Fault Location And Identification Method Of Cables Considering The Metal Sheath Structure

At present,the power cable is applied more and more widely.As the power cables are usually buried underground,it is difficult to observe,the rapid and accurate fault location and recognition for power cable is of the increasingly urgent demand.Therefore,this paper combines the actual cable structure parameters with the accurate fault location and recognition of the power cable,establishes the cable line model which fully considers the actual structure,simulates different fault types,gets simulation data.Then,the special structure and fault feature of the power cable is considered in the cable fault location and recognition method based on the double PI model.Metal sheath is the special structure of power cables,the paper analyzes the influence of the electromagnetic field on the cable line,expresses the electric field effect with the capacitance and the magnetic field with the mutual inductance.So the cable metal sheath can be equivalent to the conductor,using lumped parameter and double PI model considers the electrical effect of the metal sheath on the cable.Based on the Kirchhoff law,calculate the relationship between the head and the end electric quantity of the single-phase and three-phase single core cable model,and obtain the mathematical model of the cable with cable metal sheath layer.Analyze the characteristics of different cable fault types.Fault location could be realized based on the feather that when the single-phase fault happens,the currents before and after the fault point of the fault phase are different,but the currents of the corresponding phase are equal.We can get the fault resistance value through the calculated electrical quantity of the fault point,the fault resistance value can identify the type of the fault.The fault location and identification method based on the cable structure is studied,which can get the fault distance accurately and identify the single-phase fault type.The actual structure of the cable is considered,based on this model,the single phase fault types of the cable can be divided into core-sheath fault(main insulation fault),sheath-ground fault(external insulation fault)and the core-sheath to ground fault(single phase ground fault).When a fault occurs,according to the mathematical model of the three-phase single core cable,using the electrical quantities of the head gets the current along the line,according to the characteristics that the values of the current fault component of the non fault phase are equal,we can get the fault distance with iterative search.When the fault distance is obtained,the resistance values between the core and the metal sheath,and between the metal sheath and the ground are obtained by using the fault component information of the head end of the cable,then we can get the fault type according to the fault resistance values.The method only uses the fault component of the head end,and can realize the accurate fault location and fault type identification of the core-sheath fault,sheath-ground fault and the core-sheath to ground fault.The method is easy to implement and not affected by the fault resistance.Load impedance and fault resistance are two main factors influencing the fault location.The single ended fault location method using constant impedance load model,but the simulation analysis found that if the load side ueses the transformer model,the constant impedance load model is no longer applicable for the fault location,so it is necessary to consider the change of the load impedance value.If we use the terminal electrical quantities directly,we need to get the double end synchronous data with synchronization device,so we use the equivalent impedance model to express the cable load side.The equivalent impedance model contains the influence of complex electromagnetic change of the transformer,at the same time,it need not to get the synchronization of two terminal data.That is we get the the electrical data of the end cable,get the load impedance;using real-time load equivalent impedance accomplish fault location,we avoid the influence of the unknown or inaccurate values given by the load on the positioning accuracy.The double terminal method of the fault location is unaffected by the load impedance and the fault resistance,and does not need two terminal data synchronization.Theoretical research and simulation results have the good performance.When the cable single-phase fault occurs,considering the cable metal sheath structure and the interaction of cable internal and three-phase cable,the single ended fault location and recognition method using the non fault phase has the high precision,and is not affected by the fault distance and fault type fault resistance,fault initial angle and cable sheath grounding type effect.At the same time,the double terminal method with the improved real-time load impedance model can effectively solve the problem of the fault resistance,the load impedance model and the double end information synchronization,and can effectively improve the accuracy of fault location and recognition of the cable.