Abstracts:In this article, the degradation mechanisms of unfilled polydimethylsiloxane with different molecular weights are investigated under dry-band arcing using the inclined plane tracking and erosion test and material characterization techniques. Tracks of carbonaceous residue formed as byproducts of radical-based crosslinking are prevalent over erosion as the molecular weight increases. Erosion becomes dominant with polydimethylsiloxane that undergoes mainly depolymerization during degradation at lower molecular weights. Accordingly, tracking due to a progressive carbonaceous residue formed on the surface is the degradation pattern observed under the dry-band arcing for the polydimethylsiloxane with higher molecular weights (45 000 and 28 000 g/mol) during the inclined plane test (IPT), whereas erosion due to depolymerization is more evident on the polydimethylsiloxane surfaces with lower molecular weights (17 200 and 9400 g/mol). This study highlights the possibility to tune the tracking and erosion tendency of silicone rubber outdoor insulation by controlling the molecular weight of the organosiloxane base, without the sole reliance on fillers as commonly perceived.

Abstracts:Condition assessment is significant to the maintenance management of high-voltage (HV) cross-linked polyethylene (XLPE) power cables. This article presents a condition assessment method for corrugated aluminum sheathed cables based on evolved gas analysis (EGA). First, simultaneous thermogravimetric (TG) and mass spectrometric analyses of the pyrolysis of each cable material were conducted to characterize the gas generation caused by thermal faults. Simulated experiments were designed to investigate gas generation caused by electrical faults. The results show that different cable faults (low- and high-temperature pyrolysis of semiconductors, insulation, and buffer materials; low and high energy discharge activity across insulation materials and electrochemical corrosion of the buffer material) show distinct gas generation characteristics. Based on the evolved gases, a method of identifying faults from the key gas concentration is proposed in this study. Moreover, gas transport within the corrugated aluminum sheathed cable with a buffer layer was analyzed, based on which a gas sampling method for cables with a buffer layer is proposed. Finally, the developed method was validated by practical applications. The proposed method is comparable to conducting a blood test on a human body in that it can help discover various incipient cable faults. The fundamentals and data interpretation methods are adaptable to all XLPE cables.

Abstracts:This article presents the optimization of electric stress distribution over the live electrode of an axisymmetric vacuum interrupter which is employed for 11 kV switching operations. The optimization is carried out by a classical approach using Charnes&#x2019; Big M algorithm used for solving constrained linear programming problems (LPPs). For this purpose, the objective function is obtained by applying multiple linear regression analysis by which a mathematical relationship between the maximum resultant electric field intensity (<inline-formula> <tex-math notation="LaTeX">${E}^{text {max}}$ </tex-math></inline-formula>) over the surface of the live electrode and the critical dimensions affecting this stress. For this purpose, a set of 173 data is used which is prepared by varying the critical dimensions within the constraints of the overall dimension of the system and calculating the value of <inline-formula> <tex-math notation="LaTeX">${E}^{text {max}}$ </tex-math></inline-formula> over the surface of the live electrode by employing the boundary element method (BEM).

Abstracts:Trends toward high power density designs and emerging wide bandgap (WBG)-based semiconductors will lead to a severe temperature rise in power electronic modules. In this regard, the insulation performance of silicone gel used for power module packaging deteriorates at high temperatures and frequencies, which is a concern and has not been well studied. This study focuses on the electrical properties of silicone gel at high temperatures, including its dielectric properties, volume resistivity, and breakdown strength. A Cole&#x2013;Cole model was used to describe the dielectric response process. In particular, the dielectric loss density under pulse width modulation (PWM) voltage was calculated, and the temperature rise of the encapsulation structure caused by the dielectric loss was observed. It was found that the high-frequency dielectric constant at 200 &#x00B0;C decreases by 24&#x0025; compared to room temperature; when the temperature is raised to 180 &#x00B0;C, the volume resistivity drops by three orders of magnitude, and the 50-Hz ac and 10-kHz unipolar square wave breakdown strength of silicone gel is reduced by 50&#x0025; and 42&#x0025; compared with those at room temperature. The dielectric loss under PWM voltage increases by four orders of magnitude as the temperature increases up to 200 &#x00B0;C but does not lead to a significant temperature rise in the encapsulation structure.

Abstracts:Under natural conditions, insulators show a nonuniform pollution state, which has a great influence on the switching impulse flashover voltage of insulators. Meanwhile, the insulation level of transmission lines is selected according to the switching impulse flashover voltage of nonuniform pollution insulators. For this purpose, in this article, the wet pollution method is used to simulate nonuniform pollution conditions. Then, the switching impulse flashover tests and the switching impulse flashover tests with preloading ac voltage are conducted on two types of insulators, XP-70 and LXY-70, in natural environment. The flashover voltage of insulators is obtained, flashover performances are analyzed, and the flashover processes of insulators are photographed. Moreover, the equations of flashover voltage gradient <inline-formula> <tex-math notation="LaTeX">${E}_{{50}}$ </tex-math></inline-formula> under nonuniform pollution conditions are established. The results indicate that there is a conspicuous polarity effect and a polarity reversal phenomenon under different conditions. Besides, as the increase of the ratio of bottom surface pollution to top surface pollution, the flashover voltage changes in the opposite case of preloading and non-preloading ac voltage. Finally, the switching impulse flashover voltage of ordinary porcelain and glass suspension insulators under nonuniform pollution conditions can be calculated based on the established equations of <inline-formula> <tex-math notation="LaTeX">${E}_{{50}}$ </tex-math></inline-formula>. The results can provide theoretical support for the reasonable selection of the insulation level of transmission lines, which in turn can greatly reduce the cost of the transmission lines.

