# Sensor Technologies for Transmission and Distribution Systems: A Review of the Latest Developments
## 3 Key Transmission and Distribution System Components
### Overhead Line Insulators
Recent developments have been made to reduce these errors by using plastic optical fiber technology [92] that uses the LED output light modulated by the insulation leakage current to energize a plastic optical fiber that then transmits error signals via GPRS to a remote terminal unit. This type of fiber optic sensor has the advantages of immunity to electromagnetic interference with the use of plastic cladding along with being low cost and lightweight which makes their installation easy [93]. The monitoring of atmospheric pollutants on the insulator surface is essential for the early detection of incipient mechanical and electrical stresses. Ref. [94] uses a unique sensing system made up of multiple optical fiber sensors based on Brillouin scattering and fiber Briggs grating. Brillouin scattering gives better strain and temperature coefficients to sense the strain in overhead line insulators due to deposits of heavy pollution or ice. Ref. [95] uses a salt deposit density based optical fiber sensor with a micro-controller unit, A/D converter, quartz fiber, photodiode, laser module, data flash memory, solar panel, and sensors of temperature, humidity, and communication modules. This module can effectively monitor the presence of different densities of salt that are deposited on the transmission line insulators as pollutants with equivalent salt deposit density and non-soluble deposit density as the indicative parameters. Shearography [87] is another optical method that uses coherent light or sound waves to detect flaws, delamination, and deboned areas in composite systems. Mechanical and electrical stresses stemming from aging, poor maintenance or harsh atmospheric conditions could deteriorate the insulator surfaces causing insulation breakdown. The monitoring of occurrence and location of these discharges is essential for repair and replacement of faulty insulator components. Ultrasonic techniques-based position, vibration and optical sensors have been used to detect these defects and discharges in insulators [96]. Acoustic emissions from a faulty insulator produced by corona discharges can be monitored by acoustic sensors to detect the origins of the fault. However, these emissions are very sensitive to background noise, which makes the determination of the defect location difficult. To remedy this, a non-destructive ultrasound technique is used that relies on listening to echoes of sound signals coupled to the tested objects. The location and nature of defects are analyzed based on signal parameters such as the amplitude, time, and phase shift of the returning signal. A scanning laser acoustic microscope uses high frequency ultrasonic waves in the MHz to GHz range for detecting delamination and misorientation of fibers in fiber reinforced plastics [87]. However, due to the fixed focus and single angle beam interrogation, these ultrasonic techniques fail to detect minuscule defects at different depths of the insulator, for example under insulator sheds, etc. To remedy this, ultrasonic nondestructive testing based on the ultrasonic phased array technique has been developed that is efficient in inspections of objects having complex shapes, location of defects at different depths and identifying different types of defects on the insulators. Ref. [87] uses the technique for detecting internal defects such as air voids, paper strips, drill holes under sheds, interfacial air gaps, etc. in composite insulators. Thus, various sensor technologies developed for the detection of faults in transmission line insulators have been discussed here along with their detailed operations.