On-line dielectric response measurements on oil paper insulated transformers

Dielectric response measurements are an important maintenance and diagnostic tool for large oil filled power transformers. There are numerous operational disadvantages to performing dielectric response measurements offline, such as the need to disconnect the transformer from the grid, the time taken for the tests and the risk posed to the network. There is therefore a need to develop techniques to perform dielectric response measurements online. This paper presents the results from experimental work conducted in developing a method for performing Frequency Domain Spectroscopy (FDS) measurements on an energized three phase transformer in the laboratory. A chirp excitation voltage signal is used in this work to reduce the measurement time. Extraction of the magnitude and phase information, from the chirp signal, is accomplished using a chirp-matched fitter. Signal injection onto and out of the transformer under test is achieved via neutral connections or “virtual” neutral connections in the set-up. The experimental work establishes the feasibility of using the FDS test on an energized transformer.