Investigation on Fault Current Making in Medium Voltage Switchgear in Air

Closing a switch under a fault condition could result in extensive electrical contact wear and a possible switch failure. A load break switch (LBS), as one of the critical components in medium voltage (MV) power grids, should be able to be closed under short circuit currents of thousands of amperes. The dielectric breakdown between closing contacts creates a pre-strike arc before the contacts touch. Part of the short circuit current flows through the pre-strike arc, and the rest passes through the contacts when sliding through each other. The dissipated energy in the switching arc is partly absorbed by the electrical contact surfaces, increasing their temperature to melting and evaporation points. This increase may result in switch failure because of contacts welding when molten contact surfaces are closed. Depending on the dielectric strength of the insulation gas, the prestrike arcing time could be different. SF6 has been the most efficient insulation gas because of its high dielectric strength and thermal properties. However, it should be replaced because it is one of the most potent greenhouse gases. This PhD thesis focuses on making operations in air MV-LBS. A synthetic test circuit for making short-circuit current and a model switch are designed based on standards. Different measurement methods are employed to investigate the switch degradation and failure in short-circuit making operations. Arc emission spectroscopy is performed to characterize the pre-strike arc properties at different closing speeds. The arc dynamic motion concluded from the spatially resolved temperature profiles at different times shows that the arc gets stabilized after a few hundred microseconds in the middle of the switch between contacts. Based on the derived temperature profiles, an indirect way of estimating the arc voltage has been proposed, which was in agreement with the corrected direct measurements. Furthermore, electrical characterization of the making is performed at different stages of the operation. For the model switch, higher dissipated arc energies at the same current level result in higher contact mass loss, while an increase in the closing speed over proportionally reduces the contact degradation. The destructive energy of the pre-strike arc causing contacts degradation is measured to be at least one order of magnitude higher than the energy dissipation by ohmic loss when the current flows between sliding contacts in the closed position. Therefore, it can be concluded that the most destructive impact of making operation occurs at the first half-cycle of the short circuit current where the prestrike arc exists. The results of short circuit current making tests with one half-cycle could be extended to the test conditions according to IEC 62271-103 standard, where the minimum duration of short circuit current flow should be 0.2 s

Study of correlation between voltage variation and surface temperature during anode mode transitions in a model vacuum switch

The correlation between the voltage course and the anode surface temperature was studied by combined optical and electrical measurements. Performed analysis of the temporal evolution of electrical and optical signals reveals that in the case of constricted anode attachment a clear correlation between electrode surface temperature and arc voltage occurs. The results of the study provide new opportunities for qualification of electrode materials for switching applications

Spectroscopic Investigation of Metal Content in Prestrike Arc During Making Operation in a Low-Current Model Switch

Energy dissipation during prestrike arc is the critical factor for electrical contacts erosion and welding in medium voltage load break switches. Using air-filled devices as an alternative to SF₆ makes the switch environmentally friendly, but leads to a more challenging process due to a higher prestrike arcing time between contacts. Therefore, understanding the erosion process of electrical contacts is crucial to improve the switch lifetime. Determination of contacts surface evaporation by optical emission spectroscopy is one of the most precise methods to investigate the prestrike arc interface with the contacts. In this article, the temporal and spatial profiles of copper and tungsten emitted species during prestrike arc are presented. For this purpose, a circuit consisting of a synthetic dc high voltage part is used to initiate the arc. The temporal evolution of CuI, CuII, and WI shows evaporation of the cathode and the anode surfaces during the prestrike arc, and the spatial profiles show an inhomogeneous distribution of the vapors alongside the arc root.acceptedVersion© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works

Optical investigation on pre-strike arc characteristics in medium-voltage load break switches

Medium voltage load break switches (MV-LBS) should pass fault current while closing and be able to re-open for the next operation. Replacing SF6 as a high impact greenhouse insulating gas with air, makes the switch design more challenging because of the higher pre-strike arcing time and energy dissipation between contacts which leads to more contact surface erosion and an even higher possibility of welding. In this paper, a synthetic test circuit is used to emulate stresses applied to MV-LBS during the making of short-circuit currents. Since there are difficulties in accurate direct measurement of arc voltage because of the inherent response of the measurement system, an alternative method using optical emission spectroscopy (OES) is proposed. OES measures the pre-strike arc temperature distribution profile close to the cathode surface at a test voltage of 18 kV and a making current of 17 kA. The arc electrical characterization is achieved using the obtained spectroscopy results, Lowke's model and thermal air plasma transport properties. A maximum arc temperature of 12 500 K while the arc moves from the lower part of the cathode to the center, arc voltage of 30–58 V, and dissipated energy of 79–87 J are calculated for the pre-strike arc considering the impact of copper evaporated from the contact surfaces. Different arc behavior is observed in closing the contacts compared to free-burning arcs, which indicates gas flow blowing the arc caused by the contact movement. This investigation could be used for a better understanding of switching behavior and efficient control of the operation.submittedVersio

An Experimental Study of Short-Circuit Current Making Operation of Air Medium-Voltage Load Break Switches

