Experimental study of lightning and switching surges-induced overvoltages in low-voltage control circuits

The operating voltages of low-voltage control circuits in power stations and substations have become lower with the installation of digital control equipment. This significantly increases the risk of faults and malfunctions of the circuits resulting from overvoltages in the control circuits due to lightning and switching surges. Therefore, the characterization of such overvoltages is required to protect the circuits. In this work, to study the overvoltages arising in the control circuits, we set up a test system of low-voltage control circuits composed of two grounding grids, a gas-insulated switchgear (GIS) model with two instrumental transformers, and digital-type protection-relay equipment. Using this test system, we measured the overvoltages in the control circuits in order to study the effect of the metal sheath of a control cable in the control circuit as well as the dominant phenomena to cause the surge voltages in the following three cases: 1) injecting a lightning impulse current into the grounding grid, 2) applying an impulse voltage simulating lightning surges suppressed by lightning arresters to the center conductor of the GIS model, and 3) generating a steep-front wave simulating a very fast transient inside the GIS model.