Ester-based insulating oil provides the possibilities for controlling pollutant emissions and enhancing transformer safeties. A 2-D needle-plate electrode branching-streamer model has been established to numerically investigate the propagation characteristics of initial streamer branches and the effects from needle radius in ester-based insulating oil under lightning impulse (LI) voltage. The results indicate that by adding the source term that affects charge carrier density variation to the current continuity equation, the revised model could better capture the branching-streamer morphology in the experiment than the existing model. The accumulation of positive ions in the streamer branches promotes the electric field distortion, introducing the maximum electric field at the head of the streamer branches, which causes the appearance of the branching streamer due to the localized charge carrier surge (CCS). The electric field at the needle zone is affected by the curvature radius, and the smaller curvature radius enhances the electric field, allowing more space charge to be generated, which leads to easier distortion of the electric field at the head of the streamer branches and promotes forward propagation of the streamer branches. In addition, the smaller curvature radius contributes to the generation of the secondary streamer. 

Numerical Modeling of Branching-Streamer Propagation in Ester-Based Insulating Oil Under Positive Lightning Impulse Voltage: Effects From Needle Curvature Radius

Partial discharges can erode insulating paper, altering its surface morphology. This morphological transformation, subsequently, influences the evolution of further partial discharges. This study constructs a numeric streamer model, integrating the hydrodynamic drift-diffusion model in oil, the bipolar charge transport model in insulating paper, and the Ohmic model for interface charge transfer. The model serves to explore how changes in surface morphology affect the discharge process. Simulation results demonstrate that morphological alterations on the profiled surface accelerate partial discharge development and augment surface charge accumulation on the insulating paper. The generalized concave crater’s charge accumulation effect results in an intensity increase at the bottom of the electric field and a decrease at the top of the profiled surface. The positioning of concave and convex features on the surface primarily impacts the amplitude of local electric fields, which decreases as the distance from the arc center increases. The distortion rate of the electric field in a convex or concave surface is a time-independent characteristic, solely reliant on surface morphology. Given identical surface morphology, a greater value of surface roughness suggests a higher capability for electric field distortion. The results in this paper contribute to a better understanding of surface morphology’s effect on the streamer propagation process. 

Influence of Paper Surface Morphology on Streamer Propagation at Oil-Paper Interface

In the past three years, a number of oil‐immersed ultra‐high‐voltage shunt reactors have experienced discharge defects when being put into operation, resulting in an overload of acetylene. However, detecting and identifying these discharge defects caused by switching impulse voltage is challenging under steady‐state conditions. This poses unpredictable and difficult‐to‐assess safety risks for the longterm operation of the equipment and subsequent transient processes. Hence, comprehending the discharge behavior of oil‐pressboard (PB) insulation under switching impulse voltage and devising a method to identify defects becomes crucial. This study focuses on investigating the frequent arc reignition (FAR) pattern exhibited by typical defects under both standard and oscillation switching impulse voltages. The objective is to uncover the mechanism behind FAR and propose a defect recognition strategy suitable for transient processes. The study reveals the FAR process will occur at least once during the breakdown process; the FAR phenomenon is the weakest in the surface defect with a weak vertical electric field. The average recovery voltage percentage and the average discharge interval of the FAR process decrease with increasing impulse amplitude or oscillation frequency. Additionally, the average number of discharges decreases with higher oscillation frequency, while it initially increases and then decreases with increaseing amplitude. Based on the analysis of the number of FAR processes and their variation in terms of amplitude or discharge interval, a method for recognizing oil‐PB defects during switching transient processes is developed and successfully applied to a case study involving acetylene overload in a 1000 kV shunt reactor.

Defect recognition of oil‐pressboard insulation based on frequent arc reignition phenomenon under switching impulse voltage

Efficient metrics for the detection of partial discharges under steep-fronted voltage pulses

Ester-based insulating oil provides the possibilities for controlling pollutant emissions and enhancing transformer safeties. A 2-D needle-plate electrode branching-streamer model has been established to numerically investigate the propagation characteristics of initial streamer branches and the effects from needle radius in ester-based insulating oil under lightning impulse (LI) voltage. The results indicate that by adding the source term that affects charge carrier density variation to the current continuity equation, the revised model could better capture the branching-streamer morphology in the experiment than the existing model. The accumulation of positive ions in the streamer branches promotes the electric field distortion, introducing the maximum electric field at the head of the streamer branches, which causes the appearance of the branching streamer due to the localized charge carrier surge (CCS). The electric field at the needle zone is affected by the curvature radius, and the smaller curvature radius enhances the electric field, allowing more space charge to be generated, which leads to easier distortion of the electric field at the head of the streamer branches and promotes forward propagation of the streamer branches. In addition, the smaller curvature radius contributes to the generation of the secondary streamer.

Polarity effect of PD in oil-pressboard insulation under highly uneven electric field

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Polarity effect of PD in oil-pressboard insulation under highly uneven electric field