Power plant lightning overvoltage protection of low voltage power electronics

The effects of direct lightning strike and flashover strike to 400kV system of nuclear power plant were inspected for perspective of low voltage power electronic devices. Protection methods were simulated with PSCAD transient simulator. The devices simulated were, metal oxide protector, three phase rectifier load and clamp style protection device with over-current protection and a battery. Metal oxide protector was effective to limit over voltage but some effects were noticed still at DC bus as a rise in bus voltage. The connection of power electronic load solely already limited the overvoltages in LV AC points near the load. This effect comes from capacitance in the load to buffer the rectifier voltage. There are also protective capacitors over the rectifier components(in this case diodes) to protect them from overvoltages and they also help to dampen the overvoltages at AC and DC bus. Battery also dampened the overvoltage so much that clamp type protection did not even activate. It has to be said that there is uncertainty of battery behaviour in very fast transient phenomena as most of measurements and models do not focus on this but it is very clear that dampening effect is considerable.
Results indicate that capacitors are very effective at damping fast transient overvoltages. Because most DC rectifiers are based on active bridge technologies, it is small effort to also include protective functionalities such as overvoltage and over-current protections. Although mechanical breakers are effective devices, they have operational delay for noticing the fault and acting. Therefore some passive protective methods could be used such as clamp type protection or additional capacitors. This study points to that capacitors are good solution for lightning overvoltage protection for power electronic devices. More broad usage of them in protection concept should be studied more on many other aspects such as potential contribution to faults in probabilistic manner.