# Sensor Technologies for Transmission and Distribution Systems: A Review of the Latest Developments
## 4 Outlook  
Pathway of Sensor Development in Transmission and Distribution Systems: From Yesterday to Today and Tomorrow  
The timeline for the development of sensor technology in the power grid is depicted in Figure 3. The study of this timeline is essential not only to understand and appreciate the process of evolution of sensors to what it is today but also to take inspiration from it while building the smart sensors of tomorrow. This timeline as depicted in Figure 3 is elucidated in a detailed manner as follows. The entire timeline can be divided into four major phases. The first phase or the inception phase represents the early deployment of instruments to measure basic grid and atmospheric parameters. It starts with the mounting of basic and crude instruments to measure the current in the grid. This is because the current was recognized as a local parameter and its measurement was deemed necessary, especially during fault conditions to isolate and protect expensive grid equipment. In the early 1800s, current measurement was conducted with current clamps and coils. In the 1890s, instruments were deployed to record the atmospheric parameters that affected the operations of grid components. There are reports of the first remote sensing device, \"radiosonde\" [135], being deployed in power grids for the measurement of atmospheric parameters such as temperature, pressure, wind speed, altitude, and humidity. In the early 1900s, crude current measuring instruments developed into more sophisticated electromechanical systems such as current transformers and potential transformers. These instruments were bulky and were installed mostly on power transformers to measure the two key parameters of the power grid - the voltage and the current. Phase 2, or the growth phase, resembles the period where the importance of measuring devices in the power grid was recognized, especially for monitoring and protective purposes. Researchers aimed at developing special devices, later defined as sensors, that could provide a response to indicate the presence of various parameters in the grid. Furthermore, various other grid parameters, apart from currents and voltages, were measured or detected by these devices. One of those parameters to be considered first was the phase angle. Phase angle measurements were conducted to get a clearer estimation of real and reactive power flows in the system. The discovery of electromagnetic induction in the early 1900s paved the way for the development of magnetic sensing instruments which were unique because of their non-invasive nature [98]. In addition, in the late 1950s, multiple technologies and principles of science were incorporated into the design of new sensors. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Component** & \multicolumn{1}{c}{**Sensors**} & \multicolumn{1}{c}{**Application**} \\ \hline \multirow{8}{*}{**Underground Cables**} & 1. HFCT/Optical sensors/RF/UHF sensors [113; 114; 115; 116] & 1. Detection of leakage current, partial discharges \\ \cline{2-3}  & 2. IR sensors [117; 118; 119] & 2. Temperature monitoring \\ \cline{2-3}  & 3. Hall effect sensor/pressure sensor [121; 122; 123] & 3. Detection of cable ageing, etc. \\ \hline \multirow{8}{*}{**Distribution System** **Switches**} & 1. FBG based acoustic/vibration sensor [11; 99] & 1. Detection of defects, mechanical faults in CB \\ \cline{2-3}  & 2. Hall sensors/optical sensors [100; 101] & 2. Arc control in CB \\ \cline{2-3}  & 3. SAW/FBC based temperature sensor [2; 103] & 3. Temperature monitoring in CB \\ \hline \multirow{4}{*}{**Lightning current monitoring systems**} & 1. HFCT/TMR sensor/acoustic sensor-based LLS. [129; 130; 131] & 1. Monitoring of lightning discharge parameters including position \\ \cline{2-3}  & 2. FBG/vibration/temperature sensor [132; 134] & 2. Monitoring of external vibration, temperature, etc. \\ \hline \hline \end{tabular}
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