The gas number-density reduced AC electric breakdown field strength (E/N) bd in pure HFO1234ze(E) is investigated from 0.45 bar to 4 bar with a variable electrode gap distance of 4.88, 10.00 and 13.32 mm. The aim of the present contribution is to clarify two aspects that are contradictorily reported in literature. Firstly, the pressure dependence of (E/N) bd and secondly the fact that the first breakdown value in HFO1234ze(E) can be significantly different from subsequent breakdowns in the same arrangement. To clarify these two aspects, particular attention is paid to improving the experimental procedure and to consider the role of electrode surface roughness and solid dissociation products caused by the breakdowns. The methodology of experimentation has been improved, compared to standard breakdown experiments: a freshly polished set of electrodes is used for every breakdown measurement and the electrode surface is preconditioned with low-energy breakdowns in CO 2 to reduce the possibility of micro-protrusions. A quantitative analysis confirms the improved and reproducible surface characteristics due to the preconditioning. The (E/N) bd values measured this way, show an increase with pressure up to 3 bar in 4.88 mm gap distance. In addition, it is confirmed that HFO1234ze(E) is a non-self-restoring gas. Subsequent breakdowns can be significantly lower than the first one across the measured pressure range and this decrease in breakdown strength increases, as energy input to the breakdown becomes higher.

Elevated thermal stress-related aging is significant on the oil-impregnated paper (OIP) used as insulation in high-pressure gas cables (HPGCs). The aim of this article is to develop a cheap alternative for lab dielectric measuring and characterizing temperature-dependent parameters for OIP. First, this article derives the operating thermal conditions of the grid-aged cable based on IEC standards after analyzing the loading data using machine learning techniques to determine the elevated temperature levels for the experiments. Second, a novel lab-fabricated inexpensive electronics circuit is developed for polarization and depolarization current (PDC) measurements which can be adapted for such measurements over expensive commercial devices. From the measured parameters, an extended three-branch Debye model is optimized using a developed error function approach based on the Akaike information criterion (AIC) and goodness of fit. The model indicated a reduction in the branch resistance with temperature elevation and aging, whereas the branch capacitance revealed an increasing trend. The resultant relaxation time (RC) showed a decrease overall. Last, a short-duration frequency domain spectrum was analyzed and extrapolated to obtain parameters for a wide range of frequencies and fit in a Cole-Cole model, derived for oil-paper insulation. The time constants obtained from this model also confirmed a reducing trend across the temperature and aging variations and the model parameter, the alpha coefficient showed a decreasing trend. Last, the effect of the measured dielectric parameters is reflected with breakdown values to investigate the effect of temperature on the electrical life of insulation.

The time-to-failure for oil-impregnated paper (OIP) insulation is governed by two primary aging mechanisms: electrical and thermal. The electrical life can be represented as an Inverse Power Law, where lifetime is inversely proportional to applied electric field. The process of thermal aging on the other hand is established by Arrhenius Law, which relates the rate of aging exponentially to temperature. Due to thermal aging, the structure of insulation is altered owing to chemical changes like oxidation, polymerization, and cellulose degradation. For life estimation of a serviceaged high-pressure gas filled (HPGF) cables, electrical endurance tests are normally performed at controlled voltage levels to estimate the time to breakdown. However, it is equally necessary to investigate how thermal aging influence changes in the electrical life of insulation. Therefore, in this paper, firstly short-term ramped stress tests are carried out on elevated thermal aged OIP samples extracted from already field-aged HPGF to find a rough estimate of breakdown voltages at different temperatures. Then, long-term electro-thermal step stress tests are performed on the samples to establish a correlation of temperature on the electrical life of the OIP insulation. The long-term stress tests produce reliable breakdown statistics and Maximum Likelihood Estimation of Inverse Power Law fitted on 2parameter Weibull distributed breakdown data indicate a reduction of model parameter, n from 13.61 to 7.38 with an increase in temperature from 45 to 75 ℃ and a constant shape factor, β of 1.50. The dissipation factor, tanδ related to the aging also shows an increase with temperature across a wide frequency range and is inversely proportional to the breakdown voltage.