Towards a novel approach to the on-line diagnosis of the instrument transformer

The conventional instrument transformers (CITs) are still widely used in the power network, thanks to their high reliability, insulation capability and low drift over time and temperature. Their traditional reliability is very often used as justification for skipping a periodical calibration, which requires putting off-line the CIT, thus implying a very complex and expensive procedure. For this reason, in the last years, a growing interest has been addressed towards the diagnostic/calibration methods based on on-line procedure. The typical approach is based on the frequency response analysis that permits, under sinusoidal conditions, to detect possible deterioration of the behavior of CIT. Anyway, the real interest is to check the CIT fleet already installed and operating on the grid without requiring the disconnection from the grid. As well known the actual voltage of the grid features a not negligible harmonic distortion that combined with the intrinsically non-linearity of the CTI reduces the feasibility of the on-line diagnostic methods based on the frequency response method. This paper proposed a novel approach to the on-line diagnosis of CIT based on a non-linear simplified Volterra model of the CIT. This opens the way to a different approach to the in-site characterization process of a CIT based on the actual voltage of the grid and thus not requiring the disconnection of the CIT from the grid.