Inception Characteristics Research Articles

Inception Characteristics of Surge Corona on Line Conductors

Knowledge on the characteristics of surge corona on overhead line conductors is of great importance for studies on attenuation and distortion of lightning overvoltage surges. Impulse corona may initiate and grow in various discharge modes, namely glow, intermittent or stable streamer corona. Its inception characteristics can be described by statistical distributions, with parameters dependent on the steepness of the electric field strength around the conductor and on the atmospheric conditions. Agreement between theory and experiment on impulse corona inception characteristics is achieved based on the critical volume theory. The impulse corona performance of line conductors may differ between laboratory and outdoor testing, due to differences in the background density of negative ions. An empirical procedure yielding probability distributions for the inception characteristics of surge corona on overhead line conductors is introduced.

Partial Discharge Inception Characteristics of Twisted Pairs Under Single Voltage Pulses Generated by Silicon-Carbide Devices

Partial discharge (PD) is a major cause of degradation of motor winding insulation. It is a challenge to the successful electrification of all types of ground, sea, and aerial vehicles. However, the mechanism of PD under pulsewidth modulated voltages is not fully understood. This article studies PD inception characteristics in twisted pairs under single voltage pulses with reduced voltage overshoot, aiming to study the PD inception characteristics and mechanism without the influences of memory effects and voltage overshoot. The tests are conducted with a lab-designed 10-kV silicon carbide (SiC) device-based voltage pulse generator. The generator can produce voltage pulses with variable rise/fall times and pulse widths to emulate the output voltages of both silicon (Si) and SiC device-based power electronics converters. The voltage overshoot in this study has been limited to less than 5%. In addition, single voltage pulses are utilized as the excitation to remove charge memory effects. PD inception voltages, PD current magnitudes, and PD time delays of twisted pair samples are measured under various test conditions. It is found that the partial discharge inception voltage (PDIV) of the test sample decreases with reduced voltage rise time and increased voltage pulsewidth. Based on the test results, the impacts of voltage rise/fall times and pulse widths on PD behavior are explained using the extended volume-time theory. The relationships between PDIV, voltage rise/fall time, and pulsewidth are presented through empirical equations. Test results and analysis provide insights on the impacts of emerging wide bandgap devices in applications on motor winding insulations.

Windmilling Characteristics of a Contra-Rotating Fan

Abstract In the present experimental study, a low aspect ratio, low hub-tip ratio contra-rotating axial fan is investigated to understand its performance under windmilling conditions. Two configurations are tested; in the first configuration (event A), the front rotor of the contra-rotating fan is powered and the rear rotor is allowed to windmill; in the second configuration (event B), the rear rotor of the contra-rotating fan is powered and the front rotor is allowed to windmill. The spanwise distribution of the loading coefficient and the flow angles at different streamwise positions reveal the details of the flow development across the rotors. Though the average total pressure drops across the windmilling rotor for both the events, a small spanwise region behaves as a fan or a stirrer. Thus, a “neutral radius” on the windmilling rotor is identified for both events A and B. The neutral radius appears close to the tip for event A and it appears close to the hub for event B. Thus, the span regions close to the tip behave as a fan for event A and the span regions close to the hub behave as a stirrer for event B. It is also observed that the neutral radius shifts to a lower span location as the flow coefficient reduces. Thus, the flow coefficient is a significant parameter that decides the position of the neutral radius. Further, the unsteady pressure measurements recorded at the casing capture the fundamental phenomena during the stall inception. The paper thus relates the similarities and unveils the contrasting features of the windmilling events A and B. In summary, this paper discusses the performance, flow physics, and stall inception characteristics of a contra-rotating axial fan under windmilling conditions.

Dielectric Characteristics of Liquid Nitrogen/Synthetic Paper Composite Insulation System for Extra High Voltage HTS Cable

For extra-high-voltage high-temperature superconducting (HTS) cables, not only high electrical insulation performance but also low dielectric loss is required. Therefore, we focus on a porous synthetic paper with lower dielectric loss, e.g., Tyvek polyethylene nonwoven fabric, than that of polypropylene laminated paper (PPLP), which has been used for a conventional HTS cable insulation system. In this paper, we compare partial discharge inception characteristics for different liquid nitrogen/synthetic paper composite insulation systems. The partial discharge inception electric field strength in the composite insulation system depends on the electric field strength in the porous area of synthetic paper.

Influencing of water droplets on corona inception characteristics of composite insulator recorded by UV imager

Water droplets on the surface of insulators can distort the electric field and cause the generation of surface partial discharge or flashover. In this study, the inception and the development of surface partial discharges on a 110 kV composite insulator were evaluated by using a solar-blind ultraviolet imager. Several experiments were conducted by using water droplets differing in volume, conductivity, and position on an insulator housing. The results showed that the corona inception voltage decreases with the increasing volume of the water droplets. The relative permittivity of the water droplet decreases with the increase of the conductivity and then reduces the electric field intensity and increases the corona inception voltage. Installing the grading ring can increase the value of the corona inception voltage by 38% at most. To investigate the influence of the water droplets on the corona inception characteristics of the insulator, the sheath and shed models were proposed according to the feature of the electric field distribution. By changing the parameters of the water droplets, the corona inception voltage, discharge position, and electric field were analysed. Furthermore, the flashover voltage of the water droplets is influenced by its conductivity, while the corona inception voltage by its relative permittivity.

On the Potential Impacts of Extreme Low Temperature on Threshold Inception Characteristics of Negative Corona Discharge

The potential impacts of extremely low temperature on threshold inception characteristics of negative corona discharge are straightly estimated through the numerical algorithm and the extremely low temperature experiments. First, the inception criterion models of glow and streamer corona are solved for assessing the salient characteristics of corona inception at varying temperatures. The feasibility of the inception models applied to different temperature environments has been verified with our measurements and literature experimental data. Through introducing temperature into transport properties, the primary avalanche propagation, ionization boundary, and electron distribution are simulated at two inception modes. Computational results show that the microscopic parameter of negative corona inception has a pronounced relationship with temperature. The inception voltage of the wire-plate electrode is measured in a platform with temperature adjustable. It is observed that with the decrease of temperature, the inception field and voltage increases pronouncedly in the range from −100 °C to 30 °C, which can be attributed to the rise of effective ionization and photon absorption.

Influence of distorted inflows on the performance of a contra-rotating fan

AbstractA contra-rotating fan offers several aerodynamic advantages that make it a potential candidate for future aircraft engine configurations. Stall in a contra-rotating axial fan is interesting since instabilities could arise from either or both of the rotors. In this experimental study, a contra-rotating axial fan is analysed under clean or distorted inflow conditions to understand its performance and stall inception characteristics. The steady and unsteady measurements identified the relative contribution of each rotor towards the performance of the stage. The tip of rotor-1 is identified to be the most critical region of the contra-rotating fan. The contribution of rotor-2 to the overall loading of the stage is observed to be relatively less than rotor-1. The penalty due to distortion in the stage pressure rise is mostly felt by rotor-1, while rotor-2 also shows a reduction in performance for distorted inflows. Rotor-2 stalls at a high flow coefficient marking the initiation of partial stall of the stage, and the stall of the whole stage occurs once rotor-1 stalls. A fluid phenomenon that is attached to the blade surface marks the stall of rotor-1, and this fluid phenomenon initially rotates at a speed close to the speed of rotation of the blade. As the stage moves towards the fully developed stall, this fluid phenomenon sheds from the blade surface. The fluid phenomenon thus propagates at a speed much lower than the rotational speed of the blade during fully developed stall.

Partial Discharge Inception Characteristics of Oil Impregnated Paper Insulating System with Inserted Oil Impregnated Insulating Papers of Removed Oil Filled Cable

Partial Discharge Inception Characteristics of Oil Impregnated Paper Insulating System with Inserted Oil Impregnated Insulating Papers of Removed Oil Filled Cable

The role of time-lag in the surface discharge inception under positive repetitive pulse voltage

