Fast Frequency Characterization of Inductive Voltage Transformers using Damped Oscillatory Waves

Measurements of harmonics and, in general, of power quality in distribution grids are becoming more and more an essential task, due to the proliferation of switching power devices. Instrument Transformers (ITs), that are necessary for these kind of measurements in distribution grids, in turns, need to be frequency characterized. Although international standards about ITs do not suggest procedures for their frequency characterization, there are some characterization procedures that are acknowledged in scientific literature. They are based on the application of realistic power system waveforms to the ITs, in order to test their behaviour in conditions very similar to the actual operating conditions. A drawback of these procedures is the quite long duration of the tests, in the order of ~ 1 - 10 hours. Therefore, this paper proposes a new waveform for the frequency characterization of inductive Voltage Transformers (VTs), composed by the superposition of a fundamental component and a periodic transient phenomenon, namely a damped sine wave. With its use, it is possible, with a single waveform, to evaluate the frequency behaviour of an inductive VT, thus with a test duration in the order of ~ 1 s. The waveform is theoretically analysed and experimentally applied to two commercial VTs for medium voltage distribution grids. Results are compared with those obtained with the IT frequency characterization methods most acknowledged in scientific literature.