Table 2.

Paper Screening Methodology.

Search Index	Specific Content
Research Question	How can ageing detection in transformer oil systems be improved?
Database	RefWorks Proquest, Elsevier Science Direct, IEEE Xplore, Google Scholar, GCU. Library (host to many databases)
Article Type	Scientific articles published in peer-reviewed journals and conferences, technical papers, patents, and generic materials relevant to the field.
Search Strings	HV, Insulator, Ageing, Sensor, Transformer Oil, Superhydrophobicity, IoT
Search Language	English
Research Theme Result Ratio	96 out of 182
Screening Procedure	Relevance to research topic/question(s) judged progressively by the contents of the title, abstract, conclusion/discussion, introduction, and methodology.