# Detection Method of Partial Discharge on Transformer and Gas-Insulated Switchgear: A Review
The pre-eminent realm where faults manifest is within the winding, exhibiting a likelihood of occurrence at 37.69%. Subsequently, the tap changer constitutes the second-most plausible site of fault occurrence (31.16%), succeeded by the third delineated fault location, positioned at the bushing (17.2%). Comparatively diminutive probabilities are attributed to other fault locations, including the lead exit insulation (8.96%), core and tank (3.54%), cooling system (1.12%), and current transformer (0.37%). Instances of anomalies in these latter regions are infrequent during regular operational states. Conversely, the causes of internal faults in transformers are diverse and include factors such as axial displacement, buckling deformations, disc space variation, short-circuited turns, core insulation failure, shorted laminations, open leads, loose connections, short circuits, overheating, and other issues described in references [9; 17; 18; 19; 20]. Globally, the majority of transformer faults, approximately 70-80%, are attributed to internal faults [21]. The aforementioned deficiencies generally originate as partial discharges within the insulation of the transformer, which occur swiftly and culminate in total failure. Figure 1: Power transformer failure classification [15]. The identification of the condition of a power transformer is of utmost importance in guaranteeing the uninterrupted functioning of electrical utilities. This is because the severity of the different types of defects might increase from time to time [22]. One of the measurements considered in the preventive maintenance of power transformers is the measurement of partial discharge activities. Partial discharge is a pre-breakdown phenomenon that indicates the electrical discharge that partially bridges the dielectric medium between two conduction parts. Under the influence of high-voltage stress, partial discharge (PD) can or cannot occur adjacent to a conductor [23]. As mentioned in the standard, a PD pulse refers to the current or voltage pulse that typically occurs due to a partial discharge activity within the object under test. PD usually begins due to the imperfections of the insulation medium, such as the presence of voids or gas-containing bubbles on the dielectric materials [24]. Commonly, imperfections on the insulation medium are present due to improper manufacturing processes. Chemical by-products and particle bombardment produced by PD activity also degrade the properties of the insulating medium. Protracted PD may eventually lead to insulation breakdown. Generally, the PD testing method of power transformers can be conducted either online or offline. Offline testing is preferred compared to online testing despite its lack of dependability [2]. In order to obtain reliable outcomes from the measurement of PD activities, the appropriate calibrated PD measurement system needs to be used, and it must be performed according to the standard of commercial PD measurement techniques. Nevertheless, the PD measurement technique conducted offline has limitations, especially in accounting for the authentic electrical and thermal circumstances experienced by the insulation while the transformer is in operation [25]. Therefore, in sustaining the performance of installed power transformers, online detection of the partial discharge phenomenon must be conducted accordingly. Recently, several prevalent techniques have been conducted to monitor the PD characteristics in power transformers. As stated, PD is considered to be the primary factor responsible for the deterioration of insulation in transformers, which can ultimately result in failure [26; 27]. Various techniques for PD detection have been performed in condition monitoring to detect, identify, and diagnose PD [28].