Fault Line Selection For Ineffectively Earthed Distribution Systems Based On The Fifth Harmonic Current

When single-phase-to-ground fault occurs in the sma1l current grounding system, theshort circuit current is very small and the power supply will not be disturbed. Thus the faultedpower system can still operate for1-2hours according to the rule. However, if the fault cannotbe correctly eliminated in time, the case will become worse and even multi-point groundingfault or two-phase-to-ground fault may occur. Therefore, it is necessary to distinguish and cutthe faulted line when single-phase-to-ground fault occurs in the sma1l current groundingsystem. Line selection has great meaning and practical value for the continuous operation ofthe distribution network and stable power supply.In order to solve difficult problems of the small current grounding system’s fault lineselection, this paper analyzes the characteristics of the small current grounding system whenground fault occurs in details. The small current grounding system model is built with Matlaband the fifth harmonic current is employed to select the fault line by seeking amplitude andphase of the fifth harmonic zero-sequence current. MATLAB simulation results indicate thatthis method is effective to select the fault line.According to the above analysis results, the design of software and hardware system hasbeen completed with STC89C51. The software system mainly includes AC sampling module,fifth harmonic calculation module, fault diagnosis module and display alarming module. Thehardware system includes the analog input circuit with ADC0809, STC89C51minimumsystem, failure alarm circuit and so on. The results of system debugging proved that the fault line selection function had been realized.