## 2 Literature Review  
### Transformer Maintenance: A Review  
In the power industry, the field of maintenance expertise has started to change in recent years. It is customary to immediately shut down the transformer when a fault is found in the production area to prevent negative impacts; nevertheless, doing so typically requires significant time and financial loss and is best carried out at opportune times [8]. Utility owners have therefore developed a new maintenance action plan to prevent malfunctions and minimize financial losses. Maintenance can be divided into two types of action plans: proactive and reactive. While post-fault diagnostic conditions and economic losses are present in a reactive action plan, the major goals of a proactive maintenance action plan are to save costs and keep the system operating at its peak capacity. As a result, prognostic (i.e., predict and prevent) techniques have replaced diagnostics (i.e., fail and fix) procedures in the maintenance action plan [9; 10; 11]. Utility owners can use sensors and digital platforms to gain insights about the state of their transformers as part of the Fourth Industrial Revolution. By converting these data and comparing them with other anomalous circumstances, we can forecast both the remaining operational lifespan of the transformer in use, as well as future anomalous events. Utility owners can schedule maintenance by providing this information to the relevant departments, allowing them to resolve transformer failures that are unavoidable and prevent system breakdowns from occurring. This is a thorough examination of current and relevant research works. A wireless transformer monitoring parameter was proposed by the authors in [12; 13; 14; 15; 16; 17; 18; 19; 20]. This system's primary goal is to monitor and regulate the transformer using an RF module. Monitoring and controlling are made possible by temperature sensors, microcontrollers, and wireless communication technology, or RF transmission. Voltage, current, and temperature are the three transformer parameters that are being observed. The tracked data are then sent to a remote location after that. Data are transmitted between the client and the RF via the controller. In [21; 22; 23; 24; 25; 26; 27; 28; 29; 30] GSM technique is proposed. For systems with protection functions, this approach is effective. These technologies were developed earlier and are based on communication. In the event of network disruptions, the recommended GSM strategy can increase the system's dependability. Communication is accelerated using the GSM technique, which is independent of the distance parameter [31]. The Internet of Things could be used in conjunction with a distribution transformer monitor and control system, according to authors in [32]. The graphical signatures of the oil temperature, winding temperature, ambient temperature, current, voltage, and oil level were examined using ThingSpeak. Seven embedded sensors, a microprocessor, and ThingSpeak, which is utilized for processing, displaying, and storing, made this possible. To monitor the five power transformer parameters' current, voltage, oil level, winding temperature, and ambient temperature, the authors in [33] looked at a tool for creating graphical signatures. This was made possible in [34], by permitting early fault detection readings. Operators were alerted by Pushing Box when any parameter exceeded pre-established limits. Using the IoT, the study in [35] created the best possible protection for a power transformer. Two embedded sensors were used with the ESP32 microcontroller. Specifically, temperature and flow sensors were employed. The system proposed by [36] sends SMS messages to a central database for further processing via a GSM modem. A GSM modem, several sensors,and a chip microcontroller are combined by the system to create an online monitoring system.