Identifying Prognostic Indicators for Electrical Treeing in Solid Insulation through PD Analysis

This paper presents early results from an experimental study of electrical treeing on commercially available pre-formed silicone samples. A needle-plane test arrangement was set up using hypodermic needles. Partial discharge (PD) data was captured using both the IEC 60270 electrical method and radio frequency (RF) sensors, and visual observations are made using a digital microscope. Features of the PD plot that corresponded to electrical tree growth were assessed, evaluating the similarities and differences of both PD measurement techniques. Three univariate phase distributions were extracted from the partial discharge phase-resolved (PRPD) plot and the first four statistical moments were determined. The implications for automated lifetime prediction of insulation samples due to electrical tree development are discussed

Effect of harmonics on pulse sequence analysis plots from electrical trees

This paper investigates the effect of harmonic pollution on the Pulse Sequence Analysis (PSA) pattern. Partial discharge data was captured from electrical trees growing in epoxy resin in the presence of different harmonic regimes. These regimes included 50Hz waveforms polluted with 3rd, 5th, 7th, 11th, 13th, 23rd and 25th order harmonics, at varying levels of Total Harmonic Distortion (THD) and waveshape factor (KS). In this paper, the data has been analyzed using PSA by plotting the external voltage of consecutive PD pulses (un vs un-1). Under pure 50 Hz conditions, four clusters of data points can be identified in the plot and the formation of the clusters is discussed. Further investigation was performed by running the samples to breakdown. The results show that even in the presence of harmonics, an increase of PD occurrence and phase distribution translates into the expected PSA pattern, where clusters of data points merge to form a 45° straight line. Therefore, PSA is relatively immune to the effects of harmonic distortion when considering it only as an indicator of breakdown

Generating PD data from electrical treeing in silicone rubber for insulation lifetime modelling

Electrical treeing is a degradation phenomenon in solid dielectric material resulting from high and non-uniform electrical field or partial discharges (PD). The presence of electrical treeing, therefore, can be examined through PD monitoring by looking for characteristic features within the phase-resolved plot of PD data. As electrical trees evolve in time, time-resolved analysis of PD data may be more descriptive of the correspondence between discharges and tree propagation. Continuing partial discharges in electrical treeing may lead to catastrophic failure, but there is still a lack of understanding of the evolution of PD characteristics prior to breakdown. This paper focuses on a method of simplifying the production and growth of electrical trees in silicone rubber (SiR), an advanced insulating material that is widely used in high voltage cable accessories due to its excellent insulation and mechanical performance. Crucially, commercially available pre-formed samples of SiR are used to ensure consistency and eliminate the need for the mixing, degassing and heating process in sample preparation. The experimental methodology is described, in terms of sample preparation, applied voltage regime, and data capture. A constant 50 Hz AC voltage is applied to the samples (with a needle-plane test arrangement using hypodermic needles) at a level sufficient to induce PD, leading to breakdown within hours. Both IEC 60270 electrical method and radio frequency (RF) sensors are used to capture PD data, while a digital microscope is used for visual observation. The paper describes the features found within the PD phase-resolved plot, and evaluates the similarities and differences between the two measurement techniques. Future work aims to automatically detect those features corresponding to electrical tree growth, and give a lifetime prediction for the insulation samples being studied

Prognostic modeling for electrical treeing in solid insulation using pulse sequence analysis

This paper presents a prognostic framework for estimating the time-to-failure (TTF) of insulation samples under electrical treeing stress. The degradation data is taken from electrical treeing experiments on 25 epoxy resin samples. Breakdown occurs in all tests within 2.5 hours. Partial discharge (PD) data from 18 samples are used as training data for prognostic modeling and 7 for model validation. The degradation parameter used in this model is the voltage difference between consecutive PD pulses, which decreases prior to breakdown. Every training sample shows a decreasing exponential trend when plotting the root mean squared (RMS) of the voltage difference for 5 minute batches of data. An average model from the training data is developed to determine the RMS voltage difference during breakdown. This breakdown indicator is verified over three time horizons of 25, 50 and 75 minutes. Results show the best estimation of TTF for 50 minutes of data, with error within quantified bounds. This suggests the framework is a promising approach to estimating insulation TTF