Electrical Aging Process Research Articles

Degradation Effects of Base Oils after Thermal and Electrical Aging for EV Thermal Fluid Applications

This study presents the experimental results of the effects on base oils after thermal and electrical aging to determine key parameters of next-generation fluids for thermal management in electric vehicles. The test fluids selected were a mineral base oil API G-III, an API G-IV Polyalphaolefin (PAO), a diester, and a polyolester, all of which had similar kinematic viscosity (KV100 = 4 cSt). All were initially characterized with measurements of density, viscosity, thermal conductivity, specific heat capacity, breakdown voltage, resistivity, and dissipation factor. They underwent two separate aging processes, one thermal, heating the test fluid at 150 °C for 120 h with a copper strip as a catalyst; and the second one an electrical aging process, with the application of 1000 breakdown voltage discharges. The same properties were measured again after each aging process and compared to the initial ones. It was found that the thermal properties ranged with similar values and did not suffer major changes after the aging processes, unlike electrical properties, which vary between samples and after thermal and electrical stress. The insights gained from this study have implications for both the development of next-generation e-thermal fluids and the future standardization of these fluids for EV thermal management applications. The findings of this study underscore the significance of formulating and selecting a suitable dielectric fluid for EV thermal management. By leveraging the insights provided, researchers and engineers can advance in the development of efficient and reliable e-thermal fluids while working towards future standardization to enhance the performance and safety of EV battery systems.

Unveiling the Aging Process of Organic Light‐Emitting Devices with Langevin and Hole Trap‐Assisted Recombination Exciplex Cohosts

AbstractExciplex‐forming cohosts have gained increasing interest due to their promising performances in organic light‐emitting diodes (OLEDs). Such electroluminescent stabilities of the exciplex cohosts depend on their charge‐carrier recombination. But intrinsic relationships between the recombination schemes and the electrical aging process of the exciplex cohosts are yet to be revealed. In this case, exciplex cohosts with two different recombination schemes, namely trap‐assisted recombination (TAR) and trimolecular Langevin recombination (LR), are compared. The LR‐dominated device achieves T90 lifetime (time to 90% of initial luminance) of ≈8600 h at luminance of 1000 cd m−2, which is >2.3‐time lifetime improvement compared with the hole TAR‐dominated ones. The corresponding degradation analyses confirms that hole carriers in hole TAR‐dominated cohosts are captured by phosphorescent guests, accelerating p‐type cohost deterioration through exciton‐polaron decomposition on susceptible aromatic amine groups, which reveals the electrical aging mechanism of exciplex cohost based devices. Moreover, exciton managements of the recombination region further suppress luminous decline and release undesirable exciton–exciton annihilations. The current findings offer an in situ technique to observe exciton and charge carrier aging processes with different recombination mechanisms and provide a general guidance for the development of highly efficient and long‐lifespan exciplex cohost based OLEDs.

The Development Characteristics of Partial Discharge in Paperboard Depression Under Composite AC–DC Voltage

The converter transformer is the most important piece of equipment in ultrahigh voltage direct current (UHVDC) transmission systems. The rectifying valve side of the converter transformer is dealing with composite voltages, containing both ac and dc components. Therefore, the characteristics of the partial discharge (PD) in oil-paper insulation of such rectifying valves are different from those of rectifying valves subjected to pure ac or dc voltage. In this study, theories of hydrodynamic drift-diffusion and bipolar charge transfer were used to build the needle-plate electrode model with a paperboard depression, and experiments were used to verify the feasibility of the simulation method. The electric field and net charge distribution that can reflect the PD were analyzed. Besides, the effects of instantaneous voltage, the proportion of ac-dc components, and depression parameters on the development characteristics of PD were also discussed. Simulation results show that applied voltage, interface charge density, and model parameters can all affect the electric field strength, which in turn influences the development of PD in the depression. This research reveals the process and influencing factors of PD during the electrical aging process, which not only helps to optimize the oil-paper insulation at the rectifying valve side of the converter transformer but also provides a basis for further explaining the PD mechanism.

