Natural Current Zero Research Articles

Test Setup Design for Measuring the Conductance in the Mechanical Part of Hybrid-Circuit-Breakers Before and After Current Commutation

The number of devices either providing or demanding direct current is growing more and more. This led to the renascence of DC grids and revealed a need for research. One critical component is the DC circuit breaker. It has to handle nominal and fault currents to keep the grid stable and operable. Interrupting currents in DC grids can be challenging because of the absence of a natural current zero crossing. Therefore, complex arcing chambers are needed. Especially at low currents, the self-induced force may not be sufficient to force the arc into the arcing chamber. Hybrid circuit breakers (HCB) are one possible solution to meet theses sophisticated requirements. There are several different topologies. All have in common that mechanical switches and semiconductors are combined to create a switch with the advantages of both technologies. One simple solution consists of two mechanical switches and one bypassing semiconductor circuit. The arc is used to achieve commutation from the mechanical to the semiconductor branch. While it carries the current, the mechanical switch regains its dielectric strength. Since this recovery time is short, the semiconductor can be overloaded within the limits of its thermal capacity. Defining the recovery time can be challenging. In addition, the effect of contact distance, arcing time and current value are unknown. To research the recovery behavior a model-switching chamber with an adjustable contact gap is developed. This device under test (DUT) was implemented into a test circuit that reproduces part of the hybrid switching process. When the arc between the contacts of the DUT reaches its steady state, a parallel commutating path is activated. The residual conductance of the decaying arc was measured. The development and several iteration steps of the test circuit and DUT are laid out. Finally, some test runs and initial measurements are presented.

직류 차단기에 대한 Black-box 아크 모델 적용성 평가 및 아크 특성 매개 변수 비교 분석

Analyzing the arc interruption phenomena of circuit breakers, physical arc model is generally used, but its analysis and applications is complex because it requires understanding a lot of physical parameters such as thermodynamics and mathematical equations which occur during arc interruption process. But the black box model which simulate the dynamic arc behaviour and its interaction with network could be conveniently used to extract essential parameters to determine the performance of the circuit breaker. Black box arc model could represent the non-linear characteristics of the arc using differential equations based on the conductivity of the arc. In order to develop reliable DC circuit breaker which don’t have a natural current zero point, a transient analysis of the non-linear arc characteristics occurring during interruption is needed. In previous studies, black box arc models have been used to analyze AC arc characteristics, and there are few study applying it to DC circuit breakers. This paper simulates the applicability evaluation for DC circuit breakers using KEMA and Habedank black box arc models based on the actual DC circuit breaker test data sheet. Parameters of the black box arc model were determined by performing Parameter Sweep for the main factors, such as interrupting time, arc voltage, and arc time constant. As a result, the applicability of the black box arc model to the DC circuit breaker was verified by predicting the DC arc behavior, and more suitable model for the DC circuit breaker was suggested by comparing the DC arc waveforms.

Quasi-Three-Dimensional Computations of the Microscopic Thermal and Dielectric Characteristics of an SF6 Rotating Switching Arc.

Pure sulfur hexafluoride (SF6) is chemically inert, non-flammable, non-toxic and thermally stable, and it has excellent dielectric strength and arc-quenching and control properties. The switching-off process of SF6 arc discharges occurs at the region between the contacts during the opening sequence to interrupt the flow of excessive current in a faulty network. The arc is tolerated in a controlled manner until a natural current zero when the arc discharge is rapidly quenched to restrict the thermal and dielectric reignition to the interruption. An SF6 self-blast switching chamber combines two advantages of blowing by heat expansion of the SF6 and arc rotation by electromagnetic effect of coil to improve the switching performance on thermal and dielectric reignition. The thermal and aerodynamic behaviors of an SF6 rotating switching arc in the chamber physically are complex and difficult to understand only by measurement due to their three-dimensional effects. Since the late nineteen-eighties, significant progress has been made in the method of computational fluid dynamics describing the physical processes occurring in the switching arc. The final goal of computer simulation technology on the arc switching is to predict the switching phenomena on the thermal and dielectric reignition from an engineering point of view. In this paper, we have conducted quasi-three dimensional computations to predict the thermal and dielectric reignition of SF6 rotating arcs occurring after a current zero in the self-blast switching chamber. Through the complete work, the microscopic thermal and aerodynamic behaviors of the remnant arc column after a current zero should be good criteria to predict the thermal and dielectric reignition of the rotating switching arc in the chamber.

