Study On Current Fault Detection Mechansim For A Three-phase Five-wire Cable

In recent years,the problem of global warming has become increasingly serious.In order to achieve net zero carbon emission as soon as possible,cleaner and more efficient energy is favored all over the world.China put forward the double carbon goal of carbon peak and carbon neutralization in 2021.Electricity is an indispensable energy in life.A more environmentally friendly modern smart grid is conducive to the early realization of our dual carbon goal.Intelligent buildings,new energy vehicles and intelligent medical equipment are important components of smart grid.Three-phase AC is often used as the source of electric energy in industry and life.The current of electric equipment can be monitored and managed in real time through current sensors,which can effectively reduce the occurrence of safety accidents.It is a prerequisite for the smart grid to provide various functions to form a sensor node through current sensors,further form a sensor network,and realize the real-time monitoring and feedback of the power grid through wireless communication technology.Therefore,the development of a passive noninvasive current sensor for current detection is very important for the smart grid.Many existing current sensors have their own characteristics and advantages,but the cantilever resonant current sensor may be more suitable to meet the demanding needs.This paper presents a piezoelectric cantilever current sensor,which does not need to provide external power supply and remove the insulation layer of the cable to realize the measurement of three-phase five wire cable.It belongs to noninvasive measurement and has the advantages of simple structure,passive and noninvasive.It is the above advantages that make the cantilever current sensor have great potential in real-time current monitoring.According to Biot Savart law,this paper establishes the spatial magnetic field distribution law of a single current carrying straight wire with arbitrary length and the force analysis theory of permanent magnet in its magnetic field,and then establishes the spatial magnetic field distribution law of three-phase five wire current carrying straight wire with arbitrary length and the force analysis theory of permanent magnet in its magnetic field,And the general theoretical model of the piezoelectric voltage output of the sensor when the three-phase five wire cable is connected with any load,a method of decoupling current of multi cantilever combined structure is proposed.Aiming at the star symmetrical load connected by three-phase five wire cable,the current voltage output theoretical models of normal state and several common fault states are established.The results show that no matter whether the three-phase five wire cable is in normal state or fault state,when the sensor is measured at a fixed position,the output voltage amplitude has a linear relationship with the measured current amplitude.Through the different voltage output amplitude at the same position of the sensor,the working state of the three-phase five wire cable can be located.Only one sensor can realize the fault tracing of the current sensor to the three-phase five wire cable.In order to further improve the output of the current sensor,aiming at improving the sensitivity,this paper carries out simulation analysis in two aspects: one is to simulate the geometric structure of the sensor,so as to make its natural frequency of the first-order vibration mode close to the frequency of China’s power grid and realize resonance;Second,through the simulation of the magnetic field gradient of three-phase five wire cable,the position where the permanent magnet bears the maximum force is obtained to improve the sensitivity.Finally,the influence of the length of the current carrying straight wire on the output of the sensor at the position of the maximum magnetic field gradient is simulated and analyzed.The results show that consistent with the theory,when the wire does not meet the ideal state of infinite straight wire,the output of the sensor will be affected by the wire length,and this factor can not be ignored.Finally,the three-phase five wire cable clamp,current sensor and its clamp are designed and manufactured.By building an experimental platform,the influence of the movement of the sensor along the length of the limited current carrying wire on the output of the sensor is verified,and the voltage output of the current sensor is verified when the three-phase five wire cable is connected to the star symmetrical load in the normal state and several common fault states,At different measuring positions,the current amplitude and voltage amplitude show a good linear relationship,and the closer to the cable,the higher the sensitivity of the sensor,which is consistent with the sensing theory.Finally,the traceability mechanism of current fault diagnosis is verified,which provides a new method for the current detection(normal state and fault state)of threephase five wire cable.