# Sensor Technologies for Transmission and Distribution Systems: A Review of the Latest Developments
## 3 Key Transmission and Distribution System Components
### Underground Cables  
Recently, overhead lines are being rapidly replaced by underground cables since they provide better safety, stability, security, and reliability to the overall grid structure. Underground cables can form a part of the transmission system as well as the distribution system. Even though the chance of outage is reduced since the cables are buried into the soil, they are still vulnerable to major faults due to partial discharge, rise in temperature and cable aging causing wear and tear over time, etc. This makes fault identification and prevention and repair of cables difficult and expensive. Thus, a large variety of sensors have been developed and deployed for fault detection and the fault monitoring process in underground cables. Partial discharges are the most common faults seen in underground power cables. These faults occur due to improper installation and maintenance, the circulation of large load currents, or substantial increases in temperature inside the power cables, leading to insulation degradation around the power cables, especially at cable joints. This deterioration of insulation material causes partial discharges in the underground power cables. Partial discharges can have harmful effects and cause surface discharges, corona discharges around sharp edges, internal discharges leading to electrical treeing, etc. Hence, detection of partial discharges is essential to detect the occurrence of insulation degradation and to avoid the further breakdown of underground power cables. Various sensors have been developed for the detection and location of partial discharges in underground cables. Traditionally, ultra-high frequency coupling capacitors, vibration sensors, high frequency current transformers and Rogowski coils were used to detect partial discharges. The acoustical analysis is another method for the detection of partial discharge. Ref. [113] uses an FBG optical sensor to record the acoustic emissions caused by defects in the insulation layer. The captured acoustic waveforms are then studied with signal processing techniques and its characteristics such as loudness, fundamental frequency, pitch, and quality are investigated in the time domain and frequency domain for accurate classification of nature and source of partial discharge faults. However, since cables are buried underground, the acoustic method of sensing has a disadvantage wherein the sensors are required to be placed in the vicinity of the source and hence their installation becomes more difficult and expensive for underground cables. Acoustic signals also suffer from attenuation while transmission due to the interference by surrounding power system equipment.. Ultra-high frequency methods, on the other hand, do not suffer from the above disadvantages. These methods include the generation of RF signals to detect the location of partial discharges. [114] uses radio frequency current transformer sensors to sense the RF signals generated from the source. Here, three sensors are clamped to the three phases of the ground wire of cable joints to detect the partial discharge signals. Ref. [115] uses ultra-high frequency sensor-based antennas to capture and transmit the radio frequency signals in XLEP cables for detection of partial discharge. When the length of the cable spans to several kilometers, there might be multiple sources of partial discharge which would require the use of multiple sensors that would not be cost effective. Ref. [116] proposes a method of partial discharge detection from multiple sources on long distance cables through the time domain reflectometry method. Here, the time of arrival of the partial discharge pulses received at the sensors along with its subsequent reflected pulses, the wave propagation speed, and the total length of the cable is considered to locate the sources on the power cable. Hence, it is noteworthy to elucidate and appreciate the variety and multitude number of sensor technologies that have been developed for monitoring partial discharges in power conductors.