Early Detection And Protection Technology For Multi-level Short-Circuit Fault Based On The Terminal Voltage

So far the low voltage distribution system has the disadvantages of low reliability and low speed,and it isn’t in the early detection of short circuit to achieve selective protection.According to the characteristic which each branch terminal voltage almost drops to zero at all levels at the moment of short circuit and remains near zero zone before fault elimination,this paper introduces the wavelet packet decomposition method to extract feature of each branch terminal voltage and combines wavelet packet decomposition method with instantaneous value of terminal voltage so as to eliminate the interference of different types of load start etc.It could solve the technical bottleneck problem of existing short current early detection method,which is affected by the non-fault running state interference of different nature load start and so on.And based on the above analysis,with deeply study on characteristics of related branch terminal voltage in different levels and different types of short circuit,this paper mainly contains the following contents:1、This paper introduces the fast and early detection methods of short-circuit faults,and analyzes the effectiveness of the early detection methods of short-circuit current and the reliability of the selective protection.2、This paper uses Matlab/Simulink to establish the simulation model of short-circuit fault in low voltage,and combines with the actual experimental waveform under the same initial phase angles of short-circuit fault in order to verify the accuracy and efficiency of the simulation model.Then,the characteristics of terminal voltage signal of short-circuit fault at different levels and different types are studied.3、A novel method for short-circuit early detection of terminal voltage is proposed in the paper,which is based on the combination of the wavelet packet decomposition of the terminal voltage and the instantaneous value of the terminal voltage.By analyzing the effect of motor and different properties,different capacity load starting or switching on terminal voltage signal,this paper also studies the characteristic of the signal of the terminal voltage of various types of short-circuit fault so that the threshold value of early detection and identification of short-circuit terminal voltage is studied.Then the reliability and speed of the method is also analyzed.4、The paper develops multi-level short-fault measurement and control module and collects,storages and displays different properties,different capacity load starting terminal voltage and short-circuit terminal voltage of different levels and different types.This paper uses the NI Compact-RIO to achieve the method for short-fault early detection of terminal voltage.Practical experiments verify that the method can rapidly and effectively distinguish between normal status and fault status of different types of load starting.5、The novel multi-layer full range voltage selective protection mechanism of low voltage system is proposed based on the early detection of the terminal voltage.The key technology of the voltage selective protection mechanism is analyzed.By studying early characteristics of short-circuit correlation branch terminal voltage,the principle of accurate location of fault branch is proposed,which completes the selective protection of low-voltage distribution system,and ensures the continuity and reliability of power supply.In summary,this paper proposes a novel short early detection method based on terminal voltage,and achieves short early detection and identification of different levels and different types.On the basis of this method,a novel voltage selective protection mechanism is formed,thus realizing the selective protection quickly and reliably.