## 4 Results and Discussion
### Transformer Condition Monitoring
#### 4.8.4 Partial Discharges
Table 11 tabulates different types of transformers that were mostly preferred by researchers to conduct condition monitoring, using an IoT-based system. Table 6 shows that power transformers and distribution transformers were the two transformer types being investigated by a high number of studies. The table also shows that there is a little focus on dry-type transformers and instrument transformers from the researchers. Studies [53; 64; 73; 68; 67; 94; 114; 97; 117] did not specify the transformer types being investigated in their papers. These references provided generic terms such as \"electrical transformer\" and \"transformers\". \begin{table}
\begin{tabular}{l l} \hline \multicolumn{1}{c}{**Transformer Type**} & \multicolumn{1}{c}{**Paper ID**} \\ \hline Distribution Transformer & [2; 3; 5; 7; 10; 14; 19; 25; 26; 32; 33; 37; 38; 39; 43; 45; 46; 50; 52; 54; 66; 68; 72; 74; 76; 78; 80; 82; 84; 85; 91; 92; 98; 101; 113; 118] \\ Power Transformer & [1; 4; 6; 8; 9; 11; 13; 15; 18; 20; 31; 34; 36; 40; 41; 42; 44; 47; 49; 51; 55; 63; 65; 67; 69; 71; 79; 88; 89; 93; 95; 96; 99; 100; 115; 116] \\ Dry-Type Transformer & [77] \\ Instrument Transformer & [90] \\ Not Specified & [53; 64; 73; 83; 86; 87; 94; 97; 114; 117] \\ \hline \end{tabular}
\end{table} 
Table 11: Investigated transformer types using the IoT. Figure 15: Research distribution by transformer type. Transformers are essential electrical equipment in electrical power systems and other fields. The transformer is commonly utilized for different electrical applications to resolve electrical issues [42]. There are various ways to classify transformers. Transformers can be classified based on a number of phases. Transformers can either be classified into single-phase transformers or three-phase transformers. Transformers can also be classified according to their usage, namely step-up transformers and step-down transformers. Based on their application, they can be classified into distribution transformers, power transformers, isolation transformers, etc. Transformers can be classified based on winding configurations; there are two winding transformers and an autotransformer. Lastly, transformers can be classified based on core design. There are three types of transformers based on their core design, namely shell type, core type, and berry type, with shell and core types being commonly used [71]. The different types of transformers are briefly discussed below. Power transformers are the most employed in high-voltage transmission systems. These transformers have voltage ratings of up to 400 kV. Most of the power transformers are rated 200 MVA and above [55; 51]. They are utilized in generating power stations and transmission substations that require big transformers with higher ratings. The power transformer is constructed for optimum performance and is bigger compared to distribution transformers. Distribution transformers are designed to lower voltage for delivery to customers or for business use. This transformer offers adequate voltage control and can run 24 h a day at 50 percent load efficiency. A distribution transformer, sometimes referred to as a demand transformer, converts a low- or medium-voltage supply to the voltage required for domestic appliances and machinery in factories [7]. The dry-type transformer is one that does not use insulating oil and instead uses insulating paper and natural cooling method for its windings. The windings and core are contained in a closed tank that is previously pressurized using air. They are safe to use without insulating oil [77]. Two types of dry-type transformers that are common are Cast Resin and Vacuum Pressure-Filled transformers. Instrument transformers are intended to transmit either secondary voltage or current to instruments. The power system reticulation uses high voltages and currents, which can be detrimental to instruments, such as metering instruments, and protective relays [90].