# Sensor Technologies for Transmission and Distribution Systems: A Review of the Latest Developments
## 3 Key Transmission and Distribution System Components
### Power Transformers  
Power transformers are the most vital and expensive components of a substation. They are essential in stepping down the high transmission line voltages into lower values for the sub transmission and distribution levels. Despite a sturdy build and a prolonged life span of sixty years with routine maintenance and protective gears like surge arresters attached to them, power transformers are susceptible to many in-service failures and breakdowns which can have results from an increase in transmission losses to the complete breakdown of the transformer leading to fire and other hazards, costing millions of dollars and lives. Power transformers generally get damaged due to the wear and tear of components that mostly occurs because of ageing. Insulation degradation, moisture absorption of the transformer oil and oil leakage can also seriously damage the transformer. Furthermore, lightning, seismic and line surges can severely affect the workings of a power  
Figure 1: Key sensing areas in transmission and distribution systems. transformer [44]. To monitor these faults, various sensors have been deployed on the power transformers, as depicted in Figure 2.  
One of the most used power transformer diagnostics is the monitoring of target specific analytes that often emerge from its malfunctioning and can produce a detectable electrical signal proportional to their magnitude and corresponding to the status of the transformer using chemical sensors. Dissolved gas analysis (DGA) monitoring is conducted to assert the insulant or coolant oil degradation that corresponds to the malfunctioning of a power transformer. Although some form of it has been in use since the 1980s, the modern online dissolved gas analysis systems of chemical sensors [14; 45; 46] help in the continuous monitoring of temperature, moisture content, and concentration detection of different combustible gases that arise from the transformer oil degradation without disrupting the normal functioning of the transformer. Continuous on-line monitoring and periodic sampling of the insulating oil in the transformer is mostly conducted with dissolved gas analysis that determines the concentration of generated gases such as hydrogen, carbon monoxide, carbon dioxide, ethylene, and acetylene that are studied for the knowledge of the operation and condition of the transformer and helps with the detection and the prevention of any incipient faults. The dissolved gas analysis of load tap changers helps in its maintenance and avoids its failure and the failure of the transformer. Chemical sensors such as semiconductor gas sensors with metal oxides or metal semiconductor oxide materials, electrochemical gas sensors, optical gas sensors with infrared absorption sensors, spectrum absorptive sensors, fluorescence sensors, and fiber sensors are currently used [14]. A metal insulated semiconducting structure-based field effect gas sensor with a low-cost sensor on a chip is used for detecting the release of hydrogen or acetylene gas released during the thermal decomposition of transformer oil [14]. Gas sensors such as the SF6 sensors and, more recently, an improved version of it employing radio frequency signals in the SF6 sensors are used to monitor the gas ratios in a load tap transformer [7]. The modern on-line dissolved gas analysis DGA transformer monitoring system also has advanced chemical sensors to measure the electrochemical and optical signals to determine the condition of the insulating oil. Surface plasmon resonance principle-based optical chemo sensors are also widely used to detect transformer oil degradation. An optical sensor with a molecularly imprinted polymer layer in contact with the surface plasmon resonance active surface is used for the monitoring of an oil-filled power transformer [47].