Investigations of In-oil Methods for Pd Detection and Vibration Measurement

Usually, acoustic partial discharge (PD) and vibration measurements take place on the outside tank wall of power transformers. This contribution determines new approaches for acoustic PD measurements within the oil volume of the transformer combined with electromagnetic in-oil measurements. Scope of the work is an acoustic and ultra-high frequency (UHF) detection of PD and additional recording of transformer vibrations during service. For in-oil measurements a combined sensor is presented, which allows simultaneous detection of all three signals. After a short introduction of measurement principles, the requirements of a combined in-oil sensor are defined including mechanical demands and signal processing. A diplexer integrated into the combined sensor is introduced to separate the considered frequency spectra. Tests in a laboratory setup with a first prototype estimate if these requirements are met. Therefore, the setup uses reproducible, synthesized PD signals generated by an acoustic signal generator and an UHF pulser. Second measurements use a needle-sphere electrode emitting realistic UHF and acoustic signals. Vibrations are simulated with a small winding, which is mechanically excited. The combined sensor is compared to the UHF antennas and acoustic accelerometers. Finally, the influence of the electric field strength being unavoidable at inside-tank measurements is evaluated.