Investigation On The Surge Protective Devices Installed In The Power Supply System Of Mobile Base Station

The mobile base stations are subject to lightning strike and lightning overvoltage harm. According to the statistic date, most of lightning surges intrude the mobile base stations through the power supply lines. Therefore, lightning protection is essential for safe and reliable operation of electrical and electronic equipment inside the mobile base stations. As a whole, the measures taken for protecting the power supply system from lightning overvoltage include shielding, grounding, equipotential bonding, shunting current and voltage limitation. However, the protection function of surge protective devices (SPDs) is very important in these measures. A direct theoretical analysis for the lightning surge suppression process of SPDs is difficult to be performed, since this process is concerned with the sophisticated electromagnetic transient mechanism. The standard specifications and experiential methods have been employed to estimate the protective performance of SDS in the engineering design, while fewer achievements have been obtained in quantitative calculation and simulation analysis. With the present situation in view, this paper makes a detailed analysis to the protective performance of SPDs installed on power supply lines of the base stations in order to provide a credible design basis for the lightning overvoltage protection and reduce the equipment loss from lightning strikes.In this paper, the model of gas discharge tube was constructed. Based on the model, the operating condition of SPDs during the limitation of lightning overvoltages was inquired and the associated protective level was determined by means of computer simulation. An improvement was also made for the circuit structure of single-phase power supply SPDs. It was proved from the theoretical analysis and simulating computation that the improved circuit had a better protective performance. Furthermore, the real components were selected to assemble the SPDs and the impulse tests were carried out to measure their response characteristics. The simulated results were compared with the measured ones, which confirmed the validity of the model and the simulation procedure.Simulating computations were made respectively for the interstage coordination between the two stage SPDs and the characteristic coordination between SPDs and load impedances. The influence of two stage breakdown voltages and connecting cable length on the protective effect was investigated. For different types of load impedances, the quanlitative analysis was made for the transient voltage responses by the simplified circuit model and the quantitative simulated results was also given for the voltage protection level at the protected load terminals. The impulse experiment measurement was conducted for checking the validity of simulation method by comparison between simulated and measured results.From theoretical analysis, computation and experimental measurement, a remark was given for the key protective element, namely metal oxide varistor, on its equivalence of the through-current capability between 8/20μs and 10/350μs waveforms. The 2ms square wave was recommended as the test waveform for examining the withstand ability of SPDs under switching overvoltage. The contrast tests on metal oxide varistors under 8/20μs and 2ms square wave was carried out and the theoretical analysis result was verified by the measured ones.In order to analyze the transfer behavior of different operating states of metal oxide varistors, Markov process was employed and its state transfer diagram was drawn, from which the calculation formula of the mean time to failure of metal oxide varistors was derived. Based on this formula, the reliability of metal oxide varistors was calculated on condition that the power lines of base station was struck by lightning, and the operating lifetime of metal oxide varistors was predicted in different conditions.The research results reported in this paper can provide the reference basis for lightning protection design of power supply system of mobile base station and are of practical significance for its transient overvoltage protection。...