## 1 Introduction  
Power transformers are considered important equipment in power systems. Based on the transformer failure statistics in CIGRE A2.37, the most common cause of failure in power transformers is represented by winding and insulation faults [1]. Therefore, the online monitoring of power transformer insulation systems could avoid failures and the resulting catastrophic chain of events that increase destruction. One of the best solutions to monitor transformers is partial discharge (PD) measurement. PDs that occur in high-voltage equipment are electrical discharges that happen at points with poor insulation or a high electrical field and only partially bridge the insulation between the conductors; they may or may not occur adjacent to a conductor [2]. The PD phenomenon has several effects, such as electrical, electromagnetic, chemical, acoustic, and optical effects, and can be detected using specific sensors. In some studies, High-Frequency Current Transformers (HFCT), High-Voltage Coupling Capacitors, Ultra-High-Frequency Antennas (UHF), and acoustic sensors have been used for PD detection and measurement [3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. Based on the type of equipment that is monitored, the choice of the appropriate method can be crucial for detecting faults and avoiding false alarms. The method based on acoustic wave propagation, also known as the acoustic emission (AE) method, is a non-destructive testing (NDT) method and has the advantage of being non-invasive to the monitored equipment. AE can also be used for the localization of faults inside transformers. An advantage of acoustic detection with respect to electrical methods is its immunity to on-site electromagnetic interference (EMI). Immunity to EMI makes acoustic detection ideal for online PD detection and monitoring. Its better signal-to-noise ratio (SNR) for the acoustic signal leads to fewer false alarms. The AE method is based on detecting transient elastic waves generated by a rapid release of energy caused by the PD. The spectrum of the resulting acoustic signal can extend to several MHz [13]. These acoustic signals are detected by AE sensors located on the external wall of the transformer. However, the main disadvantage of this method is the high cost of commercial AE sensors (from 100 to 1000 dollars [14]), especially when multiple sensors are required. In this context, the objective of this paper is to present a low-cost, non-invasive method for PD detection in power transformers by applying low-cost piezoelectric diaphragms, commonly known as buzzers. This study uses two low-cost piezoelectric sensors, the lead zirconate titanate (PZT) and the microfiber composite (MFC). Several studies have previously used these sensors for PD detection and measurement. In ref. [14], a comparative study between the low-cost piezoelectric diaphragm (PZT) and a commercial AE sensor (RS15I-AST) was done. A comparison of the responses in the time and frequency domains of both sensors was carried out. The experimental results indicated that the proposed piezoelectric sensors had great potential for detecting acoustic waves generated by PDs in insulation oil. In ref. [15], a microfiber composite (MFC) and the AE sensor (RS15I-AST) were studied through parameters such as root mean square (RMS), energy criterion, Akaike criterion, power spectral density (PSD), and correlation. Most articles that showed the effectiveness of these low-cost piezoelectric sensors only analyzed the signals in the frequency-domain mode, which may hide useful information such as repetitive faults [16; 17]. In ref. [18], a comparative study between the low-cost PZT and the MFC was performed only through time-frequency analysis and RMS of the signals. The purpose of this paper was to conduct an in-depth analysis of the PD phenomenon through a comparative evaluation of captured PD signals from two low-cost piezoelectric sensors, via time-domain, frequency-domain, time-frequency, and statistical analysis.