## 4 Results and Discussion
### Study Selection Results  
A Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) flowchart that was suggested for the literature search and filtering phases of this systematic literature review (SLR) is shown in Figure 4. With the use of the keyword search terms tabulated in Table 3, the published papers were gathered from online repositories with good reputations. From these papers, data extraction was performed as discussed in Section 3.5. The inclusion and exclusion criteria were strictly applied and followed. After obtaining 262 publications, their topics and abstracts were examined. Figure 3 in Section 3 illustrates the results, with 62 articles from IEEE XPLORE, 60 papers from Google Scholar, 21 papers from Science Direct, 15 papers from Emerald Insight, 11 papers from Springer Link, 46 papers from Wiley Online Library, 27 papers from Taylor and Francis Online, 13 papers from ELESVIER, and 5 papers from other sources making up the obtained papers. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Ref. ** & **QA1** & **QA2** & **QA3** & **QA4** & **QA5** & **QA6** & **Total** & **\%** \\ \hline
[80] & 1 & 1  
As a result, 187 papers that passed the first screening stage after the screening were based on the titles and abstracts of the published papers. The remaining publications underwent a full-text review and met all the eligibility requirements included in this systematic review, while the remaining publications were eligible for full-text review. A total of 67 papers were eliminated after the full-text review was conducted, leaving 120 papers selected for this review. The 67 papers were disqualified because the studies lacked detailed information on research methodology or because they only examined the same parameters of the transformer as other published papers. Rejected papers were categorized as below 75% based on the qualifying requirements, whilst accepted papers were categorized as 75% and above. Papers that achieved 92% and above from the qualifying assessment criteria were categorized as very high level, whereas papers that achieved 75% and above were categorized as high level. Papers that achieved below 75% were rejected, and the papers that satisfied all four exclusion criteria were categorized as low level, while those that fulfilled only three were categorized as extremely low level. The quantity of papers that satisfied the inclusion and exclusion criteria is shown in Figure 4.  
The initial identification search commenced with a keyword search, as shown in Table 3, and the year of publication. The steps in the procedure were to use keyword searches for search queries, assess the standard of the published literature, identify the most important research, gather data gathering, record the review, and, finally, determine eligibility. Moreover, as it immediately defines the reference framework for this study,  
Figure 4: Proposed PRISMA flowchart. the creation of the research topic is a crucial step in the SLR process. Keywords that were applied during this stage are the Internet of Things, transformer online condition monitoring, transformer fault diagnosis, IoT review, and IoT survey. The selected paper cluster types were conference papers, journal articles, and review articles. The selected years ranged from 2014 to 2024. The identification stage discovered a total of 262 articles. The eligibility part incorporated an extraction process. During the exclusion part, to adopt and implement a selection criterion that is valid, full-length papers from journals with a high reputation were considered. This stage produced 187 papers. The last part of the screening process involved quality assessment and classification based on IoT system architecture, microcontrollers, adopted communication protocol, IoT cloud platform, security, and challenges. Finally, the inclusion/exclusion criteria were applied, and quality assessments were conducted. The last part of the screening produced 120 papers.