Arc extinction with nitrogen at 1-40 bar in a puffer-like contact configuration

To develop cost-efficient subsea switchgear for large sea depths, the extinction of arcs under high filling pressures must be understood. In this work, arc-extinction experiments have been performed with a puffer-like contact configuration using nitrogen at different filling pressures as the current interruption medium. The main finding is that, for the given contact configuration, the currentinterruption capability was lower at 20 and 40 barabs than at 1 and 10 barabs. While higher pressures result in higher cooling flow rates and longer flow times given the same puffer volume, compression spring and nozzle geometry; it does not necessarily improve the arc-extinction capability. This is probably because higher filling pressures increase the arc voltage and total energy dissipated in the arcing zone. Because the filling pressure greatly influences the flow characteristics, the puffer design should be optimized for each pressure level

Ageing of Technical Air and Technical Air with 7.5% C5-Fluoroketone by Free-Burning Arcs

This paper reports on the effect of ageing by free-burning arcs in 7.5% C5-fluoroketone (C5-FK) with 92.5% technical air in comparison to that in technical air (80% N2, 20% O2) at 1.3bar absolute pressure. The gases are aged by applying a series of arcs dissipating an accumulated energy of around 315kJ. It is found that the arc voltages in technical air and technical air with C5-FK are in the same range and do not vary significantly as a function of ageing or current amplitude (∼40-900 A). Contact erosion in both mediums is found to be similar if the discharge procedure is same. However, erosion increases significantly if ageing is performed in a short contact gap that needs more arcing operations to achieve similar level of arcing energy accumulation. Furthermore, gas decomposition by-products are analysed using gas chromatography coupled with mass-spectrometry

AIR LOAD BREAK SWITCH DESIGN PARAMETERS

Current interruption is vital in the power system, as this makes it possible to control the use of different loads, change the grid configuration, and minimize damage when faults occur. This thesis presents a study of the different switch design and test circuit parameters involved in medium voltage air load break switching and how they affect the thermal interrupting capability. Mediumvoltage load break switches are common in the distribution grid, and are a cheaper option than installing circuit breakers. Medium voltage load current ratings are typically in the range of 6 – 36 kV and 400 A up to around 1 kA (50 Hz). Air is considered an environmentally benign alternative as an interrupting medium compared to SF6 for these ratings, and is also thought to be cost-competitive compared to vacuum. However, no compact air load break switch for 24 kV is currently available for commercial use. Thus, it is necessary to have a good understanding of the design parameters involved, and how they affect the interrupting capability of the switch. This thesis addresses medium-voltage load current interruption in air. It is an empirical study based on an extensive test program in a medium voltage test lab, and the main results and contents of this thesis are presented in five papers. The two first are switch design parameter studies. Using a test switch that is simple and axisymmetric, yet in many aspects similar to commercial puffer devices, one test switch parameter has been changed at a time to find the air flow over-pressure needed for successful interruption. The test circuit settings are also varied to find how the interrupting capability changes with load currents in the range 400 – 880 A, and with a transient recovery voltage corresponding to IEC’s 24 kV ”mainly active load” test duty. Only the thermal phase of current interruption has been considered, i.e. the first tens of microseconds after current zero. The over-pressures needed to interrupt the load currents were typically from 0.2 to 0.4 bar. The third paper presents a logistic regression analysis of all the conducted interruption tests, with the goal of describing the interruption performance as a function of the main test switch design parameters and transient recovery voltage stresses. More than 3 000 interruption tests are used as input data for this analysis, which produce a mathematical expression that summarizes all the empirical results. The nozzle-to-contact diameter ratio has been found to be an important design factor. Low ratios require a substantially lower air over-pressure than high nozzle-to-contact diameter ratios in order to interrupt successfully. The choice of contact diameter important as well, where larger contact diameters require lower over-pressures, but higher mass flow rates. The nozzle length does not influence the interrupting capability very much, but the chance of successful interruption is greater when the pin contact has moved out of the nozzle at current zero. The interruption becomes more difficult with increasing current and the rate of the voltage build-up across the contacts after interruption. The other two papers are based on the current interruption experiments mentioned above, but concern details of the arc behavior and arc voltage under different currents, test design variations and air flow conditions. For typical medium-voltage and load current ratings, the arc greatly affects the air flow during current interruption. The flow through the tulip contact and nozzle is clogged during the high current part of the half-cycle, even for moderate currents and relatively large contact dimensions. The typical over-pressures needed for successful interruption correspond to air velocities that are well below supersonic level. The arc voltage is a function of several parameters, and rises with increasing air over-pressure, decreasing current, and a larger contact gap. There is also a clear visible difference in the arc appearance when it is either subjected to forced cooling or not. The typical arc voltage is a few hundred volts

