Front Time Research Articles

Influence factors and improvement scheme on the breakdown behavior of pseudospark switch

Abstract High-power pulse generators are widely used in civil and military fields. The main switch directly determines the output characteristics of the high-power pulse generators, such as the voltage front time (tf). Pseudospark switches (PSS) show a promising future for middle voltage, high repetitive frequency pulse power applications. However, how to further improve the breakdown behavior without reducing its advantages is a challenging task. In this paper, the influence of operating parameters (anode voltage UA and gas pressure p) and structural parameter (number of cathode holes) on the breakdown behavior are investigated, the related mechanism are explained, and specific improvement schemes are proposed. It is found that the tf of the single channel PSS (SCPSS) decreased significantly with increasing p, but hardly varied with UA under moderate p. However, it is not a sound solution to increase the p excessively to reduce tf. Besides, increasing the number of cathode holes can obtain a shorter tf at low pressures (which implies superior repetition frequency performance). However, at 25 Pa, the jitter (which is defined as the standard deviation of tf in multiple tests) of the 2-channel PSS is larger than that of the SCPSS. And the jitter of the 4-channel and 8-channel PSS is also greater than 6 ns and 2 ns, respectively. Through experimental and simulation analyses, it can be explained as the stepwise penetration of the virtual anode and the non-simultaneous ignition of the channels. A scheme to increase the trigger energy (ε) has been adopted to improve the simultaneous ignition probability, while shortening tf and reducing jitter. After optimization, the good ignition probability of the 4-channel PSS has been improved to 82% and the jitter has been reduced to less than 1 ns at 25 Pa and 14.7 mJ.

An expeditious calculation approach to consider the influence of power frequency voltage on the calculation of the lightning performance of transmission lines: Validation for different peak current-front time correlation expressions and cumulative peak current distributions

The approach to consider the influence of the power-frequency voltage on the calculated lightning performance of transmission lines with voltage level from 69 kV to 500 kV by means of the 0.88-multiplying-factor applied on critical currents is validated for simulation conditions that assume the correlation between lightning peak current and front time, and several cumulative peak current distributions of negative first strokes. The obtained results confirm that the relation between critical currents calculated in the presence and in the absence of the power-frequency voltage follows the 0.88-factor for all investigated cases. The root mean square errors (RMSE) calculated for the critical currents vary from 3.20 kA to 7.37 kA and for the backflashover probability are not greater than 0.02 %, ensuring the generality of application of the proposed approach. The ratio between the backflashover rates obtained by the 0.88-factor approach and by the Monte Carlo method is around 1 for most of the simulated cases, confirming the quality of the results provided by such expeditious procedure.

Partial discharge characteristics of peek utilized in press-pack IGBT under DC voltage

Modular multilevel converters (MMCs) have been utilized in solid-state transformers (SSTs). With the increasing demand in high-voltage power module in MMC-SST, press-pack IGBTs are very promising. The poly-ether-ether-ketone (PEEK) frame is utilized in the submodules of press-pack IGBT to withstand the high voltage. The electric field simulation in this paper reveals that electric field concentrates on the surface of the PEEK frame. Thus, the surface partial discharge characteristics under direct current (DC) voltage is investigated by the developed test bench. The results show that surface partial discharge model of PEEK sample has polarity effect. Under positive and negative polarity voltage, the average discharge quantity, maximum discharge quantity and discharge repetition rate of the PEEK sample all increase with the voltage amplitude. Moreover, the front time of the surface partial discharge waveform reaches 20 ns, and its frequency spectrum is concentrated in range of 10–20 MHz.

Extreme wave impacts on a suspended box above free surface

A systematic approach incorporating precision-controlled experiments and high-fidelity numerical simulations has been employed to delve deeper into the physics of extreme wave impact pressures. Using a suite of high-speed imaging and high-resolution pressure sensing techniques that are accurately synchronized with the wave generation, a consistent and well-correlated set of data providing details of the focusing incident wave front, flow velocities, and impact pressure time histories has been successfully obtained. The numerical simulations using a validated incompressible smoothed particle hydrodynamics (ISPH) provided further details that were not tractable in the experiments. Based on the results obtained, there were five broad impact scenarios identified, ranging from focusing crest impacts to plunging jet impacts with varying entrapped air. Among these impact types, the highest impact pressures were those associated with a focusing wave crest. It has been found that high impact pressures were correlated with a convergence of the horizontally impinging overturning crest and the vertical surge of the wall boundary water mass, but with the overturning crest impingement just ahead of the arrival of the wall boundary water mass, which in essence amounted to the impingement of a focusing wave front. Depending on the relative arrival times of the impinging crest and the wall boundary upsurge, the impact scenarios could vary from an up-slosh to a jet impingement with a clear entrapped air pocket, followed by a flip-through of the incident crest. For the scenario with the highest impact pressures, broadly classified as type II impact in this paper, the peak pressures could reach as high as 85ρC2, an order of magnitude higher than impact scenarios without the focusing effect, such as the scenario with a plunging jet impact with entrapped air (classified as types III and IV impact in this study). As the volume of entrapped air for this scenario was relatively small, the one-phase ISPH model used in this study was able to capture the peak pressure characteristics, including the peak pressure amplitudes. However, in scenarios with significantly highly entrapped air during impact, a two-phase model would be necessary.

Estimation of lightning overvoltages and critical flashover currents of 150 kV and 400 kV overhead lines: Effects of the CIGRE lightning current waveform parameters

This paper investigates the influence of the CIGRE lightning current waveform parameters on the computation of fast-front overvoltages and critical lightning currents causing flashover of overhead transmission line insulation. The investigation is conducted via ATP-EMTP simulations employing both negative and positive lightning current waveforms corresponding to first return strokes. Typical 150 kV and 400 kV double-circuit overhead lines are evaluated by performing shielding failure and backflashover simulations. The statistical distributions of the parameters required as input for the CIGRE lightning current waveform (front time, maximum steepness, time to half value) are considered in simulations to quantify and analyze their effects; this enables the comparison between negative and positive lightning strikes. The waveform parameters do not affect considerably the critical shielding failure flashover current for both polarities; the most influential parameter in this case is the time to half value. On the other hand, the wavetail duration influences the least the critical backflashover current; the latter is lower for waveforms with faster wavefront (lower front time and higher maximum steepness). The impact of the waveform parameters of positive lightning strikes is more marked for the backflashover case due to their larger variations compared with negative strikes.

Исследование влияния импульсной короны на проводах ВЛ 110 кВ, грозозащитном тросе на энергетические характеристики линейных ОПН во время грозовых перенапряжений

The corona effect has a significant impact on calculations of lightning protection of the high-voltage overhead transmission line. The corona effect in the calculation of the transient process during a lightning overvoltages is often neglected due to the complexity of modeling the corona effect. The issue of the corona effect influence on the operation of non-linear surge suppressors installed on overhead lines hasn’t been sufficiently studied. The aim of the research is to evaluate the energy characteristics of linear surge arresters considering the corona effect on the overhead line. Simulation of the transient process on the overhead line during lightning overvoltages is carried out in the EMTP-ATP software. There is no corona model among the basic models in EMTP-ATP software. Thus, the corona model on overhead line wires using dynamic capacitances is under consideration. A model of corona on overhead line wires in EMTP-ATP is developed. Calculations of energy characteristics of linear surge arresters on an overhead line model considering the corona and without corona effect are carried out. The comparison of the obtained energy characteristics is carried out. The corona has a significant effect on the energy absorbed by the linear surge arresters during lightning strikes to the ground-wire if the front time of the lightning current impulse has a value near 1 s. When lightning strikes to the phase wire, the energy absorbed by the linear surge arresters is so great that the influence of the corona effect is insignificant. The corona model used in calculations in EMTP-ATP software has several limitations. Therefore, in future research it’s advisable to improve the model and simplify it, when calculating lightning protection of overhead lines.

Overvoltages caused by direct lightning strokes to a hybrid overhead line

This paper presents an analysis of the overvoltages caused by direct lightning strokes to a hybrid overhead line with 138 kV, 13.8 kV, and 220 V circuits. The primary focus is on medium voltage line overvoltages and their dependence on key parameters, including ground impedance, soil resistivity, stroke current front time, conductor characteristics, lightning channel impedance, insulator capacitance, conductors' sags, and tower surge impedance. Using specific models for the transmission tower and high-voltage insulators, validated through laboratory tests and EMTP simulations, critical currents are determined for varying soil resistivity and impulse impedances. The analysis confirms the line robust performance against stroke-induced backflashovers.

