# Detection Method of Partial Discharge on Transformer and Gas-Insulated Switchgear: A Review
## 4 Partial Discharge Measurement Using Different Types of Sensors on the Power Transformer and GIS
### Mechanism in Optical Sensors  
Fiber optic cable has long been utilized as a sensor because of its many advantages, including its resistance to protection against chemical decay, electrical sparks, a wide variety of measures, and response, tolerance to high sensitivity, wide bandwidth, extreme temps, and compact size. Detection of optical properties such as wavelength, intensity, polarity, and phase are based on minute changes in optical properties. As a result, four types of optical sensors can be utilized for various applications, including spectral analysis, intensity measurements, polarization, and interferometry. The fiber optic acoustic sensor is one such sensor that combines acoustic and optical technologies. It includes a fiber optic intrinsic transducer, such as a Mach-Zehnder or Michelson interferometer, multimode fiber, and a fiber optic external device, such as a Fabry-Perot interferometric sensor. The detection method of the fiber optic acoustic sensors relies on leveraging the photo elastic effect of silica fiber to convert acoustic signals into optical signals. When a sound wave intersects with fiber optics, its shape is distorted. This distortion will influence fiber length and refractive index. As a result of this change, a passing laser beam may undergo some modulation. The silica fiber's low photo elastic effect to be increased in improving the sensitivity. PD Detection in Transformer  
In general, PD originated inside a huge oil-filled power transformer, where its detection is notoriously difficult to be performed by using an external acoustic sensor. To spot and determine the PD with sufficient sensitivity, an acoustic sensor, for example, a fiber-optic coil, is required. A previous approach to acoustic detection involved utilizing fiber optics essential interferometers, such as the fiber Michelson and Mach-Zehnder interferometers. These fiber sensors employed a single mode fiber and laser technology. A fiber coupler divides the light into two strands with intensity differences of 3 dB from the original source. One fiber serves as a reference, while the other as a sensor. To generate noise signals, either transmission, as in the Michelson interferometer, or reflection, as in the Mach-Zehnder interferometer, merges the light from the two shafts. Although the source light is in the reference shaft, the sensor arm is vulnerable to PD-induced sonic wave disturbance. The sonic wave will have affected the initial light source as viewed by the detecting arm. The essential fiber interferometer sensor achieves excessively high sensitivity when a long fiber is utilized in the detection procedure. In Figure 17, an illustration of the experimental setup is shown for monitoring PD signals using a submerged coil of fiber optic sensor into oil inside the transformer. The light source in the system is a single-mode optic fiber, an optoelectronic transducer that converts a light beam into an electrical signal, two electrodes, and a high voltage input to generate simulated PD events that would produce acoustic emission. The phase of the optical signal would alter if an acoustic wave struck the fiber optic sensor coil. As a by-product of consecutive phase differences, the visible light in the sensor coil has the tendency to be modulated. The electrical signal created by the sensor coil's light source is then amplified and inspected [125]. The Fabry-Perot interferometric sensor has a wide range of applications, including the detection of acoustic waves generated due to PD events [126]. It consists of two reflective surfaces that form a small sensing element known as Fabry-Perot cavity. The operational principle of the Fabry-Perot interferometer sensor is depicted in Figure 18. The 22 coupler was used with a light source. A single-mode fiber is attached to one of the coupler arms and fused to the sensor head. The optical signal is transformed into an electrical signal via a photodetector connected to the opposite arm simultaneously.