Contact and nozzle wear from 100 interruptions for a puffer-type air load break switch

One type test requirement for medium voltage load break switches is to interrupt 100 consecutive "mainly active loads". A puffer-type switch with axial-blown arc has been tested according to the 630 A/24 kV ratings. The nozzle and contact wear were measured regularly to investigate design requirements and the impact from nozzle wear on gas flow. The contact wear is only moderate, while the nozzle wear causes a decrease in pressure build-up, which in turn may influence the interruption performance.publishedVersio

Comparison of Different Air Flow Concepts for a Medium Voltage Load Break Switch

The research and development work towards a compact SF6-free load break switch for the medium voltage range has led to several design proposals. The interruption capability of three different nozzle and gas flow concepts with atmospheric air as the interrupting medium is compared and assessed. The three test switches are installed in circuits corresponding to the mainly active load and switch-fuse test duties of the 24 kV / 630 A load break switch standard. A pressure tank is used to provide different air flow rates, and the interruption capabilities of the different flow concepts are compared with basis in the tank pressure required to give successful interruptions. 270 current interruption tests were carried out. Air flows directed radially onto the arc or swirling along the arc turn out to result in a substantially better interruption performance than when the air flows straight and parallel to the arc. Air flows corresponding to upstream over-pressures of a few tenths of a bar seem to be sufficient for an air-based load break switch rated for 24 kV / 630 A.Comparison of Different Air Flow Concepts for a Medium Voltage Load Break SwitchacceptedVersio

Short-Circuit making of medium voltage load break switches using a grid Connected test circuit

Short-Circuit making of medium voltage load break switches using a grid Connected test circuitacceptedVersio

Ultrahigh-Pressure Nitrogen Arcs Burning Inside Cylindrical Tubes

When the pressure and temperature of a fluid exceed a critical point, the fluid enters into the supercritical (SC) region. In this region, the physical properties are believed to be in favor of a good current interruption medium. This paper focuses on the arc voltage characteristics of nitrogen arcs burning inside cylindrical tubes at different filling pressures: 1, 20, 40, and 80 bar, thus covering the SC region. Two different tube materials have been used in the experiments, alumina and polytetrafluoroethylene. Arc voltages are measured for arcs burning inside tubes of 2, 4, 8, and 15 mm inner diameters. In addition, free-burning arcs have been investigated at the same filling pressures. The arc current was 150 A at 350 Hz throughout the study. The arc voltage is found to increase with decreasing inner diameter of the tube at atmospheric pressure. At higher filling pressures (i.e., 20, 40, and 80 bar), however, such a simple relationship is not observed. The arc temperature and radius have been calculated based on the ``simple theory of free-burning arcs'' and the ``two-zone ablation arc model.'' The calculated arc radius decreases with increasing gas pressure. Furthermore, due to increased absorption of radiation at high filling pressures, ablation is found less significant for ultrahigh-pressure nitrogen arcs compared to atmospheric pressure arcs. This is in line with the observations from optical micrographs of the inner surfaces of the tubes exposed to arcs at different filling pressuresacceptedVersion© 2018 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

Arc Voltage Distribution Measurement in Tube Constricted Ultrahigh-Pressure Nitrogen Arc

This work contributes to the fundamental understanding of axial voltage distribution of the arc burning inside polytetrafluoroethylene (PTFE) tube at very high filling pressures of nitrogen. The arc peak current of 85 A at a frequency of 190 Hz with a fixed initial rate of rise of recovery voltage (IRRRV) of approximately 50 V/μs is used throughout the study. Arc burning at three different filling pressures are studied: 1 bar, 20 bar, and 40 bar. To examine the axial voltage distribution in the arc, the arc voltage at three different axial position of the arc is independently measured. For some cases, a 3 cubic centimeter heating volume is attached to the ring electrode, which produces a back flow. For the cases with a heating volume, the pressure rise in the heating volume is also measured. It is observed that the pressure rise in the heating volume increases with the filling pressure. In the presence of the heating volume at a high filling pressure (i.e., 20 bar, 40 bar), the voltage drop increases significantly near the vent due to the relatively cold gas flow

Short-Circuit making of medium voltage load break switches using a grid Connected test circuit

Short-Circuit making of medium voltage load break switches using a grid Connected test circui

Short-Circuit making of medium voltage load break switches using a grid Connected test circuit

The load break switch is an important component in the distribution grid. Besides its capability to break load currents reliably, it has to be able to make short-circuit currents according to the IEC 62271 standard. In this paper, the process of making a short-circuit current and the used test methods are described. A synthetic test circuit using semiconductors instead of a triggered spark gap is proposed and described thoroughly. The basic working principle has been validated with simulations in Simulink.acceptedVersion© 2018 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