those with low or high fault impedance and those with linear fault impedance or arcs,is verified by simulations carried out in the PSCAD/EMTDC software.Furthermore,this detection method has good performance under the circumstances with noises with low signal-to-noise ratio,which is also proved by simulations.