Stress Grading Research Articles

Potential distribution on the stress grading system of high voltage rotating machines — Part II transient measurements and simulation profiles

The authors developed a system that utilizes a Pockels sensor to measure the potential distribution along the stress grading (SG) system of a form wound coil. Potential distributions on a bar coil under a 50 Hz sinusoidal wave, 1 kHz rectangular wave, 1 kHz repetitive impulse, and two level pulse width modulation (PWM) voltage were measured using this system. The results are discussed using an equivalent circuit of the bar coil, which consists of a distributed constant type RC circuit. Calculated potential distributions with this model were quantitatively consistent with the measured results. When the penetration length of the high frequency component of the applied voltage was shorter than the length of the corona armor tape (CAT), a high-potential gradient developed in the CAT while the low-frequency component transmitted to the stress grading tape (SGT) region. In addition, the electric field distribution and power dissipation in the SG system were computed based on the finite element method (FEM). It was evident from the computed results that a frequency component of more than 200 kHz had little influence on the power dissipation in the SGT in this system.

Silicone Gel Compounds for Non-Linear Stress Grading Applications

The paper deals with a field-grading insulation usingsilicone gel as main material/binder. Additives, which providethe nonlinearity of the compounds, consist of a support materialwith metal oxide coating. The nonlinear electrical propertieswere adjusted by the addition of different dopants. In thisinvestigation the fundamental properties of new non-linearconductivematerials (NLCM) were examined. One aim of theinvestigation is to attribute the macroscopic electrical behaviorto the respective doping. For a typical compound, fieldcalculations for a cable joint model are presented and comparedto a conventional design without NLCM.

Modified stress grading system for a 13.8 kV inverter-fed rotating machine

Evaluation of thermal and electrical characteristics of a 13.8 kV rotating machine stress grading system shows that a reduced length of conductive armor tape (CAT) from the slot exit reduces the electric field and temperature rise under pulsed voltage. Although, the electric field in stress grading tape (SGT) increased slightly, with a shorter length of CAT during the rise time of the pulse voltage, it decreased during the DC portion of the pulse voltage. The temperature profile of the stress grading system under pulsed voltage at room, elevated room, and near typical operating temperatures are measured and simulated for several CAT lengths; results show lower temperatures for shorter CAT length. The partial discharge inception voltage is unchanged for shorter CAT lengths. A lower temperature rise is desirable as this leads to a longer life in the absence of partial discharges.

Surface potential measurement of model stator bar with stress grading system by field sensor

This paper proposes a novel method for measuring the potential distribution along the stress grading system of a model stator bar whose voltage is over several tens kV. The technique uses a Pockels field vector sensor by introducing an inverse calculation process from the measured field profile to the potential profile on the measured stator bar. With this system, a potential distribution along a model bar under 50 kVp, 50 Hz voltage is successfully measured. In principle, it is possible to estimate the potential either by measuring the horizontal or vertical component of the field. The utilization of the vertical component is preferable in measuring a model bar due to robustness against optical alignment errors. Through the potential distribution measurement of the model bar, although the maximum value of the electric field increases slightly with the applied voltage, it is confirmed that the electric field grading function was working due to the non-linear resistive material.

Harmonics in SiC/Alkyd resin composites with different SiC particle sizes under different temperatures