Space Charge Measurement and Modelling in Cross-Linked Polyethylene

Cross-linked polyethylene, commercially known as XLPE, is widely used as an insulating material in high voltage cables. However, space charge accumulation under the DC field is one of the most challenging problems in the further development of XLPE insulated cable. Due to the potential electrical degradation ageing process triggered by the accumulated space charges, the IEEE standard 1732 was established for measuring space charge in HVDC extruded cables as the qualification tests. Previous research has revealed that space charge originates from either charge injection at the electrodes or ionization of impurities presenting inside the bulk. In the light of this, this paper aims to simulate the accumulation of space charge in XLPE under DC stress. Space charge measurements have been carried on the fresh and degassed XLPE samples. A modified bipolar charge transport model, by considering the dissociation of impurities, has been employed to simulate the space charge behavior in XLPE. Compared with the experimental observations, the simulation results can reveal appropriate features of hetero charge formation. Both the calculated charge dynamics and field variation are consistent with the experiment results. The restrictions and potential improvements of this preliminary model are also discussed for its future application of XLPE cables.

A Study of Electrical Aging of the Turn-to-Turn Oil-Paper Insulation in Transformers with a Step-Stress Method

This study investigates the electrical aging characteristics of oil-paper composite insulation. In this paper, a coils model and test system for oil-paper transformer insulation are designed. The electrical aging tests of the model coils at room temperature and operating temperature are performed using the step-stress method. A new method for estimating the unknown parameters in the inverse power exponent model is proposed, and the inverse power model life formula expressed by voltage and electric field strength is obtained. Considering the electrical aging process at the operating temperature, the allowable field strength of the inter-turn insulation of oil-immersed transformers can be improved. The new parameter estimation method proposed in this paper can be used to calculate the unknown parameters of the step-stress solid insulation aging model. This method has a clear physical meaning and can be solved easily. When the stressed voltage is below a certain threshold value, the life of the transformer coil turn-to-turn insulation grease oiled paper model is longer at operating temperatures compared with that at room temperature; when the voltage is higher than the threshold, it presents the opposite tendency.

Mesoporous Nano-Silica Serves as the Degradation Inhibitor in Polymer Dielectrics.

A new generation of nano-additives for robust high performance nanodielectrics is proposed. It is demonstrated for the first time that mesoporous material could act as “degradation inhibitor” for polymer dielectrics to sequestrate the electrical degradation products then restrain the electrical aging process especially under high temperature conditions, which is superior to the existing additives of nanodielectrics except further increasing the dielectric strength. Polyethylenimine (PEI) loaded nano-scaled mesoporous silica MCM-41 (nano-MS) is doped into the dielectric matrix to prepare the PP/MCM-41-PEI nanocomposites. PEI provides the amines to capture the electrical degradation products while the MCM-41 brackets afford large adsorption surface, bring down the activating temperature of the absorbent then enhance the absorptive capacity. The electrical aging tests confirm the contribution of the mesoporous structure to electrical aging resistance and FT-IR analysis of the electrical degraded regions demonstrates the chemical absorption especially under high temperature conditions. Take the experimental data as examples, extending the aging durability and dielectric strength of polymer dielectrics by 5 times and 16%, respectively, can have substantial commercial significance in energy storage, power electronics and power transmission areas.

