Combined In-Oil Sensor for Vibration Measurement and Partial Discharge Detection using Acoustic and Electromagnetic Emissions SEBASTIAN COENEN Siemens AG Germany

Common acoustic partial discharge (PD) and vibration measurements use sensors attached on the outside tank wall of (power) transformers. Hereby, tank structure can lead to severe signal damping which in practice often leads to acoustic PD signals below noise level. Scope of the work is an acoustic and ultra-high frequency (UHF) detection of PD and additional recording of transformer vibrations during service without tank influences. For in-oil measurements a combined sensor is developed which is sensitive in a wide frequency range. 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 frequency spectra of vibrations PD acoustics and UHF waves. Prototype tests at laboratory conditions determine sensor qualities. Therefore, the setup uses reproducible, synthesized PD signals generated by an acoustic signal generator. UHF signals come from a RF pulser. In step with actual practice, a second setup uses a needle-sphere electrode emitting both, real UHF and acoustic signals. Vibration measurements are performed with a small winding, which is mechanically excited. The combined sensor is compared to common acoustic accelerometers and UHF antennas. Finally, the influence of the electric field strength being unavoidable at inside-tank measurements is evaluated.