A Systematic Approach to Assess the Insulation Condition of Liquid-Immersed Transformers in Time and Frequency Domains

The need to assess the condition of solid and liquid insulation in transformers is part of the history of transformer development, research, and evolution. As time passed, more applications were developed, and engineering designs were accompanied by computer technology perfecting and improving the manufacturing process with the main focus of providing a solid, reliable, and efficient machine capable of withstanding normal operation conditions and some anomalies expected in the operation of an electrical power system. For over 130 years, the use of time domain insulation diagnostics has been a fundamental way to assess the condition of the transformer insulation and it is about a century of the use of frequency domain techniques. As materials and designs progressed, the metrology and data processing capabilities of testing instruments have been evolving in parallel. Although testing equipment, materials (solid and liquid), and engineering design have reached a prominent level of complexity, some practical guides need revision to be up to date with the existing technology. Throughout this document, the authors provide a review of technical references, address potential discrepancies, and take the reader into a set of procedures and testing best practices that allow obtaining reliable and repeatable results, therefore, a better understanding of the condition of the insulation in liquid-immersed transformers. The analysis starts with the typical insulation resistance test (time domain dielectric response) where the use of a guard is thoroughly explained. Later, the analysis of the dissipation factor at line frequency as well as at a different spectrum of frequencies is described (frequency domain dielectric response). The outcome leads toward practical recommendations using laboratory analysis of oil samples (quality tests and dissolved gases analysis) or adding online monitors to alarm or evaluate the condition of a transformer in the field. The document is written considering the literature review and field experimental data.