Chunping Niu

Low-voltage circuit breaker arcs—simulation and measurements

As one of the most important electrical components, the low-voltage circuit breaker (LVCB) has been widely used for protection in all types of low-voltage distribution systems. In particular, the low-voltage dc circuit breaker has been arousing great research interest in recent years. In this type of circuit breaker, an air arc is formed in the interrupting process which is a 3D transient arc in a complex chamber geometry with splitter plates. Controlling the arc evolution and the extinction are the most significant problems. This paper reviews published research works referring to LVCB arcs. Based on the working principle, the arcing process is divided into arc commutation, arc motion and arc splitting; we focus our attention on the modelling and measurement of these phases. In addition, previous approaches in papers of the critical physical phenomenon treatment are discussed, such as radiation, metal erosion, wall ablation and turbulence in the air arc. Recommendations for air arc modelling and measurement are presented for further investigation.

Investigation on Arc Behavior During Arc Motion in Air DC Circuit Breaker

In this paper, the complicated arc phenomena in air dc circuit breakers that are widely used in rail transit and vessels are investigated with both experimental and numerical approaches. Initially, the basic arc motion characteristics in the switch are presented by the recorded waveforms and images in the carefully designed breaking experiments. Phenomena of arc stagnation and back commutation are revealed and the influence of arc chamber width on the arc stagnation is preliminarily discussed according to the experimental results. Then, to make the nature of arc motion clear and to determine the influence details of the arc chamber width on arc motion characteristics, a simplified model based on magnetohydrodynamics theory is built and four cases with the arc chambers of different widths are calculated. The simulated arc behavior is presented by the temperature distribution sequences. In addition, the formation process of back commutation is described in detail by the variation of arc column. The influence mechanism of arc chamber width on arc motion is analyzed according to the differences of pressure propagation and temperature distribution in the arc chamber. Furthermore, the calculated arc displacement curves show that the arc stagnation time could be shortened and the risk of back commutation could be reduced by decreasing the arc chamber width within the reasonable extent.

Experimental Investigation of Arc Plasma in GaInSn Liquid Metal Current-Limiting Device

This paper focuses on an experimental investigation of arc plasma in a liquid metal current-limiting device based on pinch effect. A specially designed experimental setup is employed to observe the dynamic process of arc initiation and arc evolution. From the frames obtained by high-speed photography, the mechanism of arc initiation is revealed. It indicates that the arc ignites due to the self-pinch effect, which starts from the free surface of liquid metal and develops in the form of gas cavities. According to the analysis of the subsequent stage after arc ignition, it can be concluded that the arc plasma should be a mixture of metal vapor and air. With the morphological changes of the arc, the arc evolution can be classified into four stages. The arc erosion on electrodes is observed during the arc process, and its causes are analyzed in detail. Using a liquid metal current-limiting device, the arc characteristics during the current limitation are investigated. It is found that the duration of the prearcing stage and the arc voltage are influenced by the pass-through current. As prospective current increases, the prearcing time reduces because of the acceleration of the pinch process and the arc voltage rise due to the adequate arc burning.

Dominant particles and reactions in a two-temperature chemical kinetic model of a decaying SF6 arc

This paper is devoted to the computation of the non-equilibrium composition of an SF6 plasma, and determination of the dominant particles and reactions, at conditions relevant to high-voltage circuit breakers after current zero (temperatures from 12 000 K to 1000 K and a pressure of 4 atm). The non-equilibrium composition is characterized by departures from both thermal and chemical equilibrium. In thermal non-equilibrium process, the electron temperature (T e) is not equal to the heavy-particle temperature (T h), while for chemical non-equilibrium, a chemical kinetic model is adopted. In order to evaluate the reasonableness and reliability of the non-equilibrium composition, calculation methods for equilibrium composition based on Gibbs free energy minimization and kinetic composition in a one-temperature kinetic model are first considered. Based on the one-temperature kinetic model, a two-temperature kinetic model with the ratio T e/T h varying as a function of the logarithm of electron density ratio (n e/) was established. In this model, T* is introduced to allow a smooth transition between T h and T e and to determine the temperatures for the rate constants. The initial composition in the kinetic models is obtained from the asymptotic composition as infinite time is approached at 12 000 K. The molar fractions of neutral particles and ions in the two-temperature kinetic model are consistent with the equilibrium composition and the composition obtained from the one-temperature kinetic model above 10 000 K, while significant differences appear below 10 000 K. Based on the dependence of the particle distributions on temperature in the two-temperature kinetic model, three temperature ranges, and the dominant particles and reactions in the respective ranges, are determined. The full model is then simplified into three models and the accuracy of the simplified models is assessed. The simplified models reduce the number of species and reactions by a factor of about 2, while providing results that agree closely with the full model. Thus, the physicochemical processes of SF6 arc can be characterized by relatively few species and reactions in each temperature range. It is noted that the simplified models can also be applied to a wide range of pressures, 1–16 atm, conditions which cover most circuit breaker applications. The simplified species and reactions will allow the computing time of multi-dimensional models, taking into account departures from both thermal and chemical equilibrium, to be decreased dramatically while capturing the main physicochemical processes in SF6 arcs.

