Overview of Insulation Diagnostics

A number of chemical‐based diagnostic techniques are used extensively by practicing engineers and specialists to assess the insulation condition of aged transformers. Most commonly used chemical‐based diagnostics include moisture analysis in transformer oil, dissolved gas analysis (DGA), furan analysis by high‐performance liquid chromatography (HPLC), and molecular weight assessment by degree of polymerization (DP) measurement using the viscometry method. Among electrical‐based diagnostics, insulation resistance, power frequency dissipation factor, polarization index, and partial discharge (PD) measurements have been used widely. The monitoring of furanic compounds by oil analysis using HPLC has been used widely by utilities for the last few decades. Insulation resistance and polarization index have long been used by the electricity utilities to ascertain the moisture condition of transformers. The capacitance of a transformer depends on the characteristics of the dielectric material and the physical size of the insulation system. The capacitance value can change by degradation of the insulation, contamination, and physical damage.