Study And Design Of Low Voltage Integrated Protection Device

With the continuous expansion of the scale of China's economic construction,electricity has become the lifeline of the modern economy,and the problem of safe power consuming is particularly important.The existing conventional protection devices for distribution line terminals mainly carry out overcurrent and leakage protection,but only these protection is not enough.According to statistics,the electric fire caused by fault arcs has accounted for more than 50%.Arc fault have become one of the main causes of electrical fires.However,when the series fault arcs occur,the effective value of the line current is smaller than that of the normal operation,but it is not in the range of the protection device's action.Therefore,conventional protection devices cannot cut off circuit and cause fires.Therefore,it is very important to study and design a comprehensive protection device which can protect the high current,such as overload,and the low current,such as the series fault arc.Through the fault arc protection device test,it is found that at present,the tripping ratio of these protection devices is 60%,and the accuracy rate needs to be improved.The characteristic of load is one of the reasons for these malfunctions.Therefore,starting from the research of load characteristics,the representative load is summarized and summarized based on the related literature and national standards.Secondly,because there are many combinations of load on distribution lines,it is impossible to test,so this paper chooses simulation.Through a large number of simulation tests on the arc model Mayr-Cassie by Matlab software,under different load characteristics the current waveform of normal condition and fault condition are obtained.Bycomparison,it is found that under the condition of inductive load,the characteristics of fault waveform are inconspicuous and harder to detect.Then analyze the processed data and calculate the time domain and frequency domain characteristic parameters of the current.Such as peak value,integral frequency characteristic parameters of time-domain features and harmonic components,total harmonic distortion of frequency domain features.It is proved that each time domain and frequency domain characteristic parameter of fault arcs will be affected by load characteristics.When the series fault arc occurs,the power factor is constant and the time domain characteristic parameter of the current increases,with the increase of the active power of the load.When the active power is constant,the time domain characteristic parameter decreases with the increase of power factor.The characteristic parameters of the frequency domain,the three or four harmonic component and the total harmonic distortion will increase,but they will not change with the load characteristics.Therefore,the time domain characteristic parameters are most affected.Single characteristic parameter of time domain or frequency domain can not be used to detect the series fault arcs accurately and effectively.To solve this problem,In this paper,a discriminant algorithm based on combination of multi feature parameters and BP neural network is proposed to improve the recognition rate of arc fault.Finally,based on the above identification method,the hardware and software of the integrated protector are designed by using low cost PIC18F4550.In the hardware circuit,the circuit current should be sampled and filtered so as to obtain the three or four harmonic components and peak difference,and also provide direct current for the microcontrollers and the sampling circuit.In the aspect of software,the function of arc recognition and leakage protection is completed,and the corresponding program flow chart is given for different functions.