Research on a novel two-stage direct current hybrid circuit breaker

The DC hybrid circuit breaker based on high-speed switch (HSS) and parallel connected capacitor has been widely applied in the fault current breaking of DC system. However, when the current is commutated from HSS to the capacitor according to single-stage operation, the capacitor has to absorb a large amount of energy stored in the system inductance within very short time. Meanwhile, a high over-voltage rate of rise is especially prone to be produced between the contacts of HSS, which will lead to a failed breaking. As a result, a novel DC hybrid circuit breaker based on the two-stage operation is proposed and analyzed in this paper. By controlling the thyristors in the commutation branches, the fault current is fast commutated into the capacitor, which can not only realize the arcless open of HSS, but also decrease the over-voltage rate of rise significantly in comparison to the traditional single-stage operation. The simulation model of fault current breaking under different conditions in 10 kV medium voltage DC system is constructed. The simulated over-voltages of single-stage and two-stage operations in the case of fault current breaking are compared and analyzed. Finally, the fault current breaking test in the two-stage operation is investigated experimentally, which validates the feasibility and effectiveness of the simulation model well.

Numerical analysis of the short-time current withstand performance of the DC circuit breaker

Nowadays there is the requirement for the selective protection technology of the MCCB used in the DC power system. To achieve this goal it is necessary to improve the short-time current withstand performance of the MCCB. However, the experimental test on such a performance is quite time consuming. Therefore, it is very important to evaluate the short-time current withstand performance of the MCCB by means of simulation. In this paper a simulation model was established to describe the interaction process of the contact resistance, temperature rise, and the electro-dynamic force when the short current occurs. To be more specific, the time-varying of the nonlinear contact resistance due to the electro-dynamic force and the Joule heat are considered in the model. Based on the calculation, the temperature and the current density distribution versus time are obtained, which will be instructive and meaningful for the contact system design in the DCCB. Moreover, the simulation results have been verified by the experiment.

Numerical analysis of the pre-arcing liquid metal self-pinch effect for current-limiting applications

This paper focuses on the numerical investigation of the pre-arcing liquid metal self-pinch effect in a liquid metal current limiter (LMCL). According to the typical structure of the LMCL, an experimental setup is designed to support this study. A three-dimensional magnetohydrodynamic model, based on the volume of the fluid approach, is used to simulate the dynamic self-pinch process of the liquid metal with a free surface. The distributions of volume fraction, pressure and velocity of the two-phase flow are calculated. The simulation results indicate that the depression of the liquid metal free surface during the self-pinch process is due to the downward gas flow, which eventually leads to the rupture of the liquid metal column and arc initiation in the channel. The expansion phenomenon of the free surface depression observed by both calculation and experiment is caused by the interaction of the pressure field produced by the Lorentz force and the gas flow field. The startup of the self-pinch process depends mainly on the effect of three factors, which are the pressure gradient, flow recirculation and the detaching effect of the Lorentz force.

The Development of the Arc in a Liquid Metal Current Limiter

The arc development in the liquid metal current limiter, whose operation is based on the fluid pinch effect, is researched by experimental methods, including high-speed photography. The pre-arcing time and arc voltage are related to the channel diameter and prospective current. The electrode erosion is analyzed, and an efficient method is proposed to reduce this phenomenon.

Investigation of DC hybrid circuit breaker based on high-speed switch and arc generator

A new design of DC hybrid circuit breaker based on high-speed switch (HSS) and arc generator (AG), which can drastically profit from low heat loss in normal state and fast current breaking under fault state, is presented and analyzed in this paper. AG is designed according to the magnetic pinch effect of liquid metal. By utilizing the arc voltage generated across AG, the fault current is rapidly commutated from HSS into parallel connected branch. As a consequence, the arcless open of HSS is achieved. The post-arc conducting resume time (Δ tc) of AG and the commutation original voltage (Uc), two key factors in the commutation process, are investigated experimentally. Particularly, influences of the liquid metal channel diameter (Φ) of AG, fault current rate of rise (di/dt) and Uc on Δ tc are focused on. Furthermore, a suitable Uc is determined during the current commutation process, aiming at the reliable arcless open of HSS and short breaking time. Finally, the fault current breaking test is carried out for the current peak value of 11.8 kA, and the validity of the design is confirmed by the experimental results.

Investigation of the Pinch Mechanism of Liquid Metal for the Current Limitation Application

Fault current limitation is a promising solution to protect power transmission lines and electrical apparatuses. A specially designed setup is employed to observe the pinch process of liquid metal for application of current limitation, which can be classified into three stages. The liquid metal first separates from the nonsmooth spacer channel’s surface to form a uniform air gap between them. Then, it keeps deforming as the air in the gap flows toward both ends of the channel. A moustache-shaped air cavity forms finally in the channel ends and enlarges continuously until arc ignition. Besides, the fluid pinch properties influenced by several parameters are investigated by experiments including the heights of the fluid free surface and the surface profile of the spacer’s channel. To reveal the pinch mechanism, a numerical model based on the volume of fluid method and magnetohydrodynamic theory is built finally and solved by the software FLUENT, which corresponds well with the test results. It is concluded that the formation of the moustache-shaped cavity to “cut off” the liquid metal in a liquid metal current limiter arises from several factors including the nonsmooth spacer channel’s surface, the unstable flow of galinstan, and its unique properties (considerable viscosity, surface tension, and oxidization).