Design of a Compulsator to Drive a Railgun

Compulsators are ac generators which are used for electromagnetic launch applications over the past few decades. This paper presents the work carried out by the authors to design a compulsator for a railgun which has been built and tested with a capacitor bank of 20 kJ as the pulsed-power source in the author’s laboratory. The generator topology selected for the compulsator is a single phase, two-pole machine with iron core and passive compensation. The design is based on the criterion of achieving maximum projectile exit velocity with the additional condition that natural current zero is synchronized with the instant of the projectile’s exit. The effect of varying the different parameters of the compulsator on the performance of the system has also been studied and discussed in this paper. By means of performance analysis, the necessary background for the design of the actual compulsator has also been achieved.

DC Circuit Breaker Using Superconductor for Current Limiting

The unavailability of high-capacity HVdc circuit breakers (CBs) is one of the key stoppers for building HVdc multiterminal networks. As the dc does not have a natural current zero, its interruption is very different. The objective of this paper is to propose a new type of dc CB (DCCB), including a current-limiting part and an interrupting part, which can interrupt a high short-circuit current. Experimental results show that the current-limiting part containing superconductor tapes and parallel resistance can limit the fault current from 20 to 1 or 2 kA very quickly within 1 ms. The interrupting part using the self-oscillation method can break this limited current. Therefore, this type of DCCB has the ability of interrupting a high short-circuit current.

Collisional transfer and neutral copper vapour density during a diffuse vacuum arc

Measurements of the number density of neutral metal vapour during vacuum arcs by laser-induced fluorescence (LIF) are disturbed by electronic collisions which depopulate the upper fluorescence level. To overcome this difficulty, the sinusoidal arc current was rapidly forced to zero which resulted in a rapid reduction of the collisional interaction. This is shown by measurement of the collisional transfer from the optically pumped 42P3/2 level to the neighbouring 42P1/2 level of copper. Measurements of the laser-induced fluorescence were performed shortly before and after the artificial current zero. Due to the reduction of the collisional interaction, the LIF signal after current zero rises by an order of magnitude above its value before the artificial current zero, reaches a maximum two microseconds after current zero and decays as the vapour disperses. The vapour density inferred from the maximum LIF signal is considered as an approximate value for the density before the artificial current zero. By forcing current zeros at preselected times during a 50 Hz sinusoidal half-cycle with 200 A peak current, a lower limit for the vapour density during arcing was obtained. The copper vapour density rises to a maximum value of 3.6*1018 m-3 near current crest and drops to 5*1016 m-3 at the natural current zero of the half-cycle. Afterwards it decays by about two orders of magnitude within 1 ms.

Current instability and chopping with arcs in series

Small inductive currents are normally interrupted before a natural current zero, due to an instability resulting from an interaction between the arc(s) in the circuit breaker and the capacitance in the circuit. In the paper, a circuit is discussed which consists of two or more arcs in series, a capacitance and an inductance, when the parameters are different for the arcs. Analytical methods based on equivalent circuits for the arcs, and computer simulations using dynamic arc equations give instability and chopping currents, and a time-dependent resonant-circuit model is used to demonstrate a well known statistical reduction of these currents.

Plasma Density Measurements Using a Double Langmuir Probe in a Prototype Vaccuum ARC Fault Current Limiter

Electric power systems could benefit from a device that would limit the magnitude of short-circuit current. A switched resistor fault current limiter requires a commutating switch capable of interrupting current without the benefit of a natural current zero. The unstable diffuse plasma discharge between electrodes in a cylindrical geometry might be capable of such a task but the phenomena involved in this discharge is not completely understood. To develop a detailed model of this unstable discharge, measurements of the plasma density as a function of position and time were made using a double probe. A maximum density of 5 x 1020 m-3 was measured 90 mm from the cathode of this diffuse copper vapor discharge in vacuum.