Andreas Lawall

Space-Resolved Modeling of Stationary Spots on Copper Vacuum Arc Cathodes and on Composite CuCr Cathodes With Large Grains

A self-consistent space-resolved numerical model of cathode spots in vacuum arcs is realized on the computational platform COMSOL Multiphysics. The model is applied to the investigation of stationary spots on planar cathodes made of copper or composite CuCr material with large

The impact of capacitor bank inrush current on field emission current in vacuum

({\gtrsim}{\rm 20}~\mu{\rm m})

The 3D numerical simulation of a transient vacuum arc under realistic spatial AMF profiles

chromium grains. The modeling results reveal a well defined spot with a structure, which is in agreement with the general theory of stationary cathode arc spots and similar to that of spots on cathodes of arcs in ambient gas. In the case of CuCr contacts with large chromium grains, spots with currents of the order of tens of amperes on copper coexist with spots on chromium with currents of the order of one or few amperes. The main effect of change of the cathode material from copper to chromium is a reduction of thermal conductivity of the cathode material, which causes a reduction of the radius of the spot and a corresponding reduction of the spot current.

Development of a FEM simulation of axial magnetic field vacuum arcs

Field emission current measurements during the recovery voltage are investigated to understand the origin of restrikes in vacuum interrupters in case of the interruption of capacitive loads. Measurement and analysis of very small field emission currents (0.01 – 1 mA) from the current zero crossing until the restrike are performed both in an experimental circuit as well as in a full-power test-circuits with commercially available vacuum circuit breakers (up to 36 kV rated voltage). Furthermore, the influence of pre-arcing at contact closing under inrush currents in the range of some kA and kHz on the field emission characteristics after capacitive current switching is investigated. The number of making operations as well as the amplitude of the inrush current is varied. A clear relation between inrush current during closing and field emission current after interruption was established.

Numerical simulation of multi-component arcs in high-current vacuum interrupters

Based on a transient and three-dimensional (3D) finite-volume model developed for diffuse vacuum arcs with external axial magnetic field (AMF), we analyse the characteristics of a transient vacuum arc plasma under different spatial AMF distributions in the contact gap. In particular, we discuss the influence of spatial AMF profiles typically found in commercial vacuum interrupters upon quantities of interest such as arc voltage, energy densities onto the anode, electron and ion temperatures. The dependence of the AMF field on the instantaneous current is explicitly taken into account as a function of time. The simulation results document that the distributions of current density and ion mass flow rate at the cathode have a significant effect on the quantities of the vacuum arc plasma, for example the density of the energy flux transported to the anode.

Field emission currents in vacuum breakers after capacitive switching

Axially magnetised (AMF) diffuse vacuum arcs are the heart of a major part of power distribution switchgears in the medium voltage range, and are gaining increasing interest in the high voltage sector. Although there is a good qualitative understanding of the behaviour of AMF vacuum arcs based on empirical experimental results, there is still a lack of understanding of the details of the plasma-contact interaction in high-current switching arcs. A number of different arc models has been developed by different groups, in order to investigate the behaviour of AMF arcs by means of FEM simulation models. We report on a three-dimensional, transient FEM model that is currently being developed. It is based on a magneto-hydrodynamic approach of the magnetised plasma with temperature-dependent material properties. The model is presented in detail, and first results concerning the plasma behaviour are reported.

Cathode Spot Dynamics and Arc Structure in a Dense Axial Magnetic-Field-Stabilized Vacuum Arc

A transient three-dimensional numerical model has been developed to describe a diffuse multi-component vacuum arc between copper-chromium (CuCr) electrodes under the influence of an axial magnetic field (AMF). The model is based on a two-temperature magneto-hydrodynamic approach of the plasma and is realized with commercial simulation software (CFX) and in-house extensions. The quasi-neutral plasma is described as a two-fluid system distinguishing between electrons and multiply ionized heavy particles. The heavy particles are treated as a multi-component fluid containing Cu ions and Cr ions. The model incorporates balance equations for the ion momentum, balance equations for the ion and electron energy, and transport equations for the magnetic flux density and the radiation. The plasma parameters near the cathode are specified in terms of a self-consistent space-resolved numerical model of the cathode spot on CuCr contacts taking into account the granular structure of the contact material. The simulation is performed at different times during a 50 Hz electrical current cycle. Results are presented for plasma flows under realistic conditions referring to the geometry (140 mm diameter, 11 mm gap), the material (CuCr), and the spatio-temporal AMF profiles of a cup-shaped AMF contact system in an industrial high-current vacuum interrupter (72 kA). Depending on the characteristics of the mass flow near the cathode, distinct features of the energy transport onto the anode are calculated.

Experimental investigations on cathode spots and dynamical vacuum arc structure in an axial magnetic field

Field electron emission current measurements during the recovery phase of vacuum interrupters are carried out to understand the origin of restrikes in case of interrupting capacitive loads. A special shunt is designed for this measurement. To compensate the capacitive current component due to the stray capacitance of the interrupter, hardware (electronic, on-line) as well as software (off-line) processing are developed for different applications. Measurement and analysis of field emission currents (0.01-5 mA) from current zero until the moment of restrike are performed both in an experimental circuit as well as in full-power test-circuits with commercially available vacuum circuit breakers (up to 36 kV rated voltage). In this context, the influence of capacitive breaking currents on the dielectric recovery of the vacuum gap and the contacts surface condition is studied. Furthermore, the influence of pre-arcing at contact closing under inrush currents in the range of several kilo-amperes and kilo-hertz on the field emission characteristics after capacitive current switching is investigated. