# Sensor Technologies for Transmission and Distribution Systems: A Review of the Latest Developments
## 3 Key Transmission and Distribution System Components
### Power Transformers
Hydrophone based general purpose sensors, piezoelectric accelerometers and broadband transducer-based sensors are used for the measurement of acoustic emissions in 0 to 90 kHz frequency range. Distributed acoustic and temperature sensing systems that use the temperature, strain, and vibration sensitive Rayleigh backscattering technique-based optical fiber yield more accuracy in fault detections by replacing multiple sensors with a single optical fiber sensor [50]. Alternatively, multiple acoustic sensors are integrated with a preamplifier, a high frequency current transducer, and a signal processing unit with a digital acquisition module to form a partial discharge ultrasonic detector to be clamped on the transformer tank [51]. Also, ultrasonic detection of SF6 and water content in transformer oil [52] is done with special acoustic sensors. Finally,surface acoustic wave temperature sensors with power over ethernet as the system's power and data transmission line is used for the online temperature monitoring in high voltage substations and power conductors [53]. These sensors are composed of special piezoelectric crystals. Any change in temperature affects the frequency of acoustic waves on the surface of the crystals. The measurement of this change in frequency reflects the corresponding change in the temperature. Optical fiber temperature sensors [54] can measure the actual temperature of the transformer winding by getting mounted on them directly. There are two kinds of optical fiber sensors: the surface and embedded optical fiber grating temperature sensors. Due to their compact size, they are easy to be embedded into the transformer. The use of wavelength division multiplexing technology makes multiple grating concatenated on the single fiber, realizing quasi distributed networking measurement. These sensors are also suitable for working in a high temperature and high-pressure oil and gas environment. The major disadvantage of optical fiber sensors is that they are quite expensive. Ref. [55] uses unique and cost-effective arrangements of fiber grating temperature sensors for 35 kV, 200 kVA three-phase double winding oil-immersed transformers with internal fluid-solid coupling to detect the transformer hot-spot formation and location under different load conditions. Optical fiber sensors spatially resolve temperature monitoring through mapping temperature hotspots or electrical discharges in specific locations of a transformer. Distributed optical fiber sensors with nanostructure metal oxides are actively designed for the sensing of hydrogen and methane gases. Currently, various methods of spectroscopy wavelengths and techniques such as Raman spectroscopy, transmission spectroscopy, fluorescence spectroscopy, etc. are being studied that can be made compatible with optical fiber sensors [14]. Various miscellaneous sensors have also been installed to provide complete monitoring of the transformers. Ref. [56] provides a review of various sensors placed on an on-site step-up transformer in a substation such as the brushing monitor sensor that could sense the capacitances, leakage currents, and phase-to-ground and phase-to-phase voltages in the transformer brushings. Digital transducers are used in the cooling system of the transformer to sense the temperature in the transformer oil and e water in the exchanger. Additionally, to sense the relative humidity and temperature of a transformer tank, Smart Intelligent Electronic Devices based sensors are installed directly on the transformer body for variable measurement. These sensors have communication ports that facilitate the direct connection to a communication network and make it possible to transmit the measurements directly to a server in the plant's control room. The monitoring of mechanical deformations in a power transformer winding is crucial. Aging, the effects of arching faults, or the inefficient handling of transformers can have a loose winding structure that disturbs the symmetry of short circuit forces.