A method to evaluate the accuracy of space-charge measurements by estimating the electric field distribution in the insulating walls of cables is proposed in this work. Time-domain deconvolution is used to derive the stable and fully automatic processing techniques necessary to manipulate the large data required to perform the statistical analysis. The evaluation method optimizes the deconvolution parameters and analyzes the space charge waveform, which is usually only used as a reference waveform. The proposed method was validated by analyzing pulse electroacoustic waveforms obtained from a 66 kV-class extruded cable under dc voltage application. The impulse response of the observation system was optimized to provide a 95.4% confidence interval of 0.8% ± 24.1% for the waveforms, which were observed under application of a dc voltage of 30 kV. Similar estimation accuracy was confirmed for waveforms measured under application of 45 kV. Therefore, the accuracy estimated using the reference waveform can be applied to waveforms observed at high voltages that can induce both space-charge accumulation and field distortion.
The likelihood of electrostatic discharge due to the accumulation of charge from the space environment depends strongly on the conductivity of spacecraft dielectrics. Spacecraft polymers are often filled with inorganic substances including glass fibers, glass beads, and thickening powders in order to modify their mechanical properties. This study examines the impact of these fillers on the conductivity of a few common spacecraft polymers. We show that the impact of inorganic fillers varies greatly from one usage case to another.
The potential application of eco-friendly thermoplastic material - Polypropylene (PP) for high voltage direct current (HVDC) cable has attracted significant attention. However, a better understanding of the ageing effects of on the long-term performance of PP is needed for a reliable operation of a HVDC cable. Based on previous work on thermal ageing of neat PP, an antioxidant (AO) has been introduced to investigate its influence in PP thermal-oxidative ageing for a long potential service life (>40 years) under high temperature in this work. According to chemical changes characterized using Fourier-Transform Infrared (FTIR) spectroscopy, the AO is initially ready to react with oxygen on sample surface and subsequently progresses towards the bulk of the sample over time. The space charge dynamics were measured with the pulsed electroacoustic (PEA) technique. It has been found that hetero charge formed near the cathode would further increase after adding AO, but negative charge becomes dominant with continuous poling time. After thermal ageing experiments are completed, there is an obvious charge injection near the cathode. Combined with the positive charge movement caused by the AO, a resultant charge peak can be observed. We correlate this behavior with DC breakdown strength experiments, which show an increase at an early stage of the ageing process, before decreasing as a function of ageing time.
Crosslinked polyethylene (XLPE) is a key material for power cable insulation due to its superior electrical properties. It is co-extruded with a semiconducting “semicon” layer, which is significantly more conductive than XLPE due to the incorporation of carbon black. Understanding the electrical properties of the XLPE/semicon bilayer and the interface between them is critical due to their common use. Two techniques were used to study electrical properties of XLPE and XLPE/semicon bilayers. One was current-voltage measurements, which analyzed resistivity as a function of time. The other was high-voltage polarization, which analyzed polarizability and dielectric loss as a function of electrical field. Thin films of PE and semicon were made via melt pressing. Dicumyl peroxide was infused to the beads prior to melt pressing such that XLPE was formed. All samples were degassed to remove DCP byproducts. Semicon/polyethylene bilayers were made by pressing the two layers together. LDPE and LDPE/semicon were also studied. Bulk brass electrodes were used. Current-voltage measurements showed that adding the semicon layer increased conductivity by an order of magnitude. Polarization measurements revealed that significant enhancement in the dielectric loss occurred at electric fields of 15 kV/mm and above. Polyethylene by itself and semicon bilayer samples had distinct electrical properties, and thus a proper understanding of the performance of XLPE in power cables required analyzing both. Further work is suggested that focuses on correlating changes in measured conductivity to the nature of the material interface (e.g., via enhanced charge injection and/or introduction of charge carriers).
Transformer bushing is one of the transformer current-carrying components. Transformer bushing failure will cause the oil tube lead role to be damaged, which is not conducive to the normal operation of the transformer. In order to accurately predict the bushing temperature and detect potential transformer faults in a timely manner, a temperature prediction model based on historical temperature monitoring data was proposed using Particle Swarm optimization (PSO) to optimize Long and Short Term Memory Network (LSTM). The real data was collected continuously for 13 days under the same sensor on a high voltage bushing. After cleaning, the data were used as the input data set for training and test purpose of PSO-LSTM. PSOLSTM method was used to automatically optimize the LSTM parameters, which avoids the problem of low prediction accuracy due to empirical selection of parameters, thus improving the convergence speed and prediction accuracy. The simulation results show that the PSO-LSTM algorithm can complete the bushing temperature prediction within 7s. In terms of temperature prediction accuracy, root mean square error and average absolute error, it outperforms Back Propagation (BP) neural networks, Wavelet Neural Networks (WNN) and single LSTM algorithms, which shows its good application prospect in online monitoring and fault diagnosis for electric transformers.