Abstracts:Adaptive material can alleviate the local high electric field in electrical devices, due to its nonlinear conductivity characteristics. To solve the deterioration problem caused by the high doping of large-diameter fillers in the adaptive material, carbon nanotube (CNT) was used as the second filler. And the effect of CNT doping amount on the conductivity of SiC/silicone rubber (SR) composites was explored. Experiments showed that CNT doping can improve the nonlinear conductivity. The threshold field of the material doped with 0.7 vol &#x0025; CNT decreased by 97&#x0025;, and the nonlinear coefficient was increased by 83.7&#x0025;. With the increase of CNT doping content, the decay rate of surface potential increased, which was attributed to the increase of the carrier mobility and ratio of shallow-to-deep trap density. A new field grading material (FGM) coating structure was proposed, which was compared with the traditional structure. Through finite-element simulation, the field grading effect of the two structures can be analyzed. The results showed that the field grading effects of the two coating structures were almost the same. But considering the influence of surface charge dissipation, the new coating structure was selected. The electric field of insulator surface and interface can be reduced by more than 30&#x0025; with the best CNT doping ratio. It was verified that the interface charge and leakage current of the insulator coated with the best FGM were at the compliance level.

Abstracts:Based on the experimental results of dynamic breakdown voltage and electrical breakdown time delay, the dielectric properties of insulating gas commercial gas-filled surge arresters (GFSA) were analyzed. Data for dynamic breakdown voltage obtained by the method of stepwise increase of voltage whose rise rates ranged from 1 to 10 V/s. It is shown that these values of voltage rise rate do not have a significant effect on the value of dynamic breakdown voltage. In order to monitor the processes caused by electrical breakdown on the dielectric properties of insulating gas of these components, the electrical breakdown time delay was a measure for different values of relaxation time and applied voltage. It has been shown that the presence of positive ions, which are formed during breakdown and subsequent self-sustaining discharge with recombination time of about 1 ms, has a significant effect on the reduction of the dielectric resistance of insulating gas. It has also been shown that the applied voltage whose values are close to dc-spark over-voltage has a significant effect on the delay response of these components.

Abstracts:Time-series prediction technology plays a significant role in evaluating the health status of power transformers and forecasting inchoate operation failure. This study presents a variational mode decomposition (VMD) model with the application of autoregressive integrated moving average (ARIMA) to develop a prediction technology for the dissolved gas analysis (DGA) time series. In addition, the two-step stationary test method regulates the nonstationary of the time series. The VMD decomposes the DGA time series into subcomponents to reduce the nonstationary, where the boost marine predators algorithm (BMPA) optimizes the parameters <inline-formula> <tex-math notation="LaTeX">${K}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$alpha $ </tex-math></inline-formula> of the VMD to retain the useful information of the original signal. The ARIMA predicts the future result for each subcomponent, and the ARIMA parameter is optimized by the BMPA for optimal solutions of the Bayesian information criterion (BIC). The performance of the proposed approach is measured by predicting the health status of the diurnal new values of the DGA of a power transformer. The experiment result shows that the proposed model exhibits high efficacy in predicting the DGA time series and obtains the health status of the power transformer.

Abstracts:Roof insulators are one of the key pieces of equipment to ensure the electrical safety of high-speed railways (HSRs). When the umbrella skirt of a roof insulator is seriously polluted in various environments, the risk of pollution-induced flashover rises. Field experience shows that the flashover possibility of roof insulators in coastal intercity railways is much higher than that in other regions. However, the pollution characteristics of roof insulators under those conditions are still unclear. In this work, we investigated the pollution characteristics of roof insulators used in the coastal intercity railway of electric multiple units (EMUs) of China. Aspects of the pollution spatial distribution, the statistical content, and the pollution constituent on the surface of the umbrella skirt of insulators are taken into account. The airflow field and electric field distribution around the pantograph support insulator (PSI) are also simulated based on finite element software. To reduce the formation of streak-shaped pollution, the umbrella skirt is optimized by the streamlined structure. The results indicate that a novel phenomenon of streak-shaped pollution distribution might form on the surface of the insulator, which would severely aggravate the distortion of electric field distribution. The calculated turbulence dissipation rates are found to spatially agree well with the streak-shaped pollution distribution. The streamlined structure effectively reduces the turbulence dissipation rate at the junction of the windward and leeward surfaces, to inhibit the formation of streak-shaped pollution.