# Sensor Technologies for Transmission and Distribution Systems: A Review of the Latest Developments
## 3 Key Transmission and Distribution System Components
### Overhead Line Insulators  
The major functions of overhead line insulators include reinforcing mechanical strength to the power conductors to withstand external loads and provide a high resistive path to the leakage currents. The most used types are pin type insulators, suspension type insulators, strain insulators, shackle insulators, disc insulators, stay insulators, etc. Overhead line insulator faults include the puncturing of insulators and their degradation owing to aging, mechanical stress, electrical stress, and atmospheric pollutants that can cause dielectric breakdown leading to flashover [83]. Various sensors have been deployed to monitor the above-mentioned failures in the overhead transmission line insulators. Traditionally, the monitoring of aging in the commonly used silicone rubber insulators was conducted by analyzing the shape of water droplets, known as hydrophobicity analysis, or by measuring the leakage current flowing through it [84, 85]. These types of fault analysis are being conducted by infrared thermography [83] that detects the temperature rise of the insulator resulting from partial discharge. It can also detect voids, cracks, and tracking damages. The electric field method was previously used to detect defects but mostly includes the insertion of wires and hence was rejected due to its destructive methodology [86]. Also, the diagnosis of faults in the insulator becomes difficult in humid or polluted conditions with these methods. Hence, they are being replaced with nondestructive methods such as X-rays for obtaining an integral view of interested parts or a whole insulator. However, the degree of absorption of X-rays is dependent on the density, consistency, and the thickness of the detected objects and thus it is difficult to detect smaller cracks and defects. Computer tomography was also used for the detection of smaller cracks and defects. Computer tomography detects air bubbles in glass fiber reinforced polymer cores of hollow-core composite insulators [87]. Scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermally stimulated currents are also conducted for the determination of aging [88]. However, these methods can examine only very thin layers and fail to measure the deeper layers under the surface. Recently, for accurate estimations, sensors are being used to sense the change in the crystal lattice of the structures and perform material analysis at microscopic levels. Magnetic sensors such as TMR sensors [34] are used to detect the arcing and chattering phenomenon of insulators which is an indication of wear and tear due to aging. A novel nuclear magnetic resonance sensor is employed for 1D depth profile measurements to estimate the aging behavior of a silicone rubber insulator [89]. However, these sensors employ weak static magnetic fields that cannot penetrate multiple depths. Ref. [90] uses unilateral magnetic resonance sensors to excite multiple layers and gradients of the rubber with a static magnetic field of constant gradient and a radio frequency magnetic field of fixed frequency to detect the complete change in lattice structure due to aging. Excessive electrical stress can cause the flow of leakage currents in insulators, monitoring which becomes essential for the early detection of emerging faults such as high voltage build up, flashovers and insulation material degradation. Fiber optic sensors have been used for measuring insulator leakage currents for high voltage transmission lines [91]. Here, sensors employ highly efficient LEDSs that are driven by the leakage currents flowing from high voltage lines to the ground for its detection. le. However, these sensors have several side-effects such as optical power drift due to temperature and aging, modal instability, and macro-curvature losses.