## 1 Introduction  
The primary goal of the Internet of Things (IoT) is to link physical objects with digital world together so that they can exchange data and communicate. The IoT involves embedding sensors and actuators in physical objects (including nature, people, and animals) [1]. These sensors monitor changes (e.g., temperature, motion) and communicate via wired or wireless networks with computing systems [2]. The phrase \"Internet of Things\" originated in 1999 from [PERSON], a British computer scientist. [PERSON] suggested embedding radio-frequency identification (RFID) computer chips to track products within a supply chain. The Internet of Things has made big breakthroughs in industries like pharmaceuticals and smart homes. Previously, only computers, smartphones, and tablets had access to the Internet; now, the IoT allows devices to be connected to the Internet to exchange data. The number of devices connected to the Internet is rapidly increasing. In [3] it is stated that at the end of 2023, there were approximately 16.1 billion connected IoT devices,which is expected to rise to 39.9 billion by 2033. Condition monitoring of transformers is crucial to ensure their reliability and prevent unexpected failures [3]. Current practices require that a team of technicians be dispatched on-site to manually assess the condition of transformers; this process is subject to human error and hampers continuous condition monitoring. To achieve their return on capital investment, every owner of a transformer wants to see that such crucial equipment runs as efficiently and productively as possible. Historically, transformer owners have maintained this equipment using predictive and corrective maintenance techniques. Although it requires utility in labor, downtime, and non-routine maintenance requirements, corrective maintenance ensures that transformer parts can be used without reservation. By carrying out preventive maintenance, which involves determining the lifespan of specific parts and carrying out maintenance before actual breakdown occurs, several utility owners have been able to utilize transformers for long periods of time. Although the high costs of labor, planned downtime, and underuse of the component during its operational lifespan persist, this strategy prevents unanticipated and catastrophic failures. Utility owners are now leaning toward developing a predictive maintenance model that improves the maintenance cycle and follows a corrective and preventative maintenance approach, thanks to the development of the Internet of Things (IoT). Just-in-time component replacement is a strategy used in predictive maintenance. By reducing unplanned maintenance and labor expenses, this technique extends a component's lifespan, in addition to replacing components that are closer to failure. It was not until 2014 that early research on the application of IoT technology, specifically in transformers, began to surface [4; 5]. The evaluation of IoT applications' timeliness, prediction, and visibility in comparison to traditional monitoring systems was a specific focus of this review. A total of 262 academic publications that were published between 2014 and 2024 were evaluated based on predefined criteria. PRISMA for systematic review was employed to evaluate the 262 publications. PRISMA refers to Preferred Reporting Items for Systematic Reviews and Meta-Analyses [6]. PRISMA mainly gives recommendations for the presentation of systematic reviews that evaluate the effectiveness of interventions [7]. Some critical aspects of the PRISMA 2020 checklist were adopted and used in this review paper to conduct an effective systematic literature review. After the screening process, a total of 120 publications were considered for this review. A detailed examination of the primary research elements of IoT technology in transformer condition monitoring, including the type of transformer or physical item, the examined transformer aspect, the microcontroller, the communication channel, and the cloud, was conducted.