The effect of non uniform aging on the insulation performance of transformer oil-paper insulation has not been investigated much. In this paper, a modified simulation model is developed using COMSOL Multiphysics software, and the impact of non uniform aging on the electrical properties of insulation such as electric field, current density, and space charge density is investigated by comparing the distribution of these parameters under both uniform and non uniform aging. In comparison to uniform aging, the distribution of electric field at low frequencies under non uniform aging is mainly concentrated at the outermost solid insulation with its value decreasing from the outermost to innermost insulating paper region. Also, the flow of charge carriers and their accumulation takes place at the oil-barrier gap with severe aging. Total losses with uniform and non uniform aging are also compared indicating slightly higher losses with non uniform aging.
The electromechanical performance of tri-post insulat-or is the key factor to determine whether GIL can operate safely. The structure of tri-post insulator directly affects its electromech-anical performance. In order to explore the influence of typical structural parameters on the electromechanical performance of tri-post insulator, a three-dimensional finite element simulation model is established, and the electric field calculation and the mechanical stress calculation under vertical installation are carried out. Eleven typical structural parameters (including parameters of the post, inserts, the abdomen and the metal center sleeve) were selected as independent variables, the electrical targets’ values and mechanical targets’ values under different structural parameters were calculated and the sensitivity analysis has been done. The results show that the tangential field strength is concentrated in the post, and the normal field strength is concentrated in the abdomen of the insulator. The bonding surfaces of the insulators are the locations where the mechanical stress is significantly concentrated throughout the structure. There are obvious conflicts between the rules of different structural parameters on the electric field distribution and mechanical stress distribution of insulators, which provides a reference for optimum design of insulator structure.
The degradation of transformer oil-paper insulation is one of the major concerns for utilities and transformer owners. Temperature and moisture are amongst the key variables to be considered while determining the load profiles that can be safely applied to transformers. In this paper, important parameters used to monitor the degradation of the oil-paper insulation are introduced in the existing thermal and moisture models reported in the literature. The standard IEC 60076-7 loading guide model is improved by including the viscosity of the liquids with a controlled aging history to estimate the behavior under hot-spot conditions. Similarly, moisture model is improved by including the acidity of oils and degree of polymerization of the paper. This allows a better representation of the relative moisture saturation of the liquids and the bubble inception temperature. In this work, potential models are reported to understand the moisture dynamics of cellulose paper and predict the bubbling inception temperature under specific loading profiles. The simulation analysis is also spanned to different aging conditions of mineral oil and cellulose based oil-paper insulation. The developed models may be helpful to monitor the moisture in paper and predict the bubbling temperature while taking into consideration temperature variations along with the degradation of the insulation system inside the unit.
This study was carried out in order to evaluate properties of new insulating paper (Hybrid paper or HP, aramid/cellulose composite) and thermally upgraded kraft paper (TUK) during ageing at 165 and $185 ^{\circ}\text{C}$ in natural ester (NE, corn-based oil) and mineral insulating oil (MIO). Results will be complementary to a detailed and critical study to determine Arrhenius life curve for the liquid immersed transformer insulation system composed of NE and MIO in combination with TUK and HP. Accelerated ageing tests were carried out in laboratory for the HP, according to IEEE C57.100 standard, revised in 2011. Tests were conducted with MIO and with NE. A commercially available TUK paper was used as a reference, which was subjected to the same test condition. In this study, preliminary paper properties observed in TUK/NE, TUK/ MIO, HP/NE, HP/MIO insulation systems at temperatures of 165 and $185 ^{\circ}\text{C}$ are described. Degree of polymerization (DP), tensile testing and infrared analysis were performed on aged papers. HP presented a better performance in relation to TUK, in both oils. Furthermore, in NE slower degradation was observed for both papers at 165 and $185 ^{\circ}\text{C}$. Infrared spectroscopy analysis of aged papers showed that there was no transesterification reaction in both papers aged in insulating liquids (NE or MIO).
This work reports the use of the Partial Discharge (PD) measurement technique as a tool for the investigation of High Frequency/High Voltage Pulsed (HF/HVP) transformers for motorsport application. PD technique is successfully applied to monitor the status of the internal insulation of these devices. Insight about the ageing and failure mode of the internal insulation system is obtained. PD technique proves to be a valuable diagnostic tool for HF/HVP transformers.