In order to investigate the AC response and the harmonic characteristics of stress grading material, samples of SiC/alkyd resin with 20 and 40 μm SiC particles were prepared and experiments were carried out from 30 to 100 °C. At the higher temperature, current in the composite with 20 μm SiC particles increased whereas the weak effect of temperature on current was observed with 40 μm SiC particles. In addition, the threshold voltage of volt-ampere curve decreases with increased SiC particle size. The current spectrum shows that the third harmonic current accounts for the majority of the harmonics. As the SiC particle size increases, the harmonic ratio increases and the nonlinearity of the composites becomes larger, whereas higher temperature leads to lower harmonic ratio and lower nonlinearity of composites. A power function is proposed to describe the correlation between the harmonic ratio and the nonlinearity of volt-ampere curve. These may be associated with charge hopping at low voltage and tunneling at high voltage. The lower number of barriers per unit distance in the composite with larger SiC particle size promotes the tunneling process, which leads to the decrease of the threshold voltage. Tunneling becomes the primary reason for the current after the threshold voltage and it is not influenced by temperature. Thus the composite with larger SiC particle size has current not influenced greatly by temperature after the threshold voltage. Tunneling leads to a greater increase in current and more waveform distortion, thus the composite with larger SiC particle size shows larger nonlinearity and higher harmonic ratio. Furthermore, hopping is associated with temperature while tunneling is not. The higher temperature decreases the ratio of tunneling to hopping which leads to the relative decrease in tunneling process. Thus the harmonic ratio decreases and the nonlinearity of composites becomes smaller with increased temperature.

Nonlinear resistivity in epoxy composites containing Mn‐based ferrite magnetic particles

ABSTRACTIn this study, magnetic particles were employed as nonlinear resistive materials in electric stress grading composites. Epoxy resin films containing either manganese‐based ferrite magnetic (Mn‐FM) particles or manganese‐magnesium‐based ferrite magnetic particles were prepared, and the threshold voltages required for these films to exhibit nonlinear resistivity were determined. The sample containing Mn‐FM particles having irregular shapes [Mn‐FM(i)] at 20 vol % showed the highest nonlinear resistivity. Assessments of the magnetic characteristics of these materials demonstrated a relationship between magnetic flux density and nonlinear resistivity. The magnetic flux density of the Mn‐FM(i) composite was the highest among the various specimens, indicating that magnetic flux density may affect nonlinear resistivity. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48229.

Effects of Stress Grading Materials’ Properties on the Performance of 20-kV Cable Terminations

Effects of Stress Grading Materials’ Properties on the Performance of 20-kV Cable Terminations

Effect of conductivity on the thermal and electrical properties of the stress grading system of an inverter-fed rotating machine

The thermal and electrical characteristics of a stress grading system are studied by simulations and measurements of the electric field and temperature distributions in the overhang region of a form wound medium voltage motor coil. Simulations show that a highly conductive stress grading tape (SGT) reduces the maximum electric field in this region but increases heat production and temperature. In contrast, a highly conductive armor tape (CAT) decreases the maximum electric field in this region and also reduces the heat production and temperature in the stress grading system but also produces a slight increase in the electric field in the SGT region. For the simulation studies, accurate measurements of the electrical and thermal conductivities of materials are made, taking into account the effects of the vacuum pressure impregnation (VPI) process, temperature, and tape builds. The electrical conductivity of single and double half-lap layers of CAT and SGT are measured from 30 to 100 °C. For the evaluation of the simulation model, the temperature profile along the stress grading system was measured and simulated under pulsed voltage at room and near typical operating temperatures.

Drug-Induced Liver Injury Predictions: Extended Clearance Model and Its Use for Prospective Transporter and Enzyme-Based Hepatic Cell Stress Grading

Many enzymes and transporters involved in the hepatic clearance of drugs also play an important role in endogenous compound transport. Inhibition of some of these active mechanisms has frequently been shown to be associated with Drug-Induced Liver Injury (DILI). The Extended Clearance Model (ECM) describes the complex interplay between the different processes driving hepatic clearance, namely sinusoidal uptake and efflux, canalicular secretion and intracellular metabolism. Based on the ECM, we have derived an integral concept (referred as 1/R-value approach) to quantitatively describe the overall inhibition potency of potential drug candidates on active processes involved in the transport and metabolism of endogenous and safety-relevant compounds. For a small training set of in-house compounds with largely complete in vitro inhibition and in vivo exposure data, accurate ECM-based prediction of DILI was realized. Additionally, prediction of several cases of DILI for a comprehensive validation set of external compounds was achieved with no major false-positive results. However, due to general incompleteness of the required input information available in the public space (the most probable reason for the large number of false-negatives in the test set) the overall legitimacy of ECM for large-scale prediction of cell stress mediated DILI still needs to be demonstrated. In order to advance and accelerate science in this exciting but complex field, a more transparent and open sharing of data is therefore urgently needed and should be encouraged.

