Measurement of Very Fast Transient Overvoltages in Current Transformers at Open Air HV Substations

Very fast transient overvoltages appear when switches operate in high voltage networks. In some cases, they can affect near current transformers (CT). In this paper, a new methodology for measuring these transients is proposed. It uses the stray capacitance of the CT between its high voltage winding and all other parts at ground potential. The current through this capacitance is measured by a specially designed Rogowski coil, and a reconstruction algorithm calculates the input voltage. Our proposal does not need to separate the CT from its base, as proposed in other works. Tests on a high voltage capacitor corroborate this method.