Study On The Protection And Coordination Of Surge Protective Devices In Low-Voltage Distribution Systems

In low-voltage distribution systems, over-voltage caused by lightning is a main source of threat to the safety of electronic and electrical equipments. According to the statistic data, the losses caused by lightning overvoltage in low-voltage systems are about 70 percent among all losses each year. As a main propagation path of overvoltage, the overvoltage protection problem in low-voltage distribution system has attracted more and more attentions. As a result, study on the lightning overvoltage protection problem in low-voltage distribution systems has very important practical meanings.The protection and coordination problems are core problems of lightning overvoltage protection problems in low-voltage distribution systems. And for these problems, there have lot of research works conducted home and abroad. However, these existing researching works almost all started from the view of application and mainly count on the tools of engineering experiences, software simulation, experimental analysis and simple theoretical analysis. There are lack of overall and deep theoretical analyses, and hardly to provide theoretical guide and reference for engineering practices.Aimed at the mentioned problems, in this dissertation, the deep theoretical analyses are carried out for the protection distance problem and coordination problem of SPD. The research works are mainly concentrated on the effective protection distance of SPD, cascaded coordination of SPD, the influences of line resistance on the protection distance and coordination of SPD, and the protection distance problem and coordination problem of SPD in multi-line circuit systems.In the research of protection distance problem of SPD, according to the property that the wave length of lightning wave is mucher bigger than the physical length of the line connecting SPD and equipment, the simplifiedπ–type equivalent circuit (LC oscillation circuit) is used to substitute the distributing parameter line, and analytical method is used to substitute the traveling wave method to study the oscillation phenomena of the load voltage. For different load characteristic and magnitude (resistance load, capacitive load, inductive load and integrated load), the analytical solutions of load voltage are given, and the oscillation mechanism and influence factors are analyzed. For the first time, the approximate computing eqations for determining the effective protection distance of SPD are given. The model error (usingπ–type equivalent circuit to substitute the distributing parameter line) is also studied through an example by using the simulation software. The changing trend of the model error is given. Fothermore, a direct current term and a time-variant term are used to express the ascent of the residual voltage of SPD for the first time in the research. Through theoretical computation and software simulation, it is found that the maximum load voltage is not the two times of the maximu residual voltage of SPD given in the existing research works.For cascaded coordination of SPD, the traveling wave method is used to study the turn on order of cascaded SPD. The influence of load is taken into account in coordination analysis of cascaded SPD and the judging equations are given for the first time. By analyzing the lumped-paramater equivalent circuit, the analytical solutions of the current flowing through each SPD are obtained. The influences of different turn on order on the coordination are analyzed and the equation for determining the necessary line paramaters in the coordination design of cascaded SPD is given for the first time. In the dissertation, the turn on order of SPD, the influence of line parameter on the coordination of SPD, the determination of line parameter in the coordination, the influence of surge waveshape on the coordination, the coordination of voltage switching type SPD and voltage limiting type SPD, the coordination between tow voltage limiting type SPD are studied by software simulation to verify the correctness of the theoretical analyses.In existing researches about the protection distance and cascaded coordination of SPD, to highlight the main problem and simplify the analysis process, line resistance is usually neglected and the lossless wave impedance or an inductance is used to replace connecting line. In the dissertation, the influence of line resistance on the divertion of lightning current of the end SPD is analyzed theoretically. And the computing accuracy of several line models, the error changing trends of the current flowing through the end SPD under different line paramaters and different surge magitudes are studied by using software simulation. For the influence of line resistance on effective protection distance of SPD under different load conditions, the analytical solutions of load voltages are given. The different influences for different loads are analysed.The protection distance problem and coordination problem of SPD in the multi-line circuit systems, which is more closed to the practical engineering situation, are theoretically studied and demonstrated by software simulation. The parallel effect of line impedance and load on the protection distance of SPD is studied. The parallel effects of line impedance on the coordination of different type SPD are studied. And the software simulation is used to prove the correctness of the analysis.