Visual stress grading of fibre-managed plantation Eucalypt timber for structural building applications

The aim of this study was to examine the impacts of visual characteristics, strength-reducing features (SRFs), and basic density on the mechanical properties of fibre-managed plantation Eucalypt timber to create effective structural grade groups. It is the intension that the results found in this study can lead to the development of a visual stress grading method for fibre-managed plantation timber in the future and influence the development of new applications for the resource in structural elements for the built environment. The plantation specie investigated in this study was Eucalyptus Nitens–390 sawn timber boards. The most important visual characteristics and SRFs likely to influence the mechanical properties of the boards were visually identified and measured before the boards were divided into designated groups for sampling. The impacts of the visual characteristics, SRFs, and basic density of the boards within each group on modulus of elasticity (MOE) and modulus of rupture (MOR) were determined using four-point bending test. The statistical analyses suggest three structural grade groups can categorise the resource. Strong correlations were found between MOE, MOR, and basic density with the visual characteristics and SRFs of the boards in the three grade groups.

Comprehensive electrical and thermal analysis of the stress grading system of a large hydro generator

In the slot portion of a generator core the installed stator bars are covered with a semi-conductive coating to prevent partial discharge (PD) activity between the main insulation surface and the grounded laminated sheets. Typically, this outer corona protection (OCP) layer ends a few centimeters outside the slot portion. To suppress PD-activity at the end of the OCP, a field grading is mandatory for machines with a rated voltage greater than approximately 6 kV. Figure 1 shows the effect of a stress grading layer on stator bars energized to high voltage. It suppresses potential creepage discharges at the OCP ends caused by field enhancement.

Investigation of Control Methods for Local Heating at End-turn Stress Grading System of Inverter-fed Rotating Machines

Investigation of Control Methods for Local Heating at End-turn Stress Grading System of Inverter-fed Rotating Machines

A non-contact method for the determination of fibre direction of European beech wood (Fagus sylvatica L.)

This paper introduces a non-contact method for the identification, quantification, and documentation of fibre direction of European beech wood (Fagus sylvatica L.). The developed approach is based on an automated visual analysis of the spindle pattern formed by the medullary rays, also termed wood rays. Each spindle is identified by means of image analysis technique, its position and orientation is determined, and the fibre direction of discretised elements is calculated. The individual process steps necessary to obtain an estimate of the fibre direction of a board are explained using the examples of five different failure types. In all examples, the estimated fields of fibre direction are congruent with the actual fibre direction determined by means of (1) the orientation of all present shrinkage cracks, which are established indicators for the fibre direction in wood, and (2) the fracture pattern after tensile testing. Employing the presented approach could open up new possibilities for the characterisation of European beech and other hardwood species with multi-row medullary rays in several fields of application, in particular regarding stress grading.

Modeling of power transmission and stress grading for corona protection

Electrical high voltage (HV) machines are prone to corona discharges leading to power losses as well as damage of the insulating layer. Many different techniques are applied as corona protection and computational methods aid to select the best design. In this paper we develop a reduced-order model in 1D estimating electric field and temperature distribution of a conductor wrapped with different layers, as usual for HV-machines. Many assumptions and simplifications are undertaken for this 1D model, therefore, we compare its results to a direct numerical simulation in 3D quantitatively. Both models are transient and nonlinear, giving a possibility to quickly estimate in 1D or fully compute in 3D by a computational cost. Such tools enable understanding, evaluation, and optimization of corona shielding systems for multilayered coils.

