This paper focuses on the frequency analysis of acoustic signals produced by partial discharges (PDs) in insulation oil and the positioning of the PD occurrence for application in the diagnosis of oil-insulated transformers. Three types of electrode systems; the needle-plane, the plane-plane, and the wire-wire structures were assembled to simulate the partial discharge in insulation oil. A low-noise amplifier and a de-coupler were designed to detect the acoustic signal with high-sensitivity. The frequency ranges of the acoustic signal were 60-270 kHz in the needle-plane electrode system, 45-250 kHz in the plane-plane electrode system, and 50-180 kHz in the wire-wire electrode system. Their peak frequencies were 145 kHz, I IS kHz and 121 kHz, respectively. The position of the PD occurrence was calculated from the time difference of arrival (TOA) using three acoustic emission (AE) sensors. The position was found within a 1% error in the experimental set-up. (C) 2008 Elsevier B.V. All rights reserved.