# 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
### Acoustic Method  
The mechanical explosion that causes AE in the power transformer is caused by the evaporation of oil in the area of the streamer, the electrical arc, and the mechanical vibration [85]. Ultrasonic signals, with a frequency of 40 kHz to hundreds of kHz [86; 87], can be used to estimate the location of the AE source, which sends off pressure waves with characteristics exclusive to that source. Nevertheless, the signals of PD can be affected by high-frequency signals and can be removed if using a denoising technique. The AE approach has become the preferred technique to detect PD in power transformers due to its effectiveness and low cost in eliminating electromagnetic interference. The break down of the components of the recording system which have been commonly used by the power transformer during normal operation to identify AE signals from PD is shown below [88]. Many ultrasonic systems use broadband piezoelectric transducers as their transduction element. These AE signal detectors are attached to the transformer tank at a certain point using a magnetized holder. The AE signals are then transferred into the AE analyzer, where they are amplified, filtered, and recorded. Multiple PD causes can be identified using the AE technique [86]. The AE method is often used in conjunction with others, such as UHF, optical detection, and electrical detection, to compensate for its limitations in PD-level detection and calibration. Complex acoustic emission behavior, weak detected signals, and a high price tag are all downsides of this approach. Microphones [89], piezoelectric transducers [86], acceleration sensors [86], and fiber optic (FO) sensors [90] are all examples of equipment that can be used to detect AE. Fiber optic sensors have been shown to be the most effective AE detection devices because of their high signal-to-noise ratio (SNR) and their ability to detect over a wide acoustic field. The internal design of the transformer can be optimized and noise reduced using denoising techniques, allowing for the detection of several PD sources. The biggest issue with the AE method is the inability to pinpoint the PD source in the transformer winding due to the quick attenuation of the signal as it travels through the various media [91]. In summary, we can break down the components of the recording system which have been commonly used in power transformer monitoring to identify Acoustic Emission (AE) signals from partial discharge (PD) during normal operation. 1. Test Transformer: This is a specific type of transformer used for testing and monitoring purposes. It is designed to mimic the behavior of the power transformer under normal operating conditions;
2. Transducer: The transducer is a device that converts the acoustic signals generated by the partial discharge events in the power transformer into electrical signals. It  
Figure 4: Gas chromatograph in its usual form [80]. effectively picks up the acoustic emissions and transforms them into measurable electrical signals;
3. Preamplifier: The electrical signals from the transducer are typically weak and need to be amplified for further processing. The preamplifier is responsible for boosting the weak signals to a level suitable for subsequent stages in the recording system;
4. Track Signal and Filter: After amplification, the AE signals may contain unwanted noise or irrelevant frequencies. The track signal and filter stage are used to filter out unwanted frequencies and ensure that the signals of interest are properly isolated;
5. Broadband Amplifier: The filtered AE signals are further amplified using a broadband amplifier. The amplifier boosts the desired AE signals to a level suitable for accurate measurement and analysis;
6. Measurement and Card or Analyzer: This is the core component responsible for processing and analyzing the amplified AE signals. It may be a dedicated hardware card or a specialized analyzer device designed to detect and analyze AE signals from partial discharges;
7.