Partial discharge (PD) as one of well-known signatures of defects in the insulation system of high voltage equipment has been studied over eight decades. It is of a great interest to power grid operators and owners to identify and distinguish local weak spots in insulation system of the key components of power system by using the obtained PD information to make timely decision thus to avoid any unexpected failures in the system. This article reviews the research progress of PD in high voltage apparatus, including motors and generators, cables, transformers and gas insulated equipment, with attempts to assess the severity of different types of PD initiating defects as well as key PD features that can be selected for classifying defects. Experimental and on-site test results for each type of discharge and defect associated are summarized and discussed. Also, development of these defects during operation of equipment caused by changes in physical and chemical parameters of defect and/or insulation, and its impact on phase-resolved PD pattern variation are thoroughly explained. It is expected that this paper can provide a comprehensive guideline for PD data analysis of available PD information extracted by conventional PD acquisition instruments for both engineers and asset managers of high voltage apparatus.

Bubble is a typical insulation defect in transformer oil, which can easily initiate partial discharge (PD) during the operation of transformer. Moreover, it migrates along with the oil flow and may be deformed due to the forced oil circulation and temperature difference within transformer bulk, leading to the complexity of PD characteristics and PD mechanism. However, the understanding of to this phenomenon is not clear. In order to identify it, finite element method was employed to simulate the movement process and deformation of bubbles, as well as electric field distribution under the conditions with different oil flow rates. In addition, numerous experiments were performed, mainly including the change of PD ϕ-q and ϕ-n patterns with oil flow rates. By comparing the simulation and experimental results, it is found that PDs are mainly concentrated in the negative half-cycle. Electric force causes bubbles to stretch along the electric field direction. At the stationary condition, bubbles eventually exhibit horizontal stretching and cause maximum electric field distortion. However, bubbles eventually exhibit vertical stretching in flowing oil. Flowing oil can significantly reduce PD intensity. The PD intensity declines rapidly with the increase of the flow velocity, and then increases slowly.

In electrical engineering, it is of great importance to monitor partial discharge (PD) of high voltage apparatus. However, on-site PD signal easily gets corrupted by white noise, and hence denoising the measured PD signal is necessary to acquire pure PD. In traditional method, single wavelet and regular thresholding are utilized. By combining wavelets of the same family, dual-tree complex wavelet pairs can be constructed. Specific combinations are approximately translation invariant, and they are also able to achieve better denoising effect without sophisticated computation. Meanwhile, considering the correlation between adjacent wavelet coefficients, the block thresholding is employed. In this paper, it is proposed to combine wavelet pair and block thresholding for PD denoising. There are four combinations based on single wavelet/wavelet pair and regular thresholding/block thresholding, and they are compared with each other. Based on a numerical study, the results demonstrate that the proposed algorithm outperforms the traditional method.

During the operation of power transformers, the insulating oil is actually in a moving state due to the effects of various cooling systems. Therefore, the investigation on insulation characters of moving transformer oil is necessary. In this paper, the influences of temperature on partial discharge (PD) and breakdown characteristics of moving transformer oil contaminated by metallic particles were experimentally studied at first. It was found that the PD magnitude and frequency firstly decreased and then became higher when the temperature increased from 40 °C to 80 °C, and a minimum point appeared between 70 °C and 80 °C. On the contrary, the dependence of breakdown voltage on temperature showed an opposite tendency. In order to explain experimental results, a simulation model involving multiple forces was constructed to obtain the movement trajectory of particles. The increase of oil temperature leaded to the decrease of dynamic viscosity of transformer oil, so the forces acting on particles and their movement characters changed. As the temperature increased, the collision frequency between particles and electrodes firstly decreased and then became higher, and there was a minimum point between 70 °C and 80 °C. This changing tendency was in accordance with the dependence of PD parameters on the oil temperature. In terms of the above results, the effects of temperature on PD frequency and breakdown voltage were discussed.

In this paper, the dynamic distribution of surface charges during partial discharge (PD) sequences was obtained at both polarities of direct current (DC) voltage, as well as at different levels of voltage. It is found that the decay rate of positive charges was higher than that of negative ones, and PD would take place with the existence of residual surface charges. Moreover, the curves to describe surface charges decaying discipline were obtained. Based on these, a simulation model about PD sequence at DC voltage was constructed by considering surface charge decay and discharge time lag. With the help of simulation model, microscopic physical processes, i.e. streamer development and surface charge accumulation, were obtained, as well as macroscopic parameters i.e. discharge current and discharge time. The accumulation of surface charges took a much shorter time at negative voltage than at the positive. Besides, discharge time interval at positive voltage was smaller than that at negative one, and it diminished as voltage increased, which were in accordance with the experimental results. In terms of the evolution of electric field within cavity, the effects of surface charge decay and discharge time lag on discharge time interval and PD magnitude were discussed. The exhaustive physical model is significant to clarify PD mechanism at DC voltage.

The surface charge accumulation on the basin insulator is a particular problem in high‐voltage direct current (HVDC) gas insulated switchgears (GIS) and gas insulated transmission lines (GIL), which restricts the development and application of extra/ultra high‐voltage (EHV/UHV) technology. The surface charge distribution and anti‐flashover performance are closely related to the insulator shape. To obtain the optimal basin insulator, a shape optimisation model based on artificial bee colony algorithm was established, and the minimisation objective functions based on capacitive (initial) electric field distribution and resistive (steady‐state) electric field distribution were respectively constructed. The optimisation results of the two objective functions were calculated, and the electric field and charge distribution on the surface of the insulator before and after optimisation were evaluated and compared using the simulation model. The results indicated that the optimisation based on the capacitive electric field can significantly inhibit the surface charge accumulation, and the optimised insulator can significantly reduce the surface normal electric field and make the distribution of the tangential electric field more uniform, and the maximum tangential electric field can be reduced by 10%; the optimisation based on resistive electric field can achieve similar results, but requires more computational resources. Based on the results, the ‘anti‐charge’ basin insulator structure suitable for the DC field was given.