# Detection Method of Partial Discharge on Transformer and Gas-Insulated Switchgear: A Review
## 2 Method of PD Detection Based on Different Types of Signals Emitted during PD Activities  
### Electromagnetic Method  
[PERSON] was the first to apply the EM method to a power transformer in 1997 [58]. UHF signals can be detected using conical, spiral, and Vivaldi antennas [26; 59]. UHF sensors are the subject of extensive research since they have benefits that include immunity to low-frequency signals, an insignificant impact of signals caused by internal transformer construction, and the absence of corona-free pulse interference [34; 60]. Radio interference and switching events might make UHF detection more difficult.  
For its measurements, the UHF electromagnetic technique depends on the PD activation of electrical resonance at frequencies up to 1.5 GHz. This technique can also detect and pinpoint a PD source [61; 62]. The UHF method offers multiple benefits; for instance, low decibel levels as a sequence of the transformer's shielding effect and extremely low signal losses. The measurement frequency band of 100 MHz of such a UHF process fits precisely between 300 and 1500 MHz of the entire wavelength range, permitting it to avoid local interference over the entire range. Because the UHF sensor is linked to the transformer, this technology is noise free. The secondary winding is safe and dependable against induced current since there is no electrical interrelation between a power transformer and a UHF sensor. The secondary winding of a power transformer is safe and reliable against induced current.  
Figure 2 is a circuit diagram for a power transformer that was used in [63] to analyze the impact of various PD types on UHF calibration. Researched causes of PD include surface discharge on polyethylene, surface discharge on pressboard, internal discharge, and corona discharge. There are six drain valves that facilitate the placement of various UHF probes. The optimal detection frequency range for measuring the UHF signal in terms of PD activities was introduced by the authors in [63]. They also showed how inefficient the UHF probe was because of the difficulty in reducing calibration errors caused by active transformer components. However, a maximum charge estimation approach was presented in [63], where the UHF quantifier parameter and IEC apparent charge are measured in the laboratory to achieve the least feasible ratio.  
Figure 2: Schematic depicting the effects of various PD types on UHF calibration in a power transformer reprinted by [63].  
Extensive research has been conducted on various types of current transformers, such as Rogowski coils, high-frequency current transformers (HFCTs), and radio frequency current transformers (RFCTs), for their application as PD detection sensors in power transformers [64; 65; 66]. The electromagnetic (EM) technique has proven effective in localizing multiple PD sources and identifying their unique characteristics through the use of feature extraction and denoising strategies. These strategies are used in PD detection through the EM signal approach to accurately identify and analyze PD signals in power transformers.