Study On The Devices For Limiting Fast Transient Over-voltage In Low-voltage System

Over-voltage surge is one of the main reasons for resulting in equipment failure and damage in the low-voltage system, however lightning over-voltage accounts for the main part of over-voltage surge. According to an uncompleted statistic, losses caused by lightning over-voltage each year account for more than half of the total losses. Therefore, more attention is acquired for lightning over-voltage protection. Owing to lightning transient and fast transient over-voltage exist in high-voltage laboratory, so it is necessary to protect the equipment in such a intense electromagnetic environment. Based on theoretical analysis, simulation and experiment, a systematic study on the over-voltage protection for equipment in low-voltage system is made.Surge protective device (SPD) is essential in over-voltage protection of low-voltage system. For the design of SPD used in front of equipment, characteristics of the core elements of SPD which include metal oxide varistor (MOV) and gas discharge tube (GDT) is studied. Meanwhile, a mixed type two-grade SPD differing from those in market is proposed whose protective effect is validated by simulation analysis and experimental test, which economizes cost again reduces size.Generally, a single SPD is expected to provide protection for several devices at the same time. The voltage oscillation on terminal loads is systematic studied in order to make SPD as far as possible to protect these equipment, and the effective protection distance of SPD under different loads is obtained by simulation. The simulation results show that the different loads can be protected by the SPD when they are placed in the range of 10m.More than one SPD can be installed on a line in order to protect equipment better. Initially, the cascade coordination characteristics of two SPDs are studied. Then, the electrical quantities of two SPDs are calculated and results show that "high-low" mode can achieve a better protective effect. Last, the effect of the length of cable on the cascade coordination is studied, and the simulation results confirm that the best coordination distance between two SPDs is 5m～20m.Very fast transient over-voltage (VFTO) is one of the transient over-voltages generated in the laboratory. Some researches on VFTO are carried out. Owing to the MOV under current surges of different steepness shows different characteristics, the volt-ampere characteristics of MOV is simulated under surges of steep fronts, and the simulation results consist with experimental measurements. According to the characteristics of MOV, two-grade SPD is used to limit VFTO and the protective effects under VFTO and standard lightning impulse are compared through simulation. Results show that the residual voltage under surges of steep fronts is different from that under lightning surge.