Research On On-line Monitoring And System Design Of Surge Protective Device

The surge protective device and its internal Zn O varistor will withstand various kinds of intermittent over-voltage and long-term power frequency voltage during operation,which will trigger the phenomena like increasing of leakage current and rising temperature.These phenomena will further lead to the aging and degradation of the device,impact damage and thermal damage.To cure the above problems,this paper studies the monitoring indicators of aging and deterioration of surge protective device,and designs an online monitoring system to monitor the working condition of the surge protective device in real time.The article firstly analyzes the thermal characteristics and temperature distribution of the surge protective device and its internal Zn O varistor by thermal imaging experiment and simulation.Taking temperature as a new monitoring indicator to improve the problem that the leakage current is not obvious in the early stage of device aging and degradation.Secondly,based on the equivalent circuit model of the Zn O varistor,the leakage current resistance component of the varistor is extracted and calculated by harmonic analysis method,and it is regarded as the main monitoring index.Then,the noise suppression during the extraction of leakage current is studied and analyzed to reduce the noise interference of signal extraction.Finally,based on STM32F407 series microprocessor chip,an on-line monitoring system of surge protective device is designed,which takes the resistive leakage current and temperature as the monitoring indicators.The main conclusions are as follows:1)Thermal characteristics affect the performance of Zno varistors.After lightning impulse,the temperature of varistor exceeds 65 ? and it can not effectively dissipate heat.And when it reaches 90 ?,it will lead to thermal runaway and partial hot melt because of the local uneven heating.Monitoring the temperature at high temperature zone is helpful to understand the realtime operation of the device.(2)The resistive component of the leakage current is the main index to measure the aging and degradation of the Zn O varistor.By using the extraction method of precise small resistance shunt and the improved harmonic analysis method,the resistive leakage current is taken as the monitoring index and spectrum leakage and other factors can be reduced.(3)The empirical mode decomposition(EMD)based on wavelet transform has a significant improvement in the noise filtering.Compared with the single wavelet transform denoising method,the mean square error can be reduced by 62.5%(un-aged)and 29.6%(aging).The algorithm effectively improves the accuracy of leakage current extraction.(4)Based on STM32F407 series microprocessors,an on-line monitoring system of surge protector is designed.The collected resistive leakage current and temperature can real-time understand the operation state of the device and transmit it to the rear platform.When the leakage current value exceeds 20,the resistive leakage current value exceeds 5 or the temperature value exceeds 65 ?,a warning will be given for further inspection.