C5-PFK (C₅F₁₀O), with high dielectric strength and low global warming potential, is the new environmental-friendly potential alternative gases to SF₆. In order to reduce its excessively high liquefaction temperature while retaining its excellent properties, a certain amount of buffer gases, such as N₂ can be added to it. Therefore, to understand the plasma behavior of under C₅F₁₀O-N₂ mixtures, the composition, thermodynamic properties and transport properties of the C₅F₁₀O-N₂ mixtures in a temperature range of 300K to 30000K and a pressure range of 0.1 MPa to 0.8 MPa is calculated under the assumption of local thermodynamic equilibrium. Then, the influence of mixing ratios and pressures are analyzed. The results can be used to determine the optimal proportion of C₅F₁₀O-N₂ mixtures. Several peaks of the specific heat can be observed below 5000K, which could be a favorable factor for arc quenching.

SF ₆ has strong greenhouse effect, and the research on the environmental friendly alternative SF6 gas has attracted extensive attention. In this paper, alkanes, halogenated hydrocarbons and fluorinated hydrocarbons with special groups were selected as the research objects and a nonlinear regression model for predicting insulation strength was established. The microscopic parameters of each gas molecule were calculated by density functional theory. The calculation functional is B3LYP and the basis are 6-31g/6-31g (d). The results show that the insulation strength of gas molecules increases with the number of carbon atoms in the molecule. When the number of carbon atoms is the same, the insulation strength of gas molecules containing unsaturated bonds is higher. The energy of the lowest unoccupied molecule orbit and vertical electron affinity of gas molecules are important parameters to study the insulation strength of gas, but they have a strong collinearity relationship, so only one of them can be choose in establishing the calculation model of the insulation strength of micro parameters by regression analysis. The insulation strength prediction model describes the sample value characteristics of vertical electron affinity through exponential function, and the correlation coefficient R ² is 0.735, which can preliminarily screen SF6 substitute gases.

Partial discharge (PD) can occur in cross-linked polyethylene (XLPE) insulation within high-voltage direct current (HVDC) cables with defects and the continuous development of PD will lead to the final breakdown of the cable. To study the PD behavior caused by defects in HVDC cable and provide effective guidance for the operation and maintenance of the cable, three types of defects were prepared in this paper, namely micro-void, protrusion, and interface fouling. For these three typical defects, experimental research on PD under stepped voltage boosting was conducted using the pulse current method at 30 ℃, 50 ℃, and 70 ℃, respectively. The research results indicate that the PD inception voltage (PDIV) of interface fouling is higher than that of micro-void and protrusion, while the discharge repetition rate is lower. The PD experimental results of the three types of defects all indicate that, when the defects do not develop significantly, the increase in temperature will reduce PDIV and increase their discharge frequency. The charge transfer model is employed to analyze the impact mechanisms of high temperature and electric field on the PD process, providing a rational explanation of the PD behavior during experiments. In addition, a discharge frequency evaluation method based on the equivalent circuit model of defects is proposed, which may assist in assessing defects’ destructiveness. The experimental findings and evaluation methods obtained in this study can serve as a reference for assessing the defect development process in DC cables under various operating conditions.