Polyetheretherketone is used in the packaging of high voltage power electronics as a frame material, and the surface discharge behaviors of polyetheretherketone are the main concerns for the packaging design. Pulse width modulation voltage, which shows a variable duty cycle, is applied to polyetheretherketone at working conditions. The surface discharge behaviors of polyetheretherketone under a positive repetitive pulse voltage of 50 Hz with different duty cycles were investigated in this paper. It is generally considered that the surface discharge inception is mainly related to electric field distribution; however, the pulse width of voltage could significantly affect the surface discharge inception characteristics. In this study, the surface discharge inception voltage, time-integrated streamer images, and cumulative surface discharge patterns under a positive repetitive pulse voltage of 50 Hz were obtained. It was found that the surface discharge inception voltage decreased with the increase in the duty cycle. This relationship was explained by the role of time-lag. Time-lag distribution analysis was performed, and the time-lag distribution was mainly determined by stochastic time-lag. The mean stochastic time-lag was obtained by the Laue plot, and the relationship between the mean stochastic time-lag and applied voltage could be well fitted by the inverse power law. Furthermore, the relationship between the surface discharge inception voltage and duty cycle could be exactly fitted by the inverse power law, which was derived from the relationship between the stochastic mean time-lag and applied voltage. The closely related relationships demonstrate that time-lag plays a critical role in the surface discharge inception at different duty cycles.

Numerical and experimental analysis of cavitation inception behaviour for high-skewed low-noise propellers

Numerical and experimental investigations of two highly skewed low-noise propellers with a slight alteration in the tip geometry are conducted in both wetted and cavitating flows. The performance of the propellers is measured in open water and inclined setups at different operating conditions and high speed video recordings are used to determine tip vortex cavitation inception behaviour. The numerical simulations are conducted by employing the Implicit Large Eddy Simulation on appropriate grid resolutions for tip vortex propagation, at least 32 cells per vortex diameter according to our previous studies. In order to investigate cavitation inception characteristics of the propellers, different inception prediction methods are employed and evaluated at different advance ratios.It is shown that in addition to the well-captured difference in e.g. the amount of cavitation, the simulations are capable of correctly predicting the small but crucial differences in flow features and cavitation inception characteristics of the two propeller designs. Numerical predictions of the cavitation inception charts are also compared successfully with the measured data where three different types of cavitation patterns are investigated in details. Supported by the experimental videos, the interaction between the tip vortex and trailing vortices and their impact on the pressure field and the cavitation inception are analyzed. It is shown that the numerical simulation can provide further details about the vortical flow structures, and their contributions to cavitation, and is a powerful tool in advanced propeller design stages.

Study on the streamer inception characteristics under positive lightning impulse voltage

The streamer is the main process in an air gap discharge, and the inception characteristics of streamers have been widely applied in engineering. Streamer inception characteristics under DC voltage have been studied by many researchers, but the inception characteristics under impulse voltage, and particularly under lightning impulse voltage with a high voltage rise rate have rarely been studied. A measurement system based on integrated optoelectronic technology has been proposed in this paper, and the streamer inception characteristics in a 1-m-long rod-plane air gap that was energized by a positive lightning impulse voltage have been researched. We have also measured the streamer inception electric field using electrodes with different radii of curvature and different voltage rise rates. As a result, a modified empirical criterion for the streamer inception electric field that considers the voltage rise rate has been proposed, and the wide applicability of this criterion has been proved. Based on the streamer inception time-lag obtained, we determined that the field distribution obeys a Rayleigh distribution, which explains the change law of the streamer inception time-lag. The characteristic parameter of the Rayleigh distribution lies in the range from 0.6 to 2.5 when the radius of curvature of the electrode head is in the range from 0.5 cm to 2.5 cm and the voltage rise rate ranges from 80 kV/μs to 240kV/μs under positive lightning impulse voltage.

Electrical Insulation Characteristics and Mechanisms of Low Dielectric Loss Materials for High-Voltage HTS Cables

Toward the practical development of high-voltage high-temperature superconducting (HTS) cables, high electrical insulation characteristics, and low dielectric loss become important. Dielectric loss drastically increases with the increase of operation voltage of HTS cables. Therefore, it is highly expected to find a novel insulating material with low dielectric loss for the high-voltage HTS cables. We have focused on Tyvek ® , polyethylene (PE) nonwoven fabric, and Tyvek ® /PE, a syntactic paper bonding PE-sheet on Tyvek ® by heat, as low dielectric loss materials. In this paper, we investigated fundamental partial discharge inception characteristics and their mechanisms of Tyvek ® immersed in liquid nitrogen (LN 2 ).In addition, we compared the electrical insulation characteristics of Tyvek ® , Tyvek ® /PE, and polypropylene laminated paper (PPLP ® ), and discussed their mechanisms.

