Research On Single-Phase Ground Fault Protection Method For Neutral Point Via Small Resistance Grounding System

With the urban distribution network cable rate is getting increasingly high,the resonant grounding system has many safety risks,the small resistance grounding system was adopted by many large cities with fast fault excision speed,low overvoltage level and other advantages.At present,the small resistance grounding system is mainly protected by fixed time zero sequence overcurrent protection,in single-phase high-impedance ground fault(SPHIF)and multi-feeder ground fault under zero sequence current(ZSC)is greatly weakened,the protection is very easy to refuse to move.For this reason,this paper first analyzes the fault characteristics of small resistance grounding system under SPHIF,and deduces the temporary steady-state expressions of busbar zero sequence voltage(BZSV),neutral ZSC and feeder ZSC respectively to further reveal the theoretical principle of fault characteristics.The characteristics of the amplitude ratio of the periodic and DC components in the feeder transient ZSC,the characteristics of the low-frequency component share of the feeder transient ZSC and the characteristics of the phase difference between the feeder steady-state ZSC and the fault phase voltage are obtained.The Total-LeastSquares Matrix Pencil(TLSMP)algorithm and Variational Mode Decomposition(VMD)algorithm are used to quickly and accurately extract the transient ZSC fault characteristics.SPHIF comprehensive protection method based on TLSMP-VMD is proposed,and the specific threshold value of protection is given to realize the in-situ protection.Secondly,this paper analyzes the fault characteristics of small resistance grounding system under multi-feeder ground fault,and derives the steady-state expressions of BZSV,neutral ZSC and feeder ZSC by superposition method respectively.The amplitude and phase characteristics of feeder ZSC and neutral ZSC are obtained.A centralized protection method is proposed for two lines with the same phase ground fault,and the accurate detection of two lines with the same phase fault is achieved to avoid overstep tripping of neutral transformer protection.Finally,the test results of PSCAD electromagnetic simulation software show that the proposed method can still operate accurately under a variety of extreme operating conditions with good sensitivity and reliability,and the method is easy to implement and has good engineering application value.