# Sensor Technologies for Transmission and Distribution Systems: A Review of the Latest Developments
## 3 Key Transmission and Distribution System Components
### Distribution System Switchgears & Protective Devices  
The distribution system consists of an array of instruments for protection such as circuit breakers, relays, reclosers, sectionalizers, fuses, etc. that protect the power conductors, power transformers and distribution side equipment from high voltages and fault currents. Fault currents tend to affect the protection schemes by leading to sympathetic tripping, reduction in the reach of distance relays, loss of relay coordination, etc. Traditionally, faulted circuit indicators were used to locate faulted line sections. The fault level was indicated by setting up a mechanical target like a flag or an LED. However, this process is slow and introduces a lag in the communication between the protective devices such as a recloser with the downstream devices such as fuses or other reclosers, and hence delays the time to clear faults. Ref. [97] provides literature on the development of a wireless protection sensor system that speeds up the fault detection speed. This system consists of multiple sensors that are placed per phase along distribution feeders, closer to the point of the occurrence of the fault. After sensing, they send the fault status to a collector which then sends the received status to the recloser control at high speed, securely speeding up the operation and improving the selectivity of the distribution protection scheme. The breakage of power conductors in the feeder and distribution section is a very common phenomenon that usually happens because of heavy wind flow during events such as cyclones. If the supply from the power transformer is not cut off, it can pose the risk of electrocution to the maintenance personnel. GMR-based sensors [98] mounted on feeder lines help to capture the alternating magnetic fields that correspond to the high levels of current, which is an indication of the state of energization of the cables. Circuit breakers are important switching equipment of the distribution system that performs the dual task of control and protection. Due to continuous interruption action, they undergo severe mechanical stress and wear and tear. Any physical deformations can be sensed with the help of mechanical sensors such as stress gauges. Recently, with the development of fiber optic sensors [99] vibrational fiber optic sensors have been developed for sensing wear and tear on the vacuum tubes of a circuit breaker. Sagnac interferometer-based fiber optic devices are also used to detect mechanical faults through changes in the frequency of acoustics. The wear and tear due to aging or contamination can also cause the insulating material within the circuit breaker to deteriorate, which can result in partial discharge due to the ionization process. Photo sensitive fiber-based optical sensors [11] are used to detect these partial discharges. However, these sensors can only operate in a constant electric field. Any change in electric field will generate partial discharges in the fiber. Arc control in circuit breakers is essential to construct the effect of thermal emissions and eradicate the breakdown of the anode at high current levels. This is conducted with the help of axial or radial magnetic fields that force the arc from the fault current to rotate. The motion of the arcs in lifetime sealed circuit breakers, especially vacuum circuit breakers, can be sensed through Hall sensors [100]. Other sensors fail since the vacuum is sealed with an opaque ceramic and metallic vapor shield. In addition, in a vacuum circuit breaker, knowledge of the density of electrons in the electrodes, gases and the plasma are important. A unique Shack-Hartmann type laser wavefront sensor is described in [101] that can visualize two-dimensional electron and metal vapor density distributions. Optical emission spectroscopy [102] has also been used to detect the metal gases evaporating from the electrodes. Temperature measurement and monitoring is essential in circuit breakers.