After a long-term operation, defects may occur in the composite insulators and threat the safe operation of the power transmission. To secure the power transmission system, it would be beneficial to detect the defects in their initial stage. In this paper, the terahertz technique combined with a deconvolution method was proposed, and a terahertz time-domain spectroscopy system was built. To validate the method proposed, three types of defects, namely single air gap defect, inclusion defect, and double-layer air gap defect were artificially prepared and tested. According to the test results, it was found that, compared with the traditional terahertz method, the air gap size that could lead to pulse overlapping was reduced from 1.22 mm to 0.57 mm. After the deconvolution process, air gap defect as small as 0.173 mm could be distinguished due to the shape characteristics of unipolar pulse. It was also found that when the defect was damp, the obtained waveform near the defect became smooth. The results of the present study demonstrate that terahertz time-domain spectroscopy (THz-TDS), combined with the deconvolution method, can have high potential in the detection of defects in composite insulators.

Composite insulator is prone to internal defects such as air gaps which seriously endanger the safety of power systems. To detect the internal defects in composite insulators, a terahertz (THz) imaging method based on edge detection algorithm was proposed in this article. First, air gap defect sample consisting of planar multilayers was imaged based on the envelope area of the THz reflection waveform. Then, Canny operator was applied to extract the edges of the defects. After that, the time interval of defect characteristic pulses was used to calculate the defect depth and then 3-D imaging of defects was, therefore, obtained. The experimental results showed that the imaging method based on envelope area was very effective in determining the edges of air gap defect and it had better performance compared with those using pulse amplitude or time interval as parameter for defect imaging. According to the results, the accuracy was quite satisfactory when the defect depth was above 0.339 mm. To verify the effectiveness of the proposed method, the developed THz imaging system was also applied to the rod of composite insulator. It was found that the relative error was smaller along the axial direction of the rod than the lateral direction.

Corrosion of tower foot in ultra-high voltage transmission lines is a critical issue for ensuring the safe operation of power grids. Recently, severe corrosion of tower foot has been observed on several UHV transmission lines in southern China, highlighting the emergence of this new problem both domestically and internationally. However, its underlying mechanism remains unclear and requires further investigation. This paper first surveyed the corrosion of on-site tower foot, and obtained comprehensive photographic documentation. The corrosion products at the tower foot were retrieved, and their surface morphology and material composition were analyzed by SEM, EDS, XRD, and XPS. The mechanism and cause of tower foot corrosion were inferred to be the soil corrosion resulting from the deposition of soil and dirty substances on the tower foot. Subsequently, our newly developed soil corrosion monitoring sensor was employed for on-site measurement of soil corrosion rate. It was observed that temperature and moisture exerted significant influence on soil corrosion, with its rate surpassing that of atmospheric corrosion. Therefore, the measurement results of soil corrosion also provided a reasonable explanation for the more severe corrosion observed at the tower foot compared to that at the other part of the tower, thereby offering empirical evidence for understanding the mechanism of soil corrosion at the tower foot. Ultimately, based on this understanding, an effective method for protecting against soil corrosion at the foot of the tower was proposed.

Pollution flashover is a serious accident with a wide range of impacts in power grid. Accurate and timely classification of pollution severity is both a key to preventing pollution flashovers and a crucial challenge. This paper proposes an innovative method based on PTR and MCS3VM for classification of insulator pollution severity. The introduction of time-dimension information makes it possible to achieve pixel-level image identification instead of image or region-level image identification. The PTR physical model for pollution severity measurement is established. The relevant features are extracted by using PCA. A semi-supervised classifier is proposed to solve the problem of poor generalization due to insufficient labeled samples in industrial applications. Experimental results verify the satisfactory efficiency and accuracy of the proposed method and an estimation framework for fast, accurate and nondestructive industrial application without tedious work of labeling large amounts of data is concluded.

The external insulation of the equipment in the valve hall of UHV DC converter station must be able to withstand AC-DC combined voltage influences. Investigating the surface discharge characteristics of external insulation under AC-DC combined voltage is currently a significant research topic. The investigation into the surface streamer characteristics under AC-DC combined voltage is instrumental in elucidating the mechanism of surface flashover. The streamer characteristics of different phases in a single cycle under AC-DC combined voltage were determined by conducting surface streamer discharge tests. The relationship between surface streamer characteristics and different phases under AC-DC combined voltage was summarized. By comparing the test results of surface streamer discharge under DC voltage, this study analyzed the effect mechanism of AC-DC combined voltage on surface streamer characteristics. The directional motion of space charge under AC-DC combined voltage was identified as the primary determinant of surface streamer characteristics.

Surface discharge was a common accident in power systems, significantly impacting the safe and stable operation of the power grid. The insulation structure of power equipment was intricate, and surface discharge characteristics were influenced not only by the insulation material but also by electrode geometry. In the paper, characteristics of streamer discharge along the insulation surface with embedded electrode were investigated under negative DC voltage. The streamer propagation field, velocity, and luminous intensity, were measured by photomultiplier tubes and ICCD camera. Unexpectedly, the disparity in the characteristics of streamer discharge along the insulation surface, with and without an embedded electrode, was substantial. Furthermore, the investigation also revealed that the length of the embedded electrode has a significant influence on the characteristics of surface streamer discharge. Through analysis, it had been determined that the presence of the embedded electrode did not significantly impact the electric field along the insulation surface. However, it had an influence on the charge distribution on the insulation surface, consequently affecting the characteristics of surface steamer discharge. The findings from this research would provide valuable technical support for enhancing external insulation design and operation strategies in DC bushings and other related equipment.