With the expanding scale of UHV transmission projects, the lack of high-performance insulation combinations has threatened the safe operation of the power system. Phase-change material with suitable thermal stability and insulation characteristics is gradually being applied to high-voltage power electronic devices as refrigerants. Oil-paper insulation technology has been extensively studied, but no systematic compatibility tests are focusing on refrigerants and insulation materials. In this paper, Perfluoro-compound (FC-72) and Perfluoroheptano (EC74) were selected to study the compatibility with insulating paper at room temperature. By carrying out power-frequency breakdown voltage test, dielectric loss characteristics test, and SEM detection, electrical characteristic curves at different stages and temperatures, surface and profile microscopy images of two electronic fluoride liquid-impregnated insulating paper were obtained. Whether from the macro or micro perspectives, it can be proved that compatibility of FC-72 with insulating paper were significantly better than FC-74, and it also helps to improve the electrical resistance and aging resistance of insulating paper, which means FC-72 can be used as an alternative to excellent insulating oil for transformers.
Vibration monitoring is an effective way to evaluate the mechanical condition of converter transformers. In this paper, the vibration characteristics of converter transformers under no-load condition are investigated by performing a no-load voltage rise test with DC lines in a ± 800kV converter station. Vibration features like vibration amplitude, odd-even harmonic ratio, recurrence rate, and determinism are employed to study the variation trend of converter transformers. It is found that these vibration characteristics show different variation patterns during the test, which can provide reference for evaluating the operation status of the converter transformer.
Serious icing of transmission lines threats the safe operation of power system. To reduce the complexity, most conductor icing models ignore the effect of conductor electrification, and theoretical icing is often not in line with the actual situation. In this paper, an icing model under DC electric field is proposed to study the mass and formation of rime under different electric field strength. According to the local collision efficiency (LCE), the icing mass of different elements on the icing surface is obtained, and the ice shape under different surface electric field strength is calculated iteratively. The simulation result shows that with the increase of DC electric field strength, the particles are affected by dielectrophoretic force and electric field force, and the mass of rime icing increases first and then decreases. When the surface electric field is as high as −25kV/cm, the Coulomb repulsion is dominant, the more obvious the repulsion is at the boundary, and the ice shape shrinks and concentrates towards the center.
This paper is focused on the measurement of partial discharges in the Silicon Carbide MOSFET. The latter represents a recent developed power electronics device, which has improved characteristics compared to the traditional Silicon MOSFET. The new technology has been conceived in order to satisfy the new market needs. With the ever-increasing development of the electric mobility sector, the need for ever more performing electronic power devices grows. The main features provided by the Silicon Carbide device were found in literature and concern a great thermal conductivity, wide bandgap, high power and high working temperature. However, considering the high voltage at which these devices work, also taking into account their small geometric dimensions, it is essential to evaluate the behavior from the partial discharges point of view. Based on the above, the aim of the present work is the measurement of partial discharges in ten different Silicon Carbide MOSFET devices in order to evaluate the electrical performances of the new proposed material as well as the quality of their manufacturing process. Experimental tests have been carried out by taking into account the Standard CEI EN 60270 for the realization of the measurement setup and the Standard CEI EN 61287 for the measurement procedure. In particular, the latter Standard provides the guidelines for the applied voltage waveform. Measurement results showed that no partial discharge phenomena occur in all the specimens under tests. Therefore, the SiC MOSFET passed the partial discharge test successfully.
The Phase-Resolved-Partial-Discharge pattern (PRPD) is a conventional technique used for the evaluation of partial discharges (PD) phenomena in High-Voltage-Alternating-Current (HVAC) systems. This map is constructed by plotting the peak of each detected pulses as a function of the phase angle of the supply voltage. Therefore it is obvious that this technique cannot be used for the analysis of data from PD mesaurement under different supply voltage condition (DC). The aim of this paper is to evaluate the application of the Time-Frequency map (TF map) for the analysis of a dataset obtained from PD measurement under DC voltage. A density-based clustering algorithm was also used to gain more insight from the collected data. The results show that, with this approach, it’s possible to perform a noise rejection and identify PD pulses.