Theoretical study on gas decomposition mechanism of SF6 by quantum chemical calculation

Sulfur hexafluoride has excellent insulating and arc-suppression properties, and is widely used in gas-insulated switchgear. Under high voltage discharge condition, SF6 decomposes into various by-products according to discharge scenarios. The formation mechanism of decomposition products is so far not clear. For most of the studies, experimental measurements are used to speculate the reaction mechanism, and those research is postulated with great arbitrariness. Quantum chemical calculation by using high precision simulation theory is a very effective method to reveal the microcosmic mechanism of complex chemical reaction. Therefore, in this paper, we used high level quantum chemical method (B3P86/6-31G∗∗) to study the mechanism of decomposition reaction and products, in presence of trace air or H2O. The decomposition products include SO2, H2S, and HF. For each reaction product, we provide a detailed generation pathway, including the data and energy structure of reaction intermediates and transition states. The simulation results can explain the experimental results accurately, and provide new understanding and support for the judgment of the discharge reaction state and subsequent electrical aging. These results are very helpful to build SF6/H2O/O2 reaction model, and indicate the electrical aging process from microcosmic angle.

Influence of temperature on electrical aging characteristics of polytetrafluoroethylene under nanosecond pulses

Polytetrafluoroethylene (PTFE) has been widely used in insulation equipment and high voltage apparatus due to its excellent insulating property. However, electrical aging phenomenon of PTFE may cause insulation failure under nanosecond pulses. Temperature is an important parameter that influences the electrical aging process. This paper investigated the effect of temperature on the electrical aging characteristics of PTFE under nanosecond pulses. A nanosecond pulse generator MPC50D based on magnetic pulse compression was used. The output voltage has a rise time of 30 ns and a full width at half maximum of 70 ns. The needle electrode was adopted to accelerate the aging process. The testing chamber SP-80U was used to control the temperature and humidity. The experimental results showed that the inception voltage increased and the speed of the aging process decreased when the temperature decreased. The effects of the pulse repetition rate and the impacting mechanism of temperature on electrical aging characteristics were also discussed.

The Research on the Effects of Electrical Aging on Trap Distribution in Polyethylene Doped with Nanoparticies

The photo-stimulated discharge (PSD) technology was used to characterize the trap distribution of polyethylene (PE) and the PE that doped with 5% MgO before and after electrical aging. For the PE with the aging time increasing, the internal shallow trap of the material gradually reduced and deep trap gradually increased. But for the PE that doped with 5% MgO with the aging time increasing, shallow trap gradually increased and the deep trap gradually reduced. Results show that: during the electrical aging process the PE produces a large amount of free radicals which can introduce deep traps. Adding impurity of MgO can effectively restrain charge moving that injected from electrode, thus to improve the internal electric field distortion of PE.

Comparative Study on Space Charge Distribution Measurements using PEA and PWP Methods on High Voltage Insulation

Major drawbacks to high voltage insulator especially on electrical aging process have been an issue for decades. The internal electric field distribution due to the presence of local space charge will distort, which in turn may cause the insulating material to degrade and finally can lead to accelerated aging and electrical breakdown. Comprehension of space charge buildup in solid insulator is crucial to determine how aging occurs, thus enabling the prediction of insulator lifetime. Space charge measurement has been extensively researched in recent years and has become a common method of investigating solid insulator properties. Pressure Wave Propagation (PWP) and Pulsed Electro-Acoustic (PEA) are non-destructive dielectric testing methods used to determine the space charge distribution within the insulation. This paper presents the basic principles, typical measurement setup and comparison of results from several published papers on application of PWP and PEA methods to investigate space charge accumulation. The space charge distribution in the sample is measured as current and voltage signals obtained from PWP and PEA methods respectively. Judging from extracted results of other research, both methods show the capability to detect premature aging activities in the insulator, which is necessary for condition monitoring of high voltage insulators.

Quantifying Full Field Surface Deformation in Organic Insulators under Electrical Stress with DIC

Applications of the digital image correlation method (DIC) for the determination of the full filed surface deformation, induced by a DC electrical field in organic insulator thin films, are investigated in this paper. Digital images of the film sample at different voltages were recorded and processed to obtain the full-field surface deformation associated with electrical stress changes. Considerable work has been carried out to describe the various parameters at play during the electrical ageing process of organic insulators from displacement field measurements. The result shows that the approximation method can smoothly reconstruct the experimentally obtained displacement field.

