# Sensor Technologies for Transmission and Distribution Systems: A Review of the Latest Developments
## 3 Key Transmission and Distribution System Components
### Battery Energy Storage Systems  
The battery energy storage systems are being widely used for the large scale storage of energy and form a big part of the distributed energy systems. It essentially consists of battery modules with a battery management system and an energy management system. Traditional grids, as well as microgrids and smart grids, have been integrated with battery energy storage systems to not only improve the operating reserves and ease grid congestion but also to help with frequency regulation and voltage stabilization. They can also increase the utilization of the grid by ensuring continuous and dispatchable supply despite the intermittent nature of renewable energy sources. Despite their numerous advantages and widespread use, they are susceptible to fires and explosions. The reasons can be thermal runaway, where there is an intense increase in temperature inside the battery, or mechanical runaway, which is caused by mechanical deformations to the battery unit causing short circuits inside the battery cells or the disintegration of its inner components. However, the most common reason for breakdown is caused by electrical runaway that occurs due to overcharging, undercharging or shorts caused by the interfacing inverters. Various sensors have been developed to detect these faults in a battery energy management system.  
When a sufficiently sized battery energy storage system is on fire, the DC power arc is very large, and that makes it very difficult to extinguish the initial fire. It also releases toxic combustible gases such as carbon monoxide and methane that cause severe human casualties [124]. Hence, the prevention of fire is essential. Ref. [125] uses a sensor module to detect electrolyte off-gases, which are a combination of gases of ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, and propylene carbonate that are released in lithium ion-based batteries prior to a fire, indicating a thermal runaway. The sensor module has a combination of electrochemical sensors including photo-ionization detection-based gas sensors, humidity, and temperature sensors. The main job is to monitor the target electrolyte off-gases, discontinue the charging or discharging of the battery, and to disconnect the connection from the charging devices once they are sensed above the threshold level.  
A host of sensors are used for the accurate state estimations of battery energy storage systems. Fiber optic current and voltage sensors are used for accurate estimation of state-of-charge and state-of-health estimations with over-current and over-/under-voltage protections. Islanding and anti-islanding are common features of a microgrid. However, sometimes these might initiate high circulating current that can be bidirectional in nature. Furthermore, there can be voltage and frequency fluctuations that could damage the inverter and lead to equipment failure including the battery energy storage system. Ref. [126] uses an array of sensors to monitor essential parameters such as current, voltage, frequency, active and reactive power, total harmonic distortion, etc. at the point of common coupling to provide anti-islanding protection. Furthermore, machine learning based classification techniques such as convolutional neural networks [127], support vector machines, etc. are being increasingly used for the detection of battery faults arising from aging, breakage, EMI noise, etc. by using data from the current, voltage, and temperature sensors. Thus, multiple sensor technologies and their operations have been discussed for the detection of various major faults in battery energy storage systems.