Non-uniform dampness of main insulation is a serious defect for the oil-impregnated paper bushing (OIPB) in the transformer, which causes its electric parameters unusually rising, insulation failure and explosion. Temperature gradient field is one of the main factors in the moisture diffusion in transformer bushing accidents. In this paper, by using the method of physical field finite element modeling analysis, the moisture migration patterns under different temperature gradient of oil-impregnated paper insulation bushing was studied from the most intuitionistic image observation. The damp location and diffusion process of water concentration under the temperature were obtained. The results show as follows: 1) at the temperature from 20°C to 40°C, the moisture migration speed in bushing under the action of each increasing 10°C is much less than that of 50°C to 70°C. At the same diffusion time, the increment of temperature accelerates the degree of moisture diffusion. 2) There is an obvious asymmetry in the upper and lower parts of the bushing in the concentration visual cloud picture of moisture content. 3) The aluminum foil inside the core blocks unevenly the water diffusion among multi-layer oil-paper core, forming several local dry and wet areas, which eventually leads to severe local insulation failure of the bushing.
The accurate identification and hazard assessment can effectively realize condition-based maintenance and equipment life cycle economic management. For this purpose, a novel discharge identification approach based on multi-physical information fusion is proposed considering practical applications in equipped engineering sites. Three typical discharge defect models are placed within the a SF6 gas-filled test chamber, where the high frequency current transformer (HFCT), ultra-high frequency (UHF) and acoustic emission (AE) methods-based sensors are placed for discharge pulse monitoring. Furthermore, the non-phase information of discharge pulse for each measurement are extracted by the developed host computer software, where the fingerprint characteristics, namely maximum pulse amplitude (MPA) and cumulative pulse amplitude (CPA) are further extracted. Finally, the extracted multi-physical characteristics are aggregated into feature matrix. The identification results indicated that diagnosis accuracy of the proposed model achieves an average accuracy of 99.33%, which demonstrates that the multi-physical information fusion can resulting in better diagnosis performance.
This paper investigates the effect of the test voltage level on the performance of silicone rubber in inclined plane tracking and erosion test. Silicone rubber composites, filled with either alumina tri-hydrate or ground silica to 30 wt% or 50 wt%, are tested in inclined plane tracking and erosion test under 2.5 kV, 3.5 kV, and 4.5 kV. The degradation patterns of the tested silicone rubber surfaces are found dependent on the test voltage level during the inclined plane tracking and erosion test, as the dry-band arcing on silicone rubber tends to form tracks under relatively mild test voltages and deep erosion under the critical test voltage. These findings confirm the importance of employing the critical voltage while evaluating the erosion resistance of silicone rubber in the inclined plane tracking and erosion test. In addition, the critical test voltage of silicone rubber is found dependent on the amount of filler added to the composite.
Electric power systems (EPS) for the future generation of electrified aircraft such as more electric aircraft (MEA) and all electric aircraft (AEA) are required to be high power delivery and low system mass. Due to the limited heat transfer by convection at the cruising altitude of a wide-body aircraft, designing cables for high power delivery and low system mass electric power systems (EPSs) based on typical standards e.g., IEC 60502 faces challenges such as thermal limits of the typical insulation systems. To design a low system mass cable system, aluminum should be used as the conductor and the overall diameter of the cable should be decreased. The former increases the joule losses of the cable, and the latter reduces heat transfer by radiation and convection, both resulting in exceeding the cable's maximum permissible temperature. In this paper, a multi-layer insulation system for a ±5kV cable is designed for aircraft applications. The designed multi-layer insulation system experiences higher thermal conductivity and contains high-temperature materials such as AlN, PI, and PFA to compensate for the overall heat transfer reduction caused by decreasing the cable's overall diameter. The designed multi-layer cable has a smaller thickness and a lower mass compared to insulation systems designed based on IEC 60502 standard. To determine the electric field and temperature field distributions across the designed insulation system a coupled study in COMSOL Multiphysics has been conducted. The main purpose of this study is to compare the designed multi-layer cable system's overall diameter and mass to the cables designed based on IEC 60502 standard. Moreover, the obtained electric field distribution across the designed insulation shows that the designed insulation system is electrically safe.
Size optimization of medium voltage (MV) devices, such as switchgear, requires accurate prediction of the impulse and AC power frequency breakdown voltages which is the focus of the paper. Dielectric breakdown was performed on three electrode arrangements that simulate actual switchgear for SF6 and two fluoronitrile/CO2 mixtures and the experimental data were compared with simulation result based on the streamer inception criterion. For weakly non-uniform fields, the experimental values show good agreement with those predicted using an approximation formula for the effective ionization coefficient. For strongly inhomogeneous fields, the breakdown voltage is computed based on the non-uniformity factor. A framework rule for breakdown prediction based on the streamer criterion, when applied to several switchgear electrode designs and gases, show good agreement with experimental results.
