Research On Fault Location Methods For Single-Phase-To-Earth Faults In Neutral Ineffectively Grounded Systems

The neutral ineffectively grounded systems are adopted mostly in MV power distribution networks.The advantages of this kind of system are highly reliability and small current when single-phase-to-earth fault happened.Fault location of single-phase-to-earth in neutral ineffectively grounded system has not been well resovled due to its little current and complicated connection of the branches.The thesis put forword online and offiine methods to realize the fault location based on the demand of the distribution department.The principle contents are as follows:Off-line fault location method of AC is proposed.By analyzing the effects of the inductance and distributed capacitance of the line to the AC signal injected to the line, the paper set up the lumped parameter of the line model,and draw the function between the reliability and the frequency of the AC signal,then choose 60Hz as the frequency of the signal.At last,the paper analyzes the influence of the resistor and the distributed capacitance to the reliability of this method.In order to overcome the influence of the resistor and distributed capacitance,A method of DC is put forward.The signal of DC can only flew from the injection point and the earthing point,so the method can settle all the faults of the single-phase-to earth,such as large resistor,fault line with long and short branches,and et al.The way of "section current" is adopted to settle the problem of the DC signal flew within three-phase line through the transformer.The technology of communication wireless is used to solve the issue of the detection of DC signal.The method of DC-AC is proposed in the paper.Where the product of the resistor and the length of the line is used as the target to decide which method(AC or DC) is suited under different fault location.When the target is large,the capacitance of the line will take a large percent of the AC current injected,then the reliability of method of AC is low,the method of DC should be taken.When the target is small,most of the AC current flew into the earthing point,so the method of AC is suitable.The method of DC-AC combine the accuracy of the method DC and the easily detect of the method AC together,can achieve more quickly and more accurate fault location.The method of phasor comparison of zero-sequence current is proposed to locate the fault point online for neutral non-connected grounded system.When single-phase-to-ground fault is happened,the zero-sequence current lags the zero-sequence voltage 90°on the fault route and leads the zero-sequence voltage 90°on non-fault route.This method locates the fault by the signal caused by the fault itself without no external signal,then cause no effect to the system running.The system of GPS and the technology of GPRS are used to acquire the phase between the voltage and current.Two detection means are introduced here to measure the zero-sequence current, which is mobile zero-sequence CT and mobile induct magnetic filed detection.Mobile zero-sequence CT use three precise CT of clamp mode,which socket separately on the three-phase line,and synthesis the zero-sequence current of the fault line by the discrete value of the three-phase current.This means suffer little from the environment and achieve high accuracy.Mobile induct magnetic field detection realize the current detection by compare the phase of dual sides instead of detect the amplitude of a single point,and enhance the reliability of the measure.The method of zero-sequence current change is proposed to solve the problem of fault location of resonant grounded system.The zero-sequence current will change when the inductance of arc-suppression coil is changed.The method locate the fault point based on the properties of the change on the fault line.