# 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 Electric-Electromagnetic Sensors  
When the insulating materials (i.e., solid, liquid, or gas) are subjected to a high electric field and the electric field exceeds the PD inception electric field, PD activities will occur rapidly [116]. The phenomenon of partial discharge can be explained according to the Townsend theory that stated the discharge mechanism happened due to the electrons emitted from the cathode having obtained enough energy in the electric field, which collides with gas molecules contained in the gas-contained bubbles. This effective collision will ionize the gas molecules and increase the number of charged particles by forming electrons and holes [117]. As the bubbles contain unknown gas, the dielectric constant is typically lower than the surrounding liquid dielectric. As a result, the electric field within the bubbles is greater than the local electric field formed on the liquid insulation. The liberated electron from the cathode will accelerate to the anode through the bubbles, and the gas molecules contained in the bubbles will be ionized as the molecules collided with the electron. This process also can be described as an avalanche process. However, the discharge process is a stochastic phenomenon because not every collision leads to ionizing the gas. This is because if the electron's kinetic energy is not enough, it could not be able to remove another electron upon collision. The electron with a negative charge will apparently accumulate on the bubble walls near the anode, while the ionized molecule with a positive charge will accumulate on the bubble walls nearby the cathode. The accumulation of electron and positive ion molecules will form two streamer channels with opposite polarity of charge. This streamer's channel formed by the transferred charge will create an electric field that opposed and distorted the local electric field produced by the external supply. This phenomenon is rapidly and continuously occurring until the electric field produces by the electron and positive ion in the void is greater than the specific value of the extinction electric field. Then, the partial discharge will be extinguished, which partially bridges the liquid dielectric between the two conduction parts [118].  
A coupling capacitor in the range of 0.1 nF to 1 nF can be used to detect the impulse produced during the discharge process. The PD-induced impulse's granularity, polarity, and arrival time are all recorded [119]. The signal produced by PD activity would have a  
Figure 11: Extrinsic optical sensors. (**a**) The Fabry–Perot interferometer sensor and (**b**) the microelectromechanical system sensor. Reprinted by [115].  
frequency between 300 MHz and 3 GHz (UHF band), given the nanosecond (ns) duration of PD-induced impulses. Detected PD signals can be considered to diagnose the insulation condition of the GIS, transformer, and cable with the right calibration technique. The mechanism of detection has a large detection range, excellent sensitivity, and low background noise [120]. Keeping an eye on PD characteristics is crucial for ensuring insulation performance [121]. The fast-rising duration of the PD current pulse (100 ps in SF6) [122] is a key factor in the efficacy of the UHF approach for measuring PD in the GIS. Commonly, a UHF sensor mounted near the drain/oil valve is capable to detect the PD signal upon its occurrence. Disc-shaped and cone-shaped sensors are typically used for onsite and laboratory measurements, whereas monopole sensor is used for the laboratory measurement. The input power source used in this technique must have a lower discharge rate than the value being measured.