# Detection Method of Partial Discharge on Transformer and Gas-Insulated Switchgear: A Review
## 3 Type of Sensor Used to Detect PD Signals  
### Electrical-Electromagnetic Sensor  
PD is an electrical discharge occurrence characterized by a spark or discharge that occurs when a small section of insulation is partially bridged. It happens when two conductive electrodes with high densities of positive and negative charges become separated. When the applied voltage exceeds a specific critical value called PD inception voltage, transient gas ionization leads to a localized discharge within the insulated system. PD is considered a pre-breakdown phenomenon. PD occurs on micro-/nano-sized voids/cavities/gas-contained \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Electrical Detection [102]** & **Chemical Detection [103]** & **Acoustic Detection [104]** & **Optical Detection [105]** & **UHF Detection [106]** \\ \hline \multirow{4}{*}{Advantages} & High sensitivity to detect & Good recording & \multirow{2}{*}{Non-intrusive technique} & High sensitivity to detect & High sensitivity \\  & low-level PD signals & of PD signals in the & & low-level PD signals & to detect PD signals \\  & laboratory environment & & & & \\ \cline{2-6}  & Suitable for & Suitable for & \multirow{2}{*}{Can be used for offline and online monitoring} & Suitable for & Suitable for \\  & online monitoring & & & & online monitoring \\  & & and condition assessment & & & \\ \cline{2-6}  & Can be integrated into & \multirow{2}{*}{Provides indirect evidence} & Suitable for large-scale & \multirow{2}{*}{\begin{tabular}{c} learning to \\ simulation systems \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} Can be used for both \\ electromagnetic \\ interference \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} Can be used for both \\ localized and distributed \\ PD detection \\ \end{tabular} } \\  & & & & \\ \cline{2-6}  & Provides quantitative & & \multirow{2}{*}{\begin{tabular}{c} Provides location \\ information \\ \end{tabular} } & & & \\  & measurements & & & & \\  & of PD characteristics & & & & \\ \hline \multirow{4}{*}{Dssadvantages} & Affected by noise and & Indirect detection & \multirow{2}{*}{\begin{tabular}{c} Affected by \\ method, limited to \\ specific types of PD \\ \end{tabular} } & & & & \\  & & & & & \\ \cline{2-6}  & Require complex signal & \multirow{2}{*}{\begin{tabular}{c} Requires sampling of \\ processing techniques \\ \end{tabular} } & Requires sampling of & \multirow{2}{*}{\
\begin{tabular}{c} Limited sensitivity \\ to detect low-level \\ PD signals \\ \end{tabular} } & Requires line-of-sight & Signal interpretation may \\  & & & & & \\  & & & & & be complex \\ \cline{2-6}  & & Requires specialized & \multirow{2}{*}{
\begin{tabular}{c} Requires installation \\ of UHF sensors \\ on the equipment \\ \end{tabular} } \\  & laboratory equipment & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the advantages and disadvantages of different partial discharge detection methods. Figure 6: A DGA system utilizing photo-acoustic spectroscopy [96]. bubbles even though the power transformers still can operate. Without PD detection/PD mapping/preventive action, the PD activities keep occurring on the power transformers making the voids/cavities/gas-contained bubbles more significant, and eventually forming a conductive path that completely bridges the two conduction parts; eventually, leading to the major breakdown. The identification and detection of partial discharge (PD) in power transformers are vital in industrial and power delivery systems to prevent the failure of high-voltage equipment [107; 108]. The use of UHF sensors for PD detection in gas-insulated substations (GIS) is depicted in Figure 7a. These sensors have shown to be successful in both on-site PD testing and laboratory PD measurement. The use of a UHF sensor to evaluate the compatibility of power transformers on DN50/DN80 gate valves is also demonstrated in Figure 7b.