## 3 Materials and Methods
### Search Strategy  
To find publications that presented aspects of IoT-based transformer condition monitoring experiments and transformer online monitoring, a search strategy was created specifically for this review [26; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95; 96; 97; 98; 99; 100; 101; 102; 103; 104; 105; 106; 107; 108; 109; 110; 111; 112; 113; 114; 115; 116; 117; 118; 119; 120; 121; 122; 123; 124; 125; 126; 127; 128; 129; 130; 131; 132; 133; 134]. Repetitive pilot searches were utilized to choose search phrases. Terms that failed to provide articles meeting the inclusion requirements were eliminated. The synonyms of the designated terms were identified to guarantee that the synonyms of the key terms were included. Thus, the synonyms for \"transformer condition monitoring\" include \"Transformer fault detection\" \"Transformer health index\", and \"Transformer asset management\". Defining terms for the term \"Internet of Things\" were \"IoT review,\" and \"IoT survey\", relating to transformer condition monitoring. Table 3 illustrates keyword searches utilized to facilitate the selection of relevant papers.  
Since keywords are the basis for the attainable nature of relevant material, choosing the right keywords is crucial to the process of choosing academic works for this SLR. The online match found with the above-mentioned keywords provided attainability to 262 research publications. Figure 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 ELSEVIER, and 5 papers from other sources making up the obtained papers. These results comprise 142 additional study projects that were discarded. A total of 120 research articles were gathered because of this survey.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Criteria** & **Inclusion** & **Exclusion** \\ \hline Topic & Must pertain to IoT technology in electrical & Articles unrelated to IoT technology \\  & transformer condition monitoring & for transformers \\  & or fault diagnosis & \\  & Must include a research framework or & Articles lacking a clear research framework \\  & methodology & \\ Language & Must be written in the English language & Articles published in languages other than English \\  & Must be published between 2014 and 2024 & Articles published outside the specified timeframe \\ \hline \hline \end{tabular}
\end{table}
Table 2: Proposed inclusion and exclusion criteria.  
\begin{table}
\begin{tabular}{l} \hline \hline \multicolumn{1}{c}{**Keywords Search**} \\ \hline Internet of Things (IoT) & \\ \"Transformer Monitoring\" or “Electrical Transformer Condition” or “Transformer Maintenance” or “Transformer Management”. \\ “Condition Assessment” or “Transformer Health” or “Transformer Diagnostics” or “Transformer Data Analysis”. \\ “Predictive Maintenance” or “Transformer Performance” or “Transformer Reliability” \\ \hline \hline \end{tabular}
\end{table}
Table 3: Proposed keyword search.