# Environmental Effects on HV Dielectric Materials and Related Sensing Technologies
## 2 Analyses and Discussions
The works carried out employed diverse surface techniques in tandem with HV methods [74]. These offered a comprehensive understanding of the relationship between the surface properties, droplet contents and voltage parameters that lead to electrical discharges. The stages that precede flashover events are diverse, but the main ones are contamination build-up, wetting of the surface, leakage current and Ohmic heating, spot discharges, and the deterioration of insulating materials. These stages can develop over an extended period of time or can occur rapidly. They are dictated by weather conditions and the level of pollution in the environment. One of the works that investigated the dynamics of water droplets in an electric field dates back to 1917 [75]. [PERSON] used alcohol drops at the end of a tube to examine how they evolve into a conical shape from which a thread emerges before it disintegrates into small droplets under the electrical stress. [PERSON] showed experimentally that the disintegration of water droplets is due to the instability rather than the formation of ionic currents. His criterion for instability was \(V^{2}/r_{0}T=C\), where \(V\) is the potential, \(r_{0}\) is the radius of the droplet, \(T\) is the surface tension and \(C\) is a constant. Pioneering theoretical studies into the disintegration of water droplets under electric fields also showed that water droplets elongate in an electric field [76]. The study found that the droplets become unstable when their length was 1.9 times their equatorial diameter. Using a point-to-plane configuration, corona discharge has been examined by studying the disruption of water drops in electric fields and the secondary electron emission associated with this [77]. As corona discharge commonly takes place between the sheds of HV insulators, the work explored the vertical nature of this event. Furthermore, the influence of hanging water droplets on the corona activity and flashover events was assessed for ceramic insulators. The studies show that water droplets hanging on the metallic electrodes reduce the flashover voltage significantly [78]. Droplets hanging on the upper electrode have the largest effect. The presence of water only on the lower electrode also contributes significantly to the breakdown voltage. A similar behavior was observed in the case of ceramic insulators. The water droplets can reduce the breakdown voltage significantly and cause partial discharge on 22 kV rated ceramic insulators. This can cause the aging of HV infrastructure, which ultimately leads to their failure. Partial discharge (PD) in HV technology [79; 80] can cause the failure of HV components such as capacitors, cables, and inlet bushings. PD, which takes place within or on the surface of HV insulators, is a persistent problem in the power industry. Internal PD is initiated in faults such as cavities within insulating materials, while external PD occurs on the surface of insulators or at the metal-insulator interface. Both kinds of PD can be described by a similar electrical discharge process. This discharge is known as Townsend electron avalanches, which can develop into streamers and micro-sparks [81; 82]. Diverse electromagnetic (EM) techniques were proposed for the detection and monitoring of partial discharge activity in high-voltage infrastructure [83; 84]. These methods employ EM sensors to detect the radiation emitted from PD activity and have been shown to provide valuable information on the condition of insulators. The propagation of EM radiation in isotropic media has been well explored [85]. Recent works use the finite difference time domain (FDTD) to simulate the propagation of the EM radiation emitted from PD on a distribution power line [86]. The radiation spectra along the line and in the proximity of a PD source was investigated for detection purposes. The radiation spectra of PD in dielectrics showed that the EM radiation propagates in HV insulating materials as dipoles or multipoles [87].