Study of partial discharge characteristics at repetitive square voltages based on UHF method

Due to overvoltage produced by inverter output, inverter-fed motor insulation systems often experience fast electrical aging process, especially when partial discharge (PD) is incepted. Before putting into use, the PD detection should be performed on inverter-fed motors at repetitive square voltages to avoid the PD caused insulation deterioration when the motors are collected to inverters having specific characteristics. However, unlike PD tests at AC/DC voltages proposed in IEC 60270, the PD detection at repetitive square voltages is much more complex because of serious interference generated by impulse generator. To solve the problem, ultra-high frequency (UHF) method seems recommendable for its preferable signal-to-noise ratio (SNR). The chief aim of this study is to investigate PD pulse and statistical characteristics of turn-to-turn insulation for inverter-fed traction motors. A square-shaped Archimedes antenna, specially designed for the PD detection at repetitive square voltages of fast rise times, was used to perform PD tests on turn-to-turn insulation models. Time and frequency analysis results indicate that energy component of generator disturbance and PD pulses are mainly distributed in the 0–0.5 GHz and 0.6–1.5 GHz range, respectively. Based on the results, suitable filter was designed for power disturbance suppression. Additionally, resorting to the sensor unit (i.e. antenna and filter) and the PD test system, the PD statistical features at square voltages of different frequencies were obtained. Experimental results show that higher frequency will give rise, statistically, to PD of lower magnitudes distributing at smaller phases. A reasonable interpretation of this phenomenon was presented. Lastly, according to the PD statistical features, some suggestions for the PD detection system design, generator parameter optimization and the PD pulse extracting were given. The results of this work would be beneficial to the increase of the sensitivity when performing the PD detection on insulation systems for inverter-fed motors at repetitive square voltages and thus, improving the reliability of inverter-fed motors.

A new approach to describe the electrical ageing by considering the distributed nature of processes in polymeric materials

This paper proposes to incorporate the distributed nature of processes at play in polymer insulating materials into the description of their electrical ageing process. Indeed, polymers are disordered systems showing a spatially varying morphology. Their amorphous phase is an active system in which the chain segments are continuously undergoing conformational changes characterized by different relaxation processes each with a characteristic relaxation time. Consequently, processes occurring in these materials do not have a characteristic Debye response and the parameters affecting the electrical ageing process are distributed. The already existing theories describing the ageing process of these materials are highlighted. A multivariate statistical approach is proposed to account for the complexity of the ageing process in these materials. This approach takes into consideration the local values of the different parameters of the ageing process and their distributed environments. The physical and mathematical bases of this new approach are discussed and the ageing process is described in terms of different local processes each one with a characteristic relaxation time. Some experimental techniques which allow quantifying the local values of our approach ordering parameters are proposed.

Measurement and simulation of partial discharge in oil impregnated pressboard with an electrical aging process

The continuous test on oil impregnated pressboard insulation with internal void defect was developed and the phase resolved partial discharge (PRPD) pattern of partial discharge (PD) signals during the electrical aging process was measured. Two different void structures which have different void volume were used in this experiment. It shows that the PD pattern could be classified into five stages and a great diversity in the first four stages is observed. The larger void volume leads to larger PD magnitude. The computer numerical simulation model which is based on a physical discharge process was used and the causes of PD pattern change were interpreted by comparison with computer numerical simulation results. The initial values and change tendency of gas pressure and surface conductivity were determined through experiment. The model parameters in different stages have been studied as well as the insight into the physical changes in the void during electrical aging. The results provide rules for the identification of the electrical aging stage through the partial discharge measurements.

