Improvement Of Impulse Current Method For Cable Fault Location

Power cables are widely used in power system due to its performance of safety, reliability,city beautifying and so on.However,it is very difficult to find out the fault point in case of cable failure as power cables are usually installed under ground.Because the fault conditions and installing environment of cable are complex and variable,advanced devices must be used to detect the fault point quickly and successfully.90%cable failure can be detected by impulse current method which is widely used currently.It is a great progress on cable fault detecting technology,especially in measuring high resistance and flashover fault.But,the waveform is complex and the front-of-wave of reflected impulse is difficult to be recognized when using impulse current method,so it is necessary to deeply study and improve the currently used method.The principle of impulse current method is to breakdown the cable fault point with high-voltage,sample the current wave signal by linear current coupler and then calculate the fault distance through the analysis of time needed for current wave signal’s traveling between measuring terminal and fault point.Reflected waves are not easy to be recognized because of the superposition of cable charge transient caused by capacitor and fault point discharge transient.In the paper,delay circuit is added in series with the test circuit of impulse current method.The delay circuit can prevent the fault point from reaching breakdown voltage rapidly,so the problem of wave superposition is solved by the obvious time difference between the transient generated by impulse voltage and the transient generated by fault point discharging.Delay circuit is achieved by adding inductance and resistance in series with the test circuit.The added inductance can prevent the loop voltage from leaping,so the rising speed of impulse voltage is restricted and the breakdown and discharging time of fault point is delayed.An equivalent circuit model of test circuit is established to obtain the parameters in delay circuit by solve the differential equation of inductance and resistance to delay time. According to the delay time needed and considering the effect of energy loss,the set of best matching parameters can be acquired.Wavelet analysis is applied to the improvement of impulse current method for fault data processing in order to fulfill the filtering of sampled signals.Automatically fault locating algorithm is realized by wavelet reconstructing and wavelet singularity detection theory.Vast simulations on different inductance and resistance parameters,different fault distance,and different sampling capacitance have been performed by building Electro-Magnetic Transient Program(EMTP-ATP)simulation model.The simulation results show that the improvement of impulse current method is correct and feasible.Field tests also show that the front-of-wave of reflected waveform is obvious and easy to be recognized,which further verify the validity of the proposed method.The main innovation of the paper is that the problems of complex wave and difficulty in identifying have been solved by adding delay circuit to traditional impulse current method.An automatically fault location algorithm based on wavelet analysis is also presented in the paper,which can enhance the speed of cable fault detection,reduce the professional knowledge requirements for operators and enlarge the testing area of impulse current method.It is very important to locate the fault point accurately and quickly and to enhance the power supply reliability.