Ground potential rise and lightning overvoltages in control systems of large power-plants under high soil resistivity

Power-plants are sometimes built on massive bedrock with high soil resistivity thereby often resulting in less favorable grounding conditions. While grounding for applications with power system frequency can be achieved with large electrode areas, lightning strokes usually affect smaller areas with heavier impact. As a consequence a grounding system suitable for power system frequency is not necessarily capable of handling transient phenomena sufficiently well. A critical case is the ground potential rise when a large lightning surge propagates through the grounding system. The potential differences in different parts of grounding system may damage the connected wiring and equipment, depending on how grounding connections are made. New installations can be equipped with proper equipment but existing plants, especially those subject to detailed approval processes such as nuclear power-plants, often can't be modified easily. This paper evaluates the impact of lightning on a grounding system with different lightning surge propagation properties and finally presents a variety of solutions how to properly improve the transient protection for control equipment from a system point of view.