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Much attention has been paid to different localisation algorithms to locate partial discharge (PD) sources in power transformers using the acoustic emission (AE) technique. Due to total reflection, AE sensors cannot always detect the signal of a direct wave. As a result, localisation algorithms based on an indirect wave can be more accurate. But little research has addressed arrival time (AT) methods of indirect PD signals. In this paper, a double-threshold (DT) method is introduced, and its improvement based on short-time energy and a zero-crossing rate is described. Because an indirect wave arrives earlier and has lower energy than a direct wave, current AT methods, such as the energy criterion, cross-correlation, and linear prediction residual methods, cannot determine its AT. However, the proposed DT method can determine the AT precisely. Moreover, a localisation experiment was conducted on a 35 kV transformer, and localisation results were calculated employing various AT methods and different localisation algorithms, such as the Chan, genetic, imperial competition, and improved-propagation-route-search algorithms. Characteristics of the proposed AT methods were compared and analysed to show their feasibility and accuracy.

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