Two medium size cavitation tunnel hydro-acoustic benchmark experiment comparisons as part of a round robin test campaign

The increase in marine transportation in the last decades has resulted in a rise of the different emissions linked to it. Amongst various detrimental emissions of shipping activities, underwater noise is known to affect ambient noise levels and hence threaten the life (or even survivability) of marine mammals. This concern brought the underwater radiated noise (URN) to the attention of regulators, considering the possible need to limits for commercial ships, thus resulting in a surge of interest.As being the main contributor to the URN of ships, the accurate prediction of propeller cavitation and hence associated noise in the design stage is crucial for achieving reductions in terms of emitted sound pressure levels. Whilst computational methods are developing at an exponential pace and so are prospective tools for the future, model scale experiments still represent the most reliable and largely adopted approach for the prediction of propeller radiated noise. Despite the importance of model tests, being the only reliable tool for cavitation noise prediction, a benchmark data is non-existent for facilities all over the world to compare and correlate their noise measurements.Within this framework, this paper presents the first comparison of a round robin test campaign amongst the Noise CoP (Community of Practice) of Hydro Testing Forum (HTF). Based on the extensive experiments conducted first at Emerson Cavitation Tunnel (ECT), a reduced test matrix is proposed to the forum members. University of Genoa (UNIGE) is the first member to complete the tests specified in the reduced matrix and this paper presents comparisons between the test results of ECT and UNIGE cavitation tunnel in terms of measured sound pressure levels, propeller open water performance, cavitation observations and cavitation inception characteristics. Moreover, in order to shed a light on the issue of propeller cavitation noise measurements, a series of investigations are carried out by UNIGE scrutinizing the effect of hydrophone position, oxygen content, propeller shaft revolution rate, sensitivity to thrust coefficient and cavitation number.

Negative DC corona inception in coaxial cylinders under variable atmospheric conditions: A comparison with positive corona

Characteristics of negative DC corona threshold inception in the coaxial cylindrical electrode arrangement are investigated based on experimental data and computations on avalanche growth considering a field and atmospheric conditions dependent photoelectron emission coefficient. A transition region between glow and streamer regimes of inception of the self-sustained corona discharge could be identified on the basis of space charge field effects on avalanche growth. Approximate expressions are introduced for satisfactorily estimating the negative corona inception field strength as interactively affected by conductor radius and atmospheric conditions. Polarity effects on corona inception characteristics are elucidated. The establishment of the glow corona generally occurs up to larger conductors for negative than positive corona; the opposite may also apply depending on atmospheric conditions. The inception field strength as well as its dependence on atmospheric conditions differs only slightly between negative and positive streamer corona; a critical avalanche number per unit length of ∼108 cm−1 is required for both negative and positive streamer formation.

A Study of the Dielectric Characteristics of Gaseous Nitrogen With Respect to the Electrode Material for Developing a High Voltage Superconducting Fault Current Limiter

The current lead for a high-voltage superconducting fault-current limiter is made of copper or brass, and the cryostat chamber is often composed of a nonmagnetic material, such as stainless steel. Therefore, sparkover breakdown may occur at the current lead or the superconducting coil of such a superconducting coil system. The measurement of sparkover and partial discharge inception voltages is considered an important technique for diagnosing the dielectric conditions of a high-voltage superconducting instrument. In this paper, the sparkover voltage and the partial discharge inception voltage with respect to the electrode material were examined under an alternating-current voltage. Sphere-to-plane electrode systems made of stainless steel, copper, brass, and iron were tested in dielectric experiments to verify the sparkover and partial discharge inception voltages. It is observed that the sparkover and partial discharge inception voltages in gaseous nitrogen (GN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) were strongly dependent on the electrode material. In addition, it is found that the sparkover and partial discharge inception characteristics of GN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> conform to the work function of the electrode material.