Incidence of Secondary Primary Malignancies after Long-Term Follow up of Multiple Myeloma Patients Treated with Autologous Stem Cell Transplantation - Report from Croatian Cooperative Group for Hematologic Malignancies (KroHem)

Background: Autologous stem cell transplantation (ASCT) after induction treatment is standard of care for multiple myeloma (MM) for the last 30 years. Two transplantations up-front (so-called tandem transplant) were standard of care for all MM patients. With the use of new antimyeloma agents (such as proteasome inhibitors, immunomodulators and especially monoclonal antibodies) this approach has changed and tandem transplant is now reserved mainly for patients with high-risk disease, defined by revised international staging system (R-ISS). There have been reports which cast further doubts on the value of the more aggressive tandem ASCT approach, as well as reports suggesting higher incidence of secondary primary malignancies (SPM) after high dose therapy and ASCT. Aims: To describe long-term outcomes of MM patients treated with ASCT (single, delayed, and tandem) and the rate of secondary primary malignancies. Methods: a retrospective analysis of outcomes of 577 MM patients who underwent 865 ASCT procedures between 1993 and 2017 was performed. Data were collected from 3 transplant centres and 9 other haematological centres in which MM patients were treated pre- and post-transplant. During that period treatment strategies changed significantly from single to tandem to salvage second ASCT. Results: there were 577 patients included. Median age at first transplant was 58 years (range 22 - 74). There were 295 female and 282 male patients. 346 patients received VAD (vincristine, doxorubicin, dexamethasone) and 231 received bortezomib and/or thalidomide (PI/IMiD) based regiments as induction therapy. All patients received high dose melphalan for pretransplant conditioning, 32 in combination with total body irradiation. Tandem ASCT (defined as second transplantation performed within 6 months after the first) was performed in 226 patients, single ASCT in 262 and salvage second (after relapse and/or progression) in 89 patients. Median overall survival (OS) for the entire group is 81.9 months and median progression free survival (PFS) 38.9 months. Tandem ASCT in comparison to single and second salvage transplantation resulted in statistically significant better OS (115 vs. 83 vs. 67 months respectively, p < 0.0001) and PFS (48 vs. 32. vs. 28 months respectively, p < 0.0001). There was no difference in outcomes regarding type of induction therapy, VAD vs. PI/IMiD based: median OS was 83 vs. 80 months, respectively (p=0.39) and median PFS was 39 vs. 38 months, respectively (p=0.15). 17.6% percent of patients who were treated with tandem ASCT are progression-free more than 10 years after the procedure while 10-year OS is 31.9%. 51 patients (8.8%) developed secondary malignancies (SPM). Most common SPM were lung carcinoma (10 patients), colorectal carcinoma (7 patients), myelodysplastic syndrome/acute myeloid leukaemia (6 patients), breast carcinoma (4 patients) and melanoma (4 patients). Of those 19 died due to SPM (3% of the entire patient group). There was no correlation between number of ASCT and SPM (p=0.87). Conclusion: in our group of patients tandem ASCT resulted in superior OS and PFS in comparison to single and/or salvage ASCT. Interestingly, we did not observe benefit of new drugs (PI/IMiD) in induction therapy regarding outcomes. This peculiar result might be due to rather late availability of these drugs in front line settings (bortezomib and thalidomide since 2015) and time period taken for analysis (up to 2018). SPM rate was similar to other reports and did not show significant correlation with number of ASCT. More than 15% of patients treated with tandem ASCT experienced very prolonged PFS and almost 32% had OS longer than 10 years.