Aspects of Mechanical Stress Grading for Structural Timber

The development of standard methods and criteria for the stress grading of structural timber elements by non-destructive, fast and cheap bending tests, will allow producers, distributors and customers of such elements, have a better control of the marketed product. This paper analyzes the most suitable length of free span for stress grading of structural timbers, number of points to be assayed in each element, data precision and evaluation of cost of these tests for small business.

Mental stress grading based on fNIRS signals.

In this study, we propose functional near infrared spectroscopy (fNIRS) to objectively grade different levels of mental stress. The levels of stress were set based on the difficulty of arithmetic task, time pressure and negative feedback about peer performance. We examined the proposed approach on twelve human subjects using the Montreal Imaging Stress Task. The experiment results revealed a reduction in cortical activation at prefrontal cortex when stressed, and the differences in hemodynamic response between control condition and under stress were significant for arithmetic difficulty level one, two and three, respectively, (p = 0.0023, 0.0001 and 0.0004). The experiment results thus support the suggestion of fNIRS in grading mental stress.

Functional behaviors of electric field grading composite materials

Non-linear resistive field grading materials (FGM) are used in different high voltage applications e.g. cable terminations, joints and rotating machines, where the electric stress is critical. It is already established that the use of FGM is an improved stress grading technique compared to conventional geometric methods. There is a constant endeavor within the electric power industries to extend these concepts to other power equipment. In order to achieve a wide spread use of FGM, they need to satisfy several functional requirements e.g. fire safety, tracking resistance, hydrophobicity, high thermal conductivity, besides certain dielectric properties e.g. field dependent conductivity, permittivity and low dielectric losses. In the present study ZnO filled silicon rubber (SiR) composite with different functional fillers, in total six different material compositions, are evaluated with respect to several important functional behaviors. Further, stress grading behavior of these six materials are demonstrated by finite element simulations on a generic electrode geometry and by performing inclined plane tracking test. Suitability of these material compositions to industrial applications are discussed.

Transient Simulation of Nonlinear Electro-Quasi-Static Field Problems Accelerated by Multiple GPUs

This paper focuses on the acceleration of the numerical linear algebra for solving large nonlinear electro-quasi-static problems in time domain as they occur in high-voltage applications utilizing nonlinear electric field stress grading materials. An algebraic multigrid (AMG) scheme is executed on multiple graphic processor units (GPUs) as a preconditioner for the conjugate gradients method. The speedup is further increased by exploiting the fact that the repeated solutions of almost similar systems must be obtained in the Newton–Raphson iterations of the implicit time-stepping scheme. A tailored update algorithm for the AMG preconditioner is proposed that significantly reduces the computational time due to a reduction of communication between GPUs and host.

Potential distribution on stress grading of inverter-fed rotating machines under DC biased voltage

The erosion of the stress grading system of converter-fed rotating machines may occur by partial discharges and heat generation. There remain unclear issues in the formation of the potential and temperature distributions along the stress grading system of coils. In this paper, the potential distributions on the stress grading system of the model coil bar are measured under the application of 1 kHz square wave voltages of 10 kV in peak to peak value with/without 5 kV DC biased voltage. It turns out that DC bias voltage has little effect on potential distribution on a coil surface.

Analytical approximations for the rotating machine end-turn field distribution

The end turn grading of a form-wound machine is coupled capacitively to the stator conductor so that only derivatives of the conductor voltage are relevant to the performance of the end turn grading. In other words, the key factors are the “slew rate”, i.e., the derivative of the conductor voltage with respect to time, and the capacitance per unit length between the stator conductor and conductor armor tape (CAT) and nonlinear stress grading (SG). Both the CAT and SG can participate in grading for high slew rates, and a condition is provided for such participation. The peak field along the SG, and the decay thereof as a function of time after a transition, can be estimated from fundamental considerations.