Power transformers are crucial in an electric power system. Failures in transformers can affect the quality and cause interruptions in the power supply. Partial discharges are a phenomenon that can cause failures in the transformers if not properly monitored. Typically, the monitoring requires high-cost corrective maintenance or even interruptions of the power system. Therefore, the development of online non-invasive monitoring systems to detect partial discharges in power transformers has great relevance since it can reduce significant maintenance costs. Although commercial acoustic emission sensors have been used to monitor partial discharges in power transformers, they still represent a significant cost. In order to overcome this drawback, this paper presents a study of the feasibility of low-cost piezoelectric sensors to identify partial discharges in mineral insulating oil of power transformers. The analysis of the feasibility of the proposed low-cost sensor is performed by its comparison with a commercial acoustic emission sensor commonly used to detect partial discharges. The comparison between the responses in the time and frequency domain of both sensors was carried out and the experimental results indicate that the proposed piezoelectric sensors have great potential in the detection of acoustic waves generated by partial discharges in insulation oil, contributing for the popularization of this noninvasive technique.

Piezoelectric diaphragms are low-cost acoustic components commonly used in a wide variety of electronic devices, where the typical function is to generate an audible alarm. However, scientific study concerning the use of these components in advanced applications has increased in recent years because of their low cost and widespread availability in several standard sizes. Based on a growing interest in these components, we propose a basic equivalent electromechanical circuit considering a 1-D vibration assumption of piezoelectric diaphragms for structural health monitoring applications based on the electromechanical impedance (EMI) technique, which was used in this paper to assess the sensitivity of piezoelectric diaphragms to structural damage. The proposed circuit is a three-port type and is suitable for small metal structures. The sensitivities of three diaphragms of different sizes to structural damage were assessed using the proposed equivalent circuit and compared with the sensitivity of a conventional transducer commonly used in the EMI method. In addition, tests were performed on aluminum bars to experimentally validate the equivalent circuit, and the theoretical and experimental results correspond well.

This paper presents a performance assessment of macro fiber composite (MFC) sensors for measuring acoustic emission (AE) signals from partial discharges (PD) in power transformers filled with mineral oil. MFC sensors are low-profile and flexible, allowing them to be attached to uneven surfaces, such as a transformer wall. Two types of MFC sensors were assessed: P1 (d33 effect) and P2 (d31 effect), which are optimized for different deformations in the structure, such as elongation and contraction, respectively. In addition, a conventional AE sensor, R15I-AST model from Physical Acoustics South America, was also used as a reference for comparative analysis. Four metrics were applied to the signals: root mean square, energy criterion, Akaike criterion, power spectral density, and correlation. The experimental results indicate a high similarity between the MFC sensors and the conventional AE sensor, which expands the research field in acoustic PD measurement in power transformers by using low-cost and flexible sensors.

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