# Sensor Technologies for Transmission and Distribution Systems: A Review of the Latest Developments
## 3 Key Transmission and Distribution System Components
### Lightning Current Monitoring Systems  
Lightning is a high intensity natural discharge of electromagnetic pulse that causes black-flask or shielding failures leading to transmission line trips [128]. Hence, protection against lightning is important to safety monitoring and operation in power transmission and distribution networks, and sensors are being widely used for this purpose. The accurate estimation of lightning parameters is essential to building an effective lightning protection system. Lightning location systems are automatic detection equipment that use characteristics of sound, light, and electromagnetic waves radiated by lightning strikes to estimate the lightning discharge parameters. Broadly, they are either ground based or satellite-based systems [128]. They use several types of sensors to detect lightning parameters with various methods such as acoustic methods, optical methods, and electromagnetic field methods. A detailed description of the operation of various sensors in the lightning current monitoring systems has been described in the following paragraph. Sensors are essentially mounted on the lightning current monitoring systems to detect high voltage lightning strikes. Ref. [129] depicts the working of a system with four 500 kA current transformer sensors designed to capture the magnitude of current strokes arising from the incidence of direct lightning. These sensors are also integrated with IP cameras, position sensors and electric field sensors to estimate other lightning current parameters such as position, latitude and longitude information, time of lightning strike, shape of lightning current, lightning current peak, duration, polarity, frequency, and number of components during the stroke. Ref. [130] describes a low-cost lightning detection and waveform storage system with current sensors used for recording both close and far lightning events with good amplitude resolution without saturation issues. Apart from this, other lightning monitoring methods used are the traveling wave method where the lightning position can be obtained by calculating the time difference in the arrival of transient voltage and current traveling waves at different sensors and the fault type is detected from the traveling wave shape. Ref. [131] uses a TMR magnetic sensor module-based travelling wave method. However, these methods suffer from disadvantages such as weak anti-electromagnetic interference ability, maintenance issues, high cost, influence of terrains, etc. To overcome these issues, optical fiber composite overhead ground wires [132] are being installed on transmission lines over 110 kV, which consists of distributed optical fiber sensors that capture external vibrations due to transient currents through the principle of optical time domain reflectometry. It also helps in the accurate determination of transmission line vibration, temperature rise, polarization state and lightning positioning. Magnetic tape and counter based sensors are used to calculate the number of lightning strikes. The conventional lightning arresters are made up of metal-oxides and have a limited-service life. Ref. [133] replaces the conventional arresters with a magnetic tape-based multi-chamber arresters that use a lightning event counter to calculate the number of lightning strikes and facilitates faster arc quenching with low maintenance and a longer life span. High voltage equipment sensors combined with integrated electronic devices IEDs, or lightning imaging sensors are being used to increase the accuracy and reliability of locating and measuring lightning events with measurement digitization, control networking, state visualization and function integration. Current transformer based intelligent sensors are integrated with IEDs to detect the transient rise in ground potential in the event of lightning. Another example is use of optical current transformers whose operations are based on magneto-optical Faraday effects. [134].