Earth Fault Location For Non-solidly Earthed System Based On Transient Signals

Self-heal is a key function to smart grid.Fast and reliable location is the basis for self-healing distribution network.Distribution network with non-solid grounding have some advantage over solid and low-impedance grounding.In MV system,the faults are mostly single phase to ground faults.A ground fault on a system with non-solid grounding draws a small current.Additionally the system is very complex,so it is very difficult to locate fault point.At present,earth fault identification has succeeded in projects but fault location is still in the stage of theoretical researches.There is some disadvantages for traditional methods and some methods are not available for non-solidly earthed network.In the field application,some key technique has not been solved.For example,it is difficult to acquire fault signals for overhead lines.The paper proposes transient signals to locate earth fault point.The main research works include:(1) Traditional methods and some difficulty for fault location are analyzed. At present,locating methods can not locating fault point well.The unreliable location principles and difficult measurements for fault signals are the main reasons.There are some other difficulties for field application.For example,time can not be precisely synchronized in different monitoring points and communication network can not transmit large amounts of fault data.(2) Traditional analysis methods and their disadvantages are presented in the paper. The three-phase system is uncoupled using Karrenbauer transformation,and the high-precise fault model based on distributed parameter is established.Aerial module current flows through the lines between fault point and busbar,and the aerial impedance can be simplified to inductance of those lines.The zero module network at both sides of the fault point can be simplified asπcircuit.Then a simplified fault model consisting of aerial module inductance andπcircuit is presented.The paper proposes simplified models with low-grounding resistance and high-grounding resistance to calculate the electric parameters and corresponding characteristics are analyzed.The transient signals can be decomposed into components in different frequency bands.In a selected frequency band(SFB),each of upstream and downstream feeders of the fault point can be modeled as a capacitor.So the model of zero module network in SFB is proposed in the end of chapter2.(3) A new method based on calculating approximate entropy of transient zero module current is presented in order to locate the single phase grounding fault in non-solidly earthed network.The approximate entropies of transient zero module current at both ends of healthy section are approximately equal to each other,so the ratio is close to one.On the contrary,the approximate entropies at both ends of fault section are different,and the ratio is less than that of healthy section.According to this characteristics,the earth fault location can be determined.(4) A new principle using combination admittance is presented for earth fault location in non-solidly earthed network.In selected frequency band(SFB),the weighted integral of zero module admittance is as transient combination admittance.The fault direction is calculated using combination admittance angle.According to the difference of fault direction,the earth fault location can be determined.The paper also introduces the performance.(5) It is difficult to measure fault signals for overhead lines.The paper analyses the characteristics of electric field beneath the three-phase overhead lines.The vertical electric field is basically proportional to the zero module voltage,and horizontal electric field is basically proportional to aerial module voltage.Then the zero module voltage can be measured by inducting vertical electric field,and aerial module voltage by horizontal electric field.(6) Characteristics of magnetic field beneath the overhead lines are analyzed in the paper,which show that the horizontal magnetic field is basically proportional to zero module current,and vertical magnetic field is basically proportional to aerial module current.So the zero module current can be measured by inducting horizontal magnetic field,and aerial module current by vertical component of magnetic field.(7) The paper proposes electric field sensor consisting of charge-sensitive amplifier and two parallel metal pole plates to induce electric field.The work principle and frequency characteristics of the sensor are analyzed.Some key technology for field application is introduced in the paper.(8) According to the requirement of magnetic field measurement,a new magnetic field sensor is designed.The sensor consists of a cylindrical coil and integrator.The work principle and frequency characteristics of the sensor are analyzed.Some key technology for field application is introduced in the paper.(9)The fault location system is designed.The system consists of electromagnetic sensor,fault indicator,wireless communication network,solar power system and fault location master and so on.The fault indicator keeps a large safe distance off overhead line and it doesn't have any contact with high-voltage lines,so it can be installed in normal operating conditions of power system.The system is running in laboratory well.(10)Simulations and experiments are used to verify fault location principle and fault signals measurement method.The results demonstrate the feasibility of the method presented in the paper.Innovations of the paper as follows:(1) The simplified model consisting of aerial-module inductance andπcircuit is presented.For quantitative calculation,the paper also proposes simplified model with low grounding resistance and high grounding resistance and gives the expression of mainly electric parameters correspondingly.(2)Approximate entropy is applied to earth fault location.According to the difference of approximate entropies at the both ends of healthy section and fault section,the fault location can be determined.The method of combination admittance is presented in the paper.According to the difference of admittance's angle in different detection points, fault section can be located.(3)The paper presents magnetic field induction to acquire fault current signals,and the magnetic field sensor consisting of a cylindrical coil and integrator is designed.The paper also proposes electric field induction to acquire fault voltage signals,and the electric field sensor consisting of charge-sensitive amplifier and two parallel model plates is designed.