Best practices for Dielectric Frequency Response measurements and analysis in real-world substation environment

Modern technology and developments in signal acquisition and analysis techniques have provided new tools for transformer and bushing diagnostics. Of particular interest are dielectric response measurements where insulation properties of oil-paper systems can be investigated. Dielectric Frequency Response, DFR (also known as Frequency Domain Spectroscopy, FDS), was introduced more than 20 years and has been thoroughly evaluated in a number of research projects and field tests with good results. DFR data in combination with mathematical modeling of the oil-paper insulation is proven as an excellent tool for moisture and oil conductivity assessment of power transformers. Dielectric response measurements are usually performed at a much lower voltage level than traditional power frequency tan-delta measurements. Due to this, the signal-to-noise ratio may sometimes be extremely low, especially at low frequencies and when measuring low capacitance objects e.g. bushings and instrument transformers. The interference suppression capability of the test set thus becomes an important parameter when considering different methods and instruments. In this paper investigation of the electromagnetic interference in terms of AC hum currents, induced DC currents and low frequency interference in AC as well as HVDC substations are presented. Solutions to handle these different types and levels of interference are investigated and examples of measurements under those conditions are presented. The paper also covers DFR response analysis using the XY-model. Numerical analysis using COMSOL has been performed and compared with the simplified analytical XY-model to investigate how non-ideal conditions influence geometry parameters and results.