Experimental and Simulation Study of Pulse Current Generator

8/20 µs lightning pulse current generators (LPCG) used in testing surge arresters require a laborious production process that needs a high budget. An easily repairable, portable, and affordable test system with a maximum energy of 2.9 kJ and a voltage level of 24 kV that can alter its parameter values was described in this work. Simulation and experimental studies were carried out to investigate the effects of the resistor and coil elements in the Pulse Current Generator (PCG) circuit on the amplitude, front, and tail time of the pulse current. Most components of the experimental system were created using 3D printing technology with polylactic acid (PLA) material. The production costs were significantly reduced, according to equivalent pulse generators. Tests at different pulse currents and voltage levels were performed with the experimental system. Power capacitors may often fail due to overvoltage, switching, and partial discharge reasons. The high voltage (HV) power capacitors used in the pulse generator introduced in the study were obtained with the help of low voltage capacitor stacks. As a result, a less expensive, high-capacity capacitor system that is easily and quickly repairable in case of failure was achieved.

Effects of Approximating Recorded Lightning Currents With CIGRE Waveforms on Computed Fast-Front Overvoltages and Critical Lightning Currents Causing Flashover to Overhead Transmission Lines

This work investigates through simulations with the ATP-EMTP program the application of recorded lightning current waveforms to the estimation of fast-front overvoltages and critical lightning currents causing flashover to overhead transmission lines. Typical lines are evaluated with both horizontal (single-circuit) and vertical (double-circuit) phase configurations and AC voltage from 66 kV up to 765 kV. Backflashover as well as shielding failure flashover simulations were performed for several recorded first return-stroke current waveforms of negative downward lightning flashes, as reported in literature. Their approximations with the widely-used CIGRE lightning current waveform were also employed in simulations. CIGRE waveforms, considering the statistical distributions of the front time, maximum steepness, and time to half value, were used as well. A comparison between double-peak waveforms, composed of seven Heidler functions, and their CIGRE approximations is made. The latter generally yield conservative values of overvoltages and critical backflashover currents. The leader development models used for assessing the behavior of insulators and long air gaps under fast-front overvoltages are discussed regarding the criterion for the termination of leader propagation yielding withstand of line insulation.

Study on long front time wave switching impulse of rod-plane air gap

In long-distance transmission lines, the switching overvoltage waveform differs from that of ordinary lines. The wave front time is primarily distributed above 1000 μs. And the switching overvoltage waveform has an important influence on the design of external insulation. This paper aims to deeply study the effect of various factors on the discharge of air gaps under the long front switching impulse voltage. This paper selects three kinds of rod-plane air gaps, and the switching impulse discharge experiments with different front lengths and absolute humidity under positive polarity are carried out. The effects of front time Tf, absolute humidity h, and gap distance d on the discharge characteristics of the air gap are analyzed. The results show that the U50% of an air gap is positively correlated with absolute humidity, front time, and gap distance. When considering the additional influence of humidity, the humidity additional influence factor c and the atmospheric parameter influence index n at different front times are almost unchanged. With the increase in gap distance, the influence of front time on U50% is more obvious. With the growth of the operating wave front time, the rising rate of U50% tends to slow down. The discharge time of the air gap under different conditions is mainly determined by the front time. The research results can provide a theoretical and experimental reference for the influence of front time Tf and atmospheric parameters on the discharge characteristics of air gaps under long front switching impulse.

Lightning Impulse Voltage Stresses In Underground Cables

Underground power cables are crucial for transmission and distribution. Lightning can stress their insulation, but not directly. So, impulse cable testing is studied. This research examines the cable’s transient response to standard and non-standard lightning impulse voltage waveforms. MATLAB Simulink was used to model a 132 kV wire with standard and non-standard impulse voltages. The IEC60060-1(2010) lightning impulse test uses a conventional waveform impulse voltage with a front time and a tail time of 1.2/50μs half value, while the non-standard test uses a front time and a tail time of 0.8/12s half value. Non-standard impulse waveforms are more accurate than standard waveforms. The impulse test voltage is four to five times the underground cable’s operational voltage and must withstand five applications without damage. Standard and non-standard impulse waveforms are injected with 132 kV and 550 kV to evaluate insulation failure or damage. Standard lightning and non-standard impulse voltage waveforms do not cause insulation failure or damage. When 132 kV and 550 kV are introduced into the normal and non-standard lightning impulse waveforms, the overshoot voltage increases. The peak voltage of a non-standard 550 kV impulse voltage waveform exceeds the IEC impulse withstand voltage. The finding shows that non-standard impulse voltage waves create increased cable voltage stress.