To investigate the effect of DC superimposed with opposite-polarity lightning impulse on the insulation performance of cross-linked polyethylene (XLPE) cable, this paper built a dedicated experimental platform. Based on maintaining a stable DC voltage on XLPE film samples, opposite-polarity lightning impulses with different numbers of applications were applied. The results show that as the number of impulses increases, the DC breakdown strength of XLPE insulation decreases significantly. After 300 consecutive impulses, the DC breakdown strength decreases by 48% compared to that without impulse, and the dispersion increases. Isothermal relaxation current (IRC) tests indicate that as the number of impulses increases, the trap density of the material increases significantly, and the trap energy level deepens continuously. Space charge measurements further show that the dynamic behavior of space charge inside the material becomes more complex after multiple impulses. To reveal the physicochemical mechanism of the effect of impulse number on the dielectric properties of XLPE insulation, multi-scale characterization of the samples was carried out using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and fourier transform infrared spectroscopy (FTIR). The results show that as the number of impulses increases, the initial thermal decomposition temperature and crystallinity of XLPE insulation decrease significantly, while the intensity of characteristic absorption peaks of aging functional groups such as carbonyl groups in the infrared spectrum continues to increase. Comprehensive analysis reveals that lightning impulses cause XLPE molecular chain breakage accompanied by generation of small molecular fragments. Meanwhile, the polarization loss caused by the periodic reversal of the electric field during the impulse process converts into thermal effects, thereby accelerating the thermal-oxidative aging reaction and promoting the generation of more aging products. Pearson correlation analysis further confirms a strong correlation between the evolution of trap characteristics and the degradation of breakdown performance, thus establishing a direct quantitative relationship between microstructural damage and macroscopic performance deterioration, and profoundly revealing the influence mechanism of the impulse accumulation effect on dielectric properties.

This paper investigates the evolution of insulation performance in HVDC cross-linked polyethylene (XLPE) cables located in lightning-prone areas under frequent reclosing conditions. By simulating the reclosing process using a cyclic short-circuit grounding device, the aging mechanism of the XLPE under repeated short-circuit processes was characterized through four testing techniques. The experimental results demonstrate that the gradual temperature rise enhances molecular thermal motion and accelerates the charge carrier migration process. Simultaneously, this provides the necessary energy for microstructural degradation, leading to a reduction in crystallinity. Specifically, when the electric field increases from the working electric field (WEF) (18kV/mm) to the threshold electric field (TEF) (50kV/mm), the average shape parameter decreases by 33.79%. Physicochemical testing results reveal that the influence of the electric field on the main chain structure of polymer molecules at 30 or 50°C is more pronounced. Notably, crystallinity exhibits a systematic decrease at the TEF. Compared to 30°C, the crystallinity of XLPE decreases by 26.97% at 70°C. Finally, this paper examines the correlation analysis of the electric field and temperature on space charge accumulation. Moreover, by examining microstructural changes, the relationship between microscopic defects and localized electric field distortion is elucidated, advancing the understanding of XLPE aging mechanisms under frequent short-circuit conditions and providing critical insights into XLPE insulation degradation in complex aging scenarios.

The essence of polymer breakdown is the destabilization of charge transport. Under the influence of electric field, free electric charges interact with the polyethylene (PE) matrix, leading to energy release in both radiative (luminescence) and non-radiative (thermal) forms. By recording time-resolved optical emission (OE) during DC breakdown, we provide observational constraints on charge-transport and energy-release pathways rather than asserting a single mechanism. In this study, based on the spectroscopic measurement technology, a set of spectroscopic measurement system for DC breakdown luminescence measurement of macroscale low-density polyethylene (LDPE) samples was constructed, and the optical signals were collected during the DC breakdown process of LDPE samples with thicknesses of 50 μm, 100 μm, and 200 μm. The results show that the DC breakdown electric field decreases with the increase of the sample thickness, and the spectra share a common band family at 254 nm, 270 nm, 360 nm, 430 nm, 500 nm, and 590 nm, while their relative intensities vary with thickness. Considering the finite passbands (notably a wider deep-UV channel) and the UV absorption of transformer oil, short-wave intensities are attenuated and may merge multiple processes. Accordingly, we interpret the observed features as most consistent with a mixed origin from (A) solid-state radiative recombination (including fluorescence/phosphorescence), (B) micro-plasma emission in the breakdown channel (e.g., CH/C₂/H features within the passbands), and (C) a thermal continuum. Furthermore, time-resolved optical signal analysis indicates a range of waveform patterns—including single-pulse, multi-pulse, additional multi-pulse, and hump-shaped pulses—highlighting the complex charge dynamics occurring within the LDPE matrix during DC breakdown.