Electrical ageing of polymers. The role of micro-domains

Polymer materials are disordered systems and their structure is continuously undergoing conformational changes called physical ageing. Furthermore, polymers exhibit a spatially varying morphology and a distribution of their properties and characteristics. In this article, the existence of micro-domains into polymer materials is proved, using an optical strain measurement technique. Then after, the role of these micro-domains is taken into consideration for a phenomenological description of their electrical ageing process. The different parameters affecting the ageing process, such as the local structure evolution and the local field value, are presented in a distributed way. In a first analysis the ageing process is described in a local scale. Afterwards, a statistical description of the totality of processes at play is necessary to present the global effect of the applied stress over the material structure which leads to the failure mechanism.

Resistive Switching Memory Based on CuSCN Films Fabricated by Solution-Dipping Method

Cuprous thiocyanate (CuSCN) films are fabricated through liquid-solid interface reactions and are investigated for resistive nonvolatile memories. An enhancement of memory switching properties was specified after an electrical aging process. The Cu-doped CuSCN solid-state electrolyte is suggested to be involved in the formation-rupture of localized and tamable conducting channels. The mechanisms for the narrow dispersion of resistances and thermal degradation are also suggested. More than 3000 cycles of write-read-erase-read are achieved, and operating cycles can also be performed 1000 cycles at 85°C in air.

Effect of Surface Morphology and Adhesion Force on the Field Emisson Properties ofCarbon Nanotube Based Cathode

The effects of the field emission property in relation to the surface morphology and adhesion force were investigated. The single-wall-nanotube-based cathode was obtained by use of an in-situ arc discharge synthesis method, a screen-printing method and a spray method. The morphologies of the formed emitter layers were very different. The emission stability and uniformity were dramatically improved by employing an in-situ arc discharge synthesis method. In this study, it was confirmed that the current stability and uniformity of the field emission of the cathode depend on the surface morphology and adhesion force of the emitters. The current stability of the field emission device was also studied through an electrical aging process by varying the current and electric field.

Electrical ageing of carbon nanotube composite cathode layers

Effect of electrical ageing (EA) on the field emission parameters of thin multiwall carbon nanotube composite (t-MWCNTs-composite) was studied. Initially, t-MWCNTs were mixed with α-terpineol and ethyl cellulose and subjected to three roll milling process to obtain t-MWCNTs-composite. Following this, the composite was screen printed on a conducting substrate, annealed for 10 min and employed to the electrical ageing process for a period of 6 h. The ageing, on each cathode layer, was repeated for five times and J– E characteristics have been collected before and after each ageing attempt. The analysis revealed that, the magnitude of threshold turn-on-field gradually increased from its virgin value of 1.223 to 1.968 V µm − 1 and corresponding mean field enhancement factor, γ m, gradually decreased from 2700 ± 210 to 1940 ± 30 with a sequential increase in the ageing attempts. The degradation rate, δ J/δ t, estimated for untreated and EA samples, indicated that the magnitude of δ J/δ t reached to an equilibrium value of ~ 0.785 μA cm − 2 min − 1 , which shows a stable emission state of the emitters. To investigate the effect of EA on the physical state of the emitters, a few virgin and all EA samples were subjected to scanning electron microscopy, micro Raman spectroscopy and X-ray photoelectron spectroscopy. The details of the analysis are presented.

Space charge analysis of electrothermally aged XLPE cable insulation

Cross-linked polyethylene (XLPE) is currently widely used as an insulating material for power cables due to its good physical properties, however when in use it undergoes an electrical ageing process. Its ability to trap electric charge can give rise to space charge accumulation in the bulk of the polymer and produce localised electric stresses that can lead to cable failure, since the electric field will be increased above the design stress in some regions favouring the initiation of degradation there. In this work the PEA (pulsed electroacoustic) method was used to compare the charge dynamics in three samples (XLPE cable peelings) aged in different ways (electrothermally in the laboratory, field aged in service and thermally aged in the laboratory). Very different transient behavior was found depending upon the ageing history. This is related to differences in the migration of chemical species in the insulation layer, which are known to act as charge traps. All materials showed heterocharge peaks when the space charge reached stability, the magnitude of which seems to be related to the severity of the ageing.