Replacing SF 6 , the most potent greenhouse gas, with an alternative gas is a challenge faced by medium-voltage load break switches (MV-LBS). Air is a possible alternative, but there are some challenges regarding low dielectric strength leading to high arcing time and dissipated energy. Therefore, understanding the switching processes for both interruption and making operations in air MV-LBS is crucial to designing efficient compact switchgear. This work focuses on making operations in air MV-LBS. A synthetic test circuit and a model switch are designed based on the standards to simulate making operations under fault conditions similar to a real test. The test condition is set to achieve high pre-strike arcing time and energy. The results show that the most destructive impact of the making short-circuit current occurs in the first half-cycle of the load current when the pre-strike arc is formed. For an average short-circuit current with a peak of 22 kA and a breakdown voltage of 20 kV, the switch failed to re-open due to the arcing contacts welding after four successive making operations without main contacts and seven successive operations with main contacts. It has been shown that the total arcing contact mass loss occurs mainly during the pre-arcing time.Increasing the closing speed could be a possible solution to minimize the impact of arcing.submittedVersio

Impact of pre-strike arc on contacts degradation after short cir- cuit current making operation in medium voltage air load break switches

Medium voltage load break switches are required to perform a number of making operation while passing of short circuit current that could be more than tens of kiloamperes. Using air-filled devices as an alternative to SF6, which is a high impact greenhouse gas, makes the switch more environmentally friendly but leads to more challenging making operation due to higher arcing times and dissipated energies between the contacts. In this case, the prestrike arc could lead to contacts welding and degradation, which is highly undesirable. This paper reports on an investigation of the pre-strike arc impact on erosion and welding of copper/tungsten (20/80) arcing contacts during short-circuit making operations. For this purpose, a synthetic test circuit consisting of a high current source in combination with a high voltage one is used. Experiments are conducted for different operation voltages, while the short circuit current is kept constant at 22 kA. Mass loss measurement and visual inspection of eroded/welded contacts are examined with regard to pre-strike arc impact on their degradation. The contacts are welded by three times repeating the test at operation voltage of 20 kV and short-circuit current of 22 kA and failed to re-open. Besides, an increase in the contacts’ mass loss with arcing time is observed while the making current is constant. This is an indication that the pre-strike arc energy highly impacts the switch reliability and service life.submittedVersio

Optical Diagnostics of Anode Surface Temperature in Vacuum Interrupters with Different CuCr Compositions

The requirements for contact material in vacuum interrupters are demanding because the arc medium is formed by electrode evaporation during the operation. Copper chromium alloy (CuCr) is widely used as contact material. Different compositions could result in a different rate of successful interruption. This work compares vacuum arc properties for two CuCr compositions - 75/25 and 50/50 weight percent. The contact surface temperature is measured during and after the vacuum arc with an AC current flow at 2kA magnitude. Quantitative characterization of the contact surface temperature was obtained with near-infrared spectroscopy using Planck curve fitting and spectrally filtered high-speed photography. Evaluation of the anode surface temperature has shown a slower surface cooling for CuCr 50/50 compared to 75/25. Despite this, CuCr 75/25 has experienced higher temperature with dispersion in a larger area during the arcing time, in a successful current interruption

Correction: Analysis of the transient response of a dual-fed RC transmission line

This is an Erratum for Analysis of the transient response of a dual-fed RC transmission line. PLoS One 10(3):e0116993-1-e0116993-11 Article number 2 2015. DOI: http://doi.org/10.1371/journal.pone.0116993.The transient analysis of a uniform transmission line of finite length is considered in this paper. For the first time this paper provides an analytical expression for the time-domain response of an RC transmission line, which is stimulated by a step function that is fed into both ends of the transmission line. In particular, we find an analytical expression for the step response at the center of the transmission line, in order to determine the worst-case rise time. This is of interest, for example, in large charge-coupled device (CCD) arrays, where long polysilicon lines are dual-fed in order to mitigate degradation in rise time. The analytical expressions for the RC transmission line are supported by computer-simulated lumped RC models.Mohsen Dorraki, Gregory K. Cambrell, Derek Abbot

Downregulation of Caspase 8 in a group of Iranian breast cancer patients – A pilot study

Acknowledgments Author contributions: MA performed related laboratory work, helped with sample collection, analysed the data and drafted the manuscript. ND and FAJ helped with sample collection and laboratory work. ASF and MG confirmed the diagnosis and provided the appropriate specimens. AP conceived and designed the study, supervised the project and edited the manuscript. The authors would like to thank the research council of Mashhad University of Medical Sciences, Mashhad, Iran for the financial support (Grant Number: 940936).Peer reviewedPublisher PD

Parameter identification using moment of velocity

Many physical systems can be adequately modelled using a second-order approximation. Thus, the problem of system identification often reduces to the problem of estimating the position of a single pair of complex–conjugate poles. This paper presents a convenient but approximate technique for the estimation of the position of a single pair of complex–conjugate poles, using the moment of velocity (MoV). The MoV is a Hilbert transform based signal processing tool that addresses the shortcomings of instantaneous frequency. We demonstrate that the MoV can be employed for parameter identification of a dynamical system. We estimate the damping coefficient and oscillation frequency via MoV of the impulse response.M. Dorraki, M.S. Islam, A. Allison and D. Abbot