An Equation-Based Dynamic Nonlinear Model of Metal-Oxide Arrester and Its SPICE Implementation

The existing models of the Metal-Oxide arresters (MOA) have difficulty covering the behavior prediction under surges with front time ranging from microseconds to nanoseconds. This paper presents an equation-based dynamic nonlinear model and its circuit implementation, which can describe the behavior of the MOA in a wide range of frequencies and amplitudes with the consideration of the piecewise nonlinearity and the dynamic nonlinearity characteristics. The model can be easily constructed based on the manufacturer’s data and a multiplication factor table for the dynamic characteristic. Two types of MOA used in the power distribution systems are tested to validate the proposed model. The simulated responses of MOAs are in good agreement with the measured results when the MOAs are applied to impulses with front time ranging from 20 ns to 30 ls. Both the errors of the peak voltage and the absorbed energy between simulated and measured results are less than 6%.

Measurement and Analysis of the Switching Transients in 500 kV Pumped Storage Power Plants

Pumped storage power plants (PSPPs) are among the most efficient and practical large-scale energy storage systems. In recent years, several transformer failures have occurred in PSPPs, and transient overvoltage is considered a potential reason for failure. However, there is a lack of field measurement results for transient analysis. In this paper, a switching transient measurement system and multipoint synchronous triggering system were developed. Field measurements were carried out in three PSPPs. According to the measurement results, the maximum amplitude of the switching overvoltage was 630.6±46.5 kV. The shortest front time was 76±7.6 ns. The largest steepness was 1130±140.8 kV/μs. A simulation model was built to analyze three phenomena in the transient, including multistep waveform, overvoltage peak, and power transformer demagnetization. The measurement and analysis results are beneficial for the improvement of simulation modeling. It is helpful to pinpoint potentially damaging transients for equipment manufacturers and designers of PSPPs.

Varistor coupling method and recommendation parameters for combination wave test

AbstractVaristor coupling is recommended by International Telecommunication Union in combination wave test waveforms of surge protective devices (SPD), although the varistor coupling method and recommendation parameters for the combination wave test are not well understood. The waveform influencing characteristics introduced by varistor coupling are studied in detail in this paper. Low‐voltage varistors, with maximum operating voltages ranging from 130 to 750 V, are adopted here in the OrCAD/PSpice environment to analyze the influence of varistor coupling on the output characteristic of the combination wave generator (CWG) in test levels of 0 to 20 kV. Simulation results show that, owing to varistor coupling, the reverse oscillation phenomena will arise in the open‐circuit voltages (OCVs). Also owing to varistor coupling, compared with those generated directly by a CWG, the peak value, the front time, and the tail time for both OCV and short‐circuit current will decrease, whereas the virtual impedance will increase. Moreover, the lower the test level, or the higher the maximum operating voltage of the varistor, the more serious this effect. Experiments have also been conducted here, and our simulation results are found to match the experimental ones well. It seems that, compared with the conventional capacitor coupling, the range of the voltage of power supply networks and the levels of combination wave test by varistor coupling have to be limited to a great extent. Varistor coupling for combination wave test of SPD should be only used in special cases and special test levels where there are no strict waveform limitations.

Female Collegiate Dancers' Physical Fitness across Their Four-Year Programs: A Prospective Analysis.

Dance is physically demanding, requiring physical fitness (PF) that includes upper body, lower body, core fitness, and balance for successful performance. Whether PF changes as dancers advance from when they enter (freshmen) to when they graduate from their collegiate program (seniors) is unclear. We prospectively compared collegiate dancers' freshman-to-senior PF. We recorded PF in regard to upper body strength endurance (push-ups), core strength endurance (front, left-side, right-side, and extensor plank hold times), lower body power (single leg hop-SLH-distances % height; Leg Symmetry Index: LSI = higher/lower × 100, %), and balance (anterior reach balance, % leg length, LL; LSI balance = higher/lower × 100, %) in 23 female collegiate dancers (freshman age = 18.2 ± 0.6 years). Repeated measures ANOVAs (p ≤ 0.05) were used to compare measures from freshman to senior years. Across their collegiate programs, dancers' PF remained unchanged. Specifically, their upper body strength endurance push-up numbers (p = 0.93), their core strength endurance plank times (left: p = 0.44, right: p = 0.67, front: p = 0.60, p = 0.22), their SLH distances (left: p = 0.44, right: p = 0.85), and their symmetry (p = 0.16) stayed similar. Also, dancers' right leg (p = 0.08) and left leg balance (p = 0.06) remained similar, with better balance symmetry (p < 0.001) in seniors. Overall, dancers' PF did not change across their collegiate programs. Thus, female dancers' freshman PF may be an adequate baseline reference measure when devising rehabilitation programs and determining readiness-to-return-to-activity post injury.

