Graphene quantum dot (GQD) is a new type of carbon nanometer material. In addition to the excellent properties of graphene, it is superior due to the quantum limit effect and edge effect. Because of its advantages such as water solution, strong fluorescent, small size, and low biological toxicity, it has important application potential in various fields, especially in sensors and biomedical areas, which are mainly used as optical electrical sensors as well as in biological imaging and tumor therapy. In addition, GQDs have very important characteristics, such as optical and electrical properties. There are many preparation methods, divided into top-down and bottom-up methods, which have different advantages and disadvantages, respectively. In addition, the modification methods include heterogeneous doping, surface heterogeneity, etc. There are still many challenges in developing GQDs. For example, the synthesis steps are still hard to conduct, but as the inquiry continues to deepen, GQDs will be revolutionary materials in the future. In this work, the literature concerning research progress on GQDs has been reviewed and summarized, while the key challenges of their application have been pointed out, which may bring new insights to the application of GQDs.

Frequency domain reflectometry (FDR) technologies could be applied to locate defects in cables effectively. Owing to the large space span and complex operating environment of the actual XLPE distribution cables, segmented thermal aging phenomenon occurs easily, causing the nonuniform distribution of electrical parameters along cables, which might affect the defect location accuracy ultimately. In this paper, the segmented thermal aging cable models were established and the effect of segmented-aging in XLPE distribution cables on defect location accuracy was explored based on simulation. It was found the floating range of defect location deviation in segmented thermal aging cable models could reach 10% of the whole cable length, which could not be ignored. Further research and analysis revealed that the defect location deviation in the segmented-aging cable mainly attributes to the differences of distributed wave velocities in the cable. On this basis, a correction coefficient of defect location $M$ was put forward for the segmented-aging cable models established, which has achieved remarkable results. Finally, a segmented-aging cable model with a length of 119 m was built in the laboratory, and the validity for improving defect location accuracy by considering the differences of wave velocities was verified through experiments. Results show the defect location deviation can be reduced from 2.19% to 0.39% by introducing $M$ .

To address the challenges of real-time insulation monitoring and accurate partial discharge (PD) location in cable joints. A distributed Optical Fiber sensing system with polarization compensation module is established to detect the ultrasonic signal of partial discharge. The locating range of optical time-domain reflectometer (Φ-OTDR) is analyzed, and a variance variable mathematical model based on Rayleigh scattering coherent light signals is established to denote weak ultrasonic characteristic signals of partial discharge in cable joints. The polarizing beam splitter (PBS) is devised in the Φ-OTDR detection circuit to improves the stability of polarization states. Four Optical Fiber ring sensors are wound on the cable joint to locate and detect the partial discharge point of the cable joint. Then, the signal characteristics of partial discharge measured by electrical measurement method and Optical Fiber acoustic measurement method are compered, and the positions of four Optical Fiber ring sensors are calibrated based on piezoelectric sensors. It shows that when the test fiber length is 600m or 1125m respectively, the locating deviation is less than 1.9m, which is verified a high accuracy of Optical Fiber sensing system in detecting partial discharge. It provides a feasible monitoring method for real-time sensing of insulation status of transmission equipment.

The lifetime of ethylene propylene rubber (EPR) insulated cables will decrease because of complex aging processes. From the safety perspective, insulation condition assessment of the cable is essential to maintain an efficient and reliable operation. As a nondestructive and online evaluation method, a hardness retention rate was used to estimate the lifetime of cable. First, accelerated thermal aging tests in the laboratory were performed to measure the elongation at break retention rate (EAB%) and a hardness retention rate at different temperatures. Second, the aging values were processed by the Arrhenius equation and time temperature superposition to assess aging lifetime of insulation at different temperatures and end levels. As the insulation condition assessment of the cable by hardness retention test has no approved standard, the EAB% data were correlated with hardness retention to provide an evaluation basis. The results show that when EAB% picks out the time corresponding to a certain amount of 50% degradation, 10% of hardness retention was chosen as the termination index.