# Sensor Technologies for Transmission and Distribution Systems: A Review of the Latest Developments
## 3 Key Transmission and Distribution System Components
### Underground Cables
Temperature monitoring in underground cables is conducted to detect any abnormal increase in temperature that could be a precursor to many faults. It is often very difficult to mitigate rising temperatures in underground systems because the power cables are not surrounded by air but rather by soil with very low humidity. Also, the underground power cable tunnel has very poor ventilation and very low natural heat convection. In such cases, if the rising temperature goes unhindered, it will decrease the efficiency of power transmission and lead to insulation failures, short circuits, the malfunctioning of equipment and even explosions leading to fires. Traditionally, cable temperature monitoring was conducted with point type or thermal infrared technology [117]. Point type technology had temperature sensors installed on key points on the conductor for small distance monitoring purposes. The second technology had infrared sensors to sense the light emitted from the source to convert it into heat to measure temperature [118]. However, this method is dependent on the emissivity of the source as well as the interference from the surrounding objects which makes its accuracy lower. To mitigate this, optical sensors based on fiber Briggs grating and Raman scattering are preferred [119]. Autonomous robots are also used in the temperature monitoring of underground cables. Ref. [120] designed an IoT based temperature monitoring system with LM35 temperature sensors with a battery system and wireless transmission protocol with smart cloud service analysis. Cable aging is another important parameter, and its consideration is essential in fault monitoring systems. Cable aging can be caused by thermal, mechanical, or electrical stress or moisture ingression on the cable over time. Since the power conductors are buried underground, the visual inspection of cables is not possible. Undiagnosed cable aging can lead to insulation failure, short circuits and power conductor breakdown, hampering the transmission of energy. Therefore, it will be a difficult and expensive task to replace the entire cable since it is buried underground. The detection of cable aging has been conducted through various ways. Traditionally, off-line methods such as the study of dielectric response and impedance phase detection were used, but currently various on-line methods have been developed. One of the methods used in [121] uses magnetic sensors to measure the common mode leakage current since the aging of cables is accompanied by an increase in relative permittivity that in turn increases the insulation capacitance and conductance of the cable, thereby increasing the flow of leakage current. Ref. [122] uses pressure sensors to monitor the change in pressure around XLEP-SR cable joints, indicating mechanical stress leading to cable aging. The dielectric loss angle is another crucial parameter indicating the aging of cables. Ref. [123] uses current and voltage sensors to calculate the dielectric loss angle along with the Grubbs criterion and adaptive weighted data fusion algorithm-based data processing for accurate aging estimation of an XLEP cable. Thus, multiple sensor technologies have been discussed for the detection of various major faults in underground power cables.