Study On Lightning Surge Protection Of Low Voltage AC Power System

Over-voltage caused by lightning surges is one of the main reasons for resulting in electrical equipments failure and the systems are not functioning in the low-voltage AC power systems. According to statistics, the economic losses due to lightning overvoltage in low-voltage systems are about 70 percent with respect to the total losses of the low-voltage equipment. As a main propagation path of overvoltage, invasion the overvoltage along the low-voltage AC power lines has been concerned a lot.The design and coordination problems of surge protection devices (SPD) are important to lightning overvoltage protection in low-voltage AC power systems. At first, we must have knowledge of the protective characteristics and make a reasonable chose for SPD. In the surge protective devices used in low-voltage AC power system, the main protective elements are metal oxide varistors (MOV) and gas discharge tube (GDT). In this paper, the transient behavior of surge protective devices is simulated by using electromagnetic transient analysis software (PSCAD/EMTDC). A few simulation models have been reported previous literature, which can be employed to carry out a transient analysis to perform comparison between simulated and measured results. As for GDT, attention is paid on research of the response characteristics of simulation model, while the investigation on the simplified circuit formed by GDT and MOV is made by PSCAD/EMTDC and coordination performance is obtained to confirm the validity of simulation procedure by contrast to the measured results.According to refraction and reflection principle of the traveling waves, when lightning surges travel through series inductor and shunt capacitor, the surge gradient can be reduced due to the refraction wave, which is favorable to the SPDs. Several protective circuits are simulated by PSCAD/EMTDC. We investigate the protective effect of two kinds of protective circuits including different filters with MOVs under the case of single stage. The protective performance of two-stage protection circuit is obtained by PSCAD/EMTDC simulation and the validity of the simulating algorithm is verified by comparing the simulated result with the measured one. Then, the protective effect of single-stage and two-stage protection circuits in three-phase AC power lines is also introduced.From the view point of protective reliability, reasonable cascaded protection is very important since the protective elements have distinguished non-linear voltage-current characteristics and response time. PSCAD/EMTDC is used to investigate the energy coordination of two-stage voltage limiting type SPDs in low-voltage power systems. A number of practical factors, such as the amplitude of lightning surge, residual voltage of protective devices and separation distance between two protective devices etc, are taken in to account to analyze their influence on the protective performance of cascaded protective systems. With the length of cable connecting SPDs in the range of 5-20m, it is shown that the high-low coordination is appropriate for the cascaded protective systems whether the lightning surges are introduced in differential mode or common mode.