# Sensor Technologies for Transmission and Distribution Systems: A Review of the Latest Developments
## 3 Key Transmission and Distribution System Components
### Overhead Line Power Conductors  
Overhead line power conductors are essential for transporting power over long distances and are prone to high voltages and severe atmospheric conditions that lead to its deterioration. Overhead line power conductors form a part of the transmission systems as well as the distribution systems. Some of the causes of such deterioration are explained as follows. Conductor corrosion is a common phenomenon occurring from galvanization due to atmospheric pollution. Vibration damage such as aeolian vibration, galloping, or sub conductor oscillations put severe mechanical strain on the power conductors. Severe atmospheric conditions such as heavy wind, rain and snow also have variable effects on the power conductors, such as sagging and breaking of the conductors [18]. Various sensors have been developed for monitoring these faults and are discussed in the following paragraphs. The monitoring of overhead line power conductors is conducted in one way by measuring the changes in mechanical structure of the conductors that can occur due to aging, corrosion or vibration damage leading to loss of material, putting extra strain on the conductors, and reducing the transmission capacity. Magnetic sensors help in the detection of magnetic field enhancement because of defects in the cable insulation. Air gaps and protrusions in XLPE insulation [69] lead to enhancement in the semiconductor layers that alter the magnetic field, and its detection is very useful in the degradation monitoring and evaluation of medium voltage power cables in transmission lines. In [70], TMR sensors are mounted atop support towers of high voltage transmission lines to sense the change in magnetic field with the change in sag of the power conductors as well as conductor motions such as swinging and galloping effects caused by changes in weather conditions. These sensors are synchronized with the current transformers to give real values of data synchronized in time and space. Among other methods of overhead line power conductor monitoring, unmanned aerial vehicle (UAV) inspection [71] has been developing recently. Fixtures of multiple sensors on UAVs can help sense the electric and magnetic field around the transmission line and the distribution of line power conductors. Ref. [72] uses TMR sensors on UAVs for magnetic field measurements and the reconstruction of position and current parameters of the transmission line. Current monitoring is also conducted to detect leakage current due to partial discharge in the form of corona or gap discharges, breakage of conductors, etc., that would otherwise lead to unnecessary dissipation, reducing the transmission capacity. Various sensors such as Hall effect-based magnetic sensors, current sensors, and optical sensors are used for measuring the power conductor fault currents. Fiber optic current sensors [18] are based on optical reflection sensing where the reflection of light estimates the leakage currents. These sensors are compact, non-invasive and replace bulky and lossy current transformers at substations. These sensors can be installed in remote sites with large bandwidth, greater signal to noise ratio and no saturation issues. An advanced version of it, an optically interrogated Rogowski coil, is developed by interfacing an optical low voltage sensor and a piezoelectric stack transducer with a fiber Bragg grating to the output of a Rogowski coil for effective operations at higher power conductor harmonics [73]. Piezoelectric-based acoustic sensors are also used for the monitoring of partial discharges. The increase in conductor temperature can be caused by lightning surges, overloads, or short circuit faults. Temperature monitoring is essential to identify and limit sag as well as partial discharges, corona discharges, etc., as well as to check breakage, elongation and increases in tensile strength of the conductor.