Influence of Rain on the Support Insulation Electrical Strength

The article presents the results of studying the influence of artificial rain on the electrical strength of an extra high voltage support insulation column containing insulators of different heights when subjected to positive polarity switching impulses. It has been found that, in general, rain has an adverse effect on the support insulation discharge characteristics, which is associated with the occurrence of cascading overlaps from the screen fittings along the insulation structure components. The adverse effect caused by rain tends to aggravate with increasing the column upper element structural height, the switching impulse front time, and rainwater conductivity. As a result, the support insulation electrical strength may drop by as much as 40%. The reason for such a significant effect is the external electric field redistribution along the columns. On the one hand, the presence of a relatively high conductivity water film on the insulator surface when subjected to impulses with a front time of several milliseconds leads to an almost uniform distribution of voltage across insulation structural elements. On the other hand, sharp field strength spikes of the opposite polarity arise on the water droplets and trickles formed at the edges of neighboring sheds, which facilitates the occurrence of spark overlaps across a number of shed spacing distances, weakening of air insulation, and a decrease of structural discharge voltage. Measures to increase the support insulation electrical strength under rain conditions are proposed.

Study on a novel shielded double-winding Rogowski coil for measurement of nano-second current pulses

A novel shielded double-winding Rogowski coil for measurement of nanosecond current pulses is studied. Theoretic analysis of the proposed coil is carried out, and the attenuation factor(reciprocal of the sensitivity) of the Rogowski coil is acquired. It is found that The proposed coil has an advantage on reducing the interference caused by annular displacement current due to the mirror-symmetry structure. A double-winding coil is developed for discharge current pulse of the high power pulse generator. A compact connector in which sampling resistor with resistance as low as 11.3 Ω could be integrated is also developed. According to calibration of the proposed coil, the calibration attenuation factor conforms to the theoretic result. The proposed coil is utilized for measurement of discharge current pulse of the pulse generator. The test result shows that the proposed coil responds well for rectangular pulses with front times of 4.3 ns.

Light front synchronization and rest frame densities of the proton: Electromagnetic densities

We clarify the physical origin and meaning of the two-dimensional relativistic densities of the light front formalism. The densities are shown to originate entirely from the use of light front time instead of instant form time, which physically corresponds to using an alternative synchronization convention. This is shown by using tilted light front coordinates, which consist of light front time and ordinary spatial coordinates, and which are also used to show that the obtained densities describe a system at rest rather than at infinite momentum. These coordinates allow all four components of the electromagnetic current density to be given clear physical meanings. We explicate the formalism for spin-half targets, obtaining charge and current densities of the proton and neutron using empirical form factor parametrizations, as well as up and down quark densities and currents. Angular modulations in the densities of transversely polarized states are explained as originating from redshifts and blueshifts due to quarks moving in different longitudinal directions.

Research on Lightning Current Shunting Characteristics Between Tower and Ground Wires Based on Field Test and Numerical Simulation

When lightning strikes the transmission tower, lightning current is shunted between the ground wires and the tower. The shunting changes the current waveform in the tower, which may affect the potential rise of the tower and the grounding device. A field test is conducted on a 220 kV transmission line and the characteristics of lightning current shunting are obtained in this paper. The test results are compared with the simulation results, which verifies the feasibility of the commonly used simulation models. The main influencing factors of the shunting are discussed. The results show that the current in the tower changes obviously in our case compared to the applied impulse current. The high frequency components of the impulse current mainly enter the tower, which leads to a steeper current waveform in the tower. The difference is closely related to the waveform of the applied impulse current. The slower the impulse current waveform, the greater the difference between the front time and amplitude of the current in the tower and that of the original impulse current. Although part of the current flows along ground wires, this phenomenon may put forward higher requirements for the tower grounding device.