# Sensor Technologies for Transmission and Distribution Systems: A Review of the Latest Developments
## 2 Need of Sensors in Transmission and Distribution Systems  
The electric power grid is one of the most expansive and diversified assemblies of expensive and long-standing machineries. The transmission and distribution systems are vital assets of the electrical grid. This is because they essentially 'transport and dispense' energy from the generation plants to the consumers. This is realized through a substantial number of electrical, mechanical, thermal, chemical, and electromagnetic hardware that are geared to carry electrical currents at high levels of voltage and thermal ratings over a long span of distance. Hence, these systems face wide ranging issues and challenges with their structures and functionalities. Extreme weather conditions have a detrimental effect on them. For instance, temperatures increase conductor sagging while heavy winds lead to the swinging and galloping of power conductors. Thunderstorms can generate strong transient magnetic fields in the vicinity of transmission towers [33]. Excess moisture and air pollutants can cause contamination of the surface of the insulators, creating a conducting path and resulting in arcs and flashovers [34]. Growing vegetation and even animal infestations can cause short circuits in the transmission system networks. Instruments like power transformers and breakers can malfunction due to wear and tear, unwarranted power surges, moisture issues, and even overloading [24]. Today, sensors have become one of the most essential components of an outage management system in the electrical power grid. The real time monitoring capability of sensors helps in total prevention or in reducing the impact of disruptions in the transmission and distribution networks. Sensors make condition-based maintenance of the power grid possible, which can be cost effective and more efficient than the normal scheduled maintenance in certain cases. Sensors also form a very crucial input facing part for the most control and monitoring systems, as they are solely responsible for the raw fault data acquisition. Various grid parameters collected from the sensors are important for classification and forecasting purposes to provide better predictive, adaptive, and corrective analysis [35] of the nature of functioning and breakdown of instruments in the transmission and distribution lines. The utilization of assets of the power grid is highly increased with the probabilistic risk assessment or contingency analyses using sensors [5,36]. Today, the increase in demand for utilities with the increase in population has put more stress than ever on the already maturing technologies on transmission and distribution networks. This increase in demand also calls for an increase in the reliability, efficiency, and security of the systems. Therefore, it has become imperative for the traditional power grid to expand its capacity and make space for penetration of newer technologies such as distributed generation, renewable energy sources, power electronic devices, intelligent control systems, sizeable storage devices, and electric vehicles. Through these changes, the traditional power grid is expected to transform into distributed microgrids and smart grids in future. However, the upcoming age of DC- based grids and smart grid structures [37] have certain unique features and challenges such as the bidirectional flow of power, the need for continuous synchronization, and the integration of multiple heterogeneous and non-linear power electronic devices and loads that cause the circulation of harmonic currents and reactive power. Furthermore, the integration of distributed energy sources, especially non-dispatchable generation such as PV parks and wind parks, poses various challenges to the power system security of the grid. The intermittent nature of non-dispatchable generation results in unpredictable voltage, current, and frequency profile deviations causing power fluctuations and imbalances in the grid [38].