Effects of Gas Nucleus on Cavitation Inception and Bubble Characteristics inside V-Throttling Groove

The paper combines CFD simulation and high-speed observation experiments to analyze the forming mechanism of high-speed cavitation flowing inside V-groove. It is found that the formation and development of cavitation are greatly affected by the cavitation nucleus, and subjected to two flow patterns, shear flow and vortex. When the groove is shallow or valve opening is small, cavitation inception occurs in the strong shear region near the throttling edge, which is related to the separation of boundary layer and the wall attachment effect. As the depth of the valve port increases, the reflux becomes stronger at the back of the throttling edge, which complements the low pressure area generated from the fluid separation. At the same time the pulsation is formed in the downstream due to the large longitudinal velocity gradient, resulting in vortex, and the area of ​​low pressure in the center of vortex also leads to the generation of cavitation.

Measurement and simulation of partial discharge in oil-paper insulation under the combined AC–DC voltage

Depending on unique operation characteristics for voltage applied on valve side winding of the converter transformers, it is extraordinarily significant to study the partial discharge (PD) behavior with oil-paper insulation under combined AC–DC voltage. Therefore, this paper investigated PD inception characteristics by pulse current methods with needle-plate electrode system under combined AC–DC voltage. Furthermore, 3D electric field distributions versus combined AC–DC voltage in different ratios were calculated by Finite Element Analysis (FEA). An experimental conclusion was drew that AC partial discharge inception voltage (PDIV) in pure oil would decrease linearly with the DC component increasing but the inception voltage with oil-paper insulation appeared to be independent of DC voltage and dependent of AC voltage. And 3D electric field distribution deduced from simulation provided a supplementary proof on the experimental results. Moreover, high speed photography was used to capture emitted light produced by discharge, estimate streamer velocity (1.8 km/s) and record streamer initiation and propagation process in oil gap. Previous studies have shown that the prebreakdown phenomena involving the generation and propagation of vapor channels through the oil could be divided into a three-stage process.

축류압축기의 입구안내깃 각도에 따른 스톨선구신호 특성 연구

Stall inception characteristics are researched to understand stall well. To realize different stall inception patterns, IGV stagger angle was changed. At design IGV stagger angle, spike, which is short length scale, is observed. Decreasing IGV stagger angle, spike changes to mode, which is long length scale, and further decreasing get multi cell. Compressor maps for each IGV stagger are shown to compare different stall inceptions. The characteristics of both spike and mode are confirmed in this experiment. Furthermore, transient from spike to mode is find. multi cell has 4cells and is little bit faster than mode. and multi cell shows 2nd, 3rd characteristics on compressor map.

Measurement of partial discharge inception characteristics in sub-cooled liquid nitrogen

Partial discharge (PD) measurement is one of the effective diagnostic techniques to predict abnormal high voltage dielectric insulation conditions of the electric equipments. PD diagnostic techniques were also could be utilized to evaluate the conditions of cryogenic dielectric insulation media of high temperature superconducting electric equipment in liquid nitrogen. Generally, liquid nitrogen at 77K is used as cryogenic and dielectric media for high temperature superconducting devices for high voltage electric power systems. But due to generation of bubbles during quench conditions which cause harmful effect on the properties of liquid nitrogen insulation, sub-cooled nitrogen under 77K was also employed to suppress bubble formation. In this work, investigation of PD characteristics of sub-cooled liquid nitrogen was conducted in order to clarify the relation between PD inception and the temperature of liquid nitrogen. It was observed that measured PDIV (PD inception voltage) shows little differences according to the sub-cooled temperature of liquid nitrogen, but the magnitude and total numbers of PD has been slightly decreased according the decrease of cooled temperature of liquid nitrogen. From experimental results, it was deduced that the sub-cooled liquid nitrogen from 68K to 77K, could be applicable without any considerations of the variation of PDIV.

Partial Discharge Inception Characteristics and V-t Characteristics of Sheath Material of Cables for High Voltage Inverter Systems

In inverter drive systems, surge voltage between cable shield and ground (generally 0V in AC drive systems) caused from inverter switching is induced if cable length is longer and dv/dt of the inverter output voltage is much higher. This surge voltage isn't observed under AC drives. This unexpected surge voltage may cause partial discharge at cable sheaths and finally breakdown if the surge voltage exceeds partial discharge inception voltage (PDIV) of the sheaths. Therefore insulating characteristics of cable sheaths under inverter surge should be understood for optimal design of inverter-motor systems. This report states about insulating characteristics of the cable sheath, especially PDIV and V-t characteristics under AC and high dv/dt pulse voltage.