Accelerating Dielectric Response Measurements on Power Transformers—Part I: A Frequency-Domain Approach

Dielectric frequency response, also known as frequency-domain spectroscopy (FDS), was introduced about a decade ago as the preferred method for measuring moisture content in the cellulose insulation of power transformers. A conventional FDS measurement is carried out as a frequency sweep down until 100 μHz, thus causing unavoidable large measuring time due to the very low-frequency oscillations. Nevertheless, FDS is the favored choice for onsite measurements due to its robustness against noise. This contribution proposes a newly developed modification to the FDS measurement technique. In Part I of this two-part series, we propose reducing the measuring time by measuring multiple sinusoidal oscillations simultaneously. Discrete Fourier transformation is used to separate the individual oscillations in the frequency domain. The capacitance and dissipation factor (tan  delta) are then calculated for each individual oscillation. The proposed advancement described in part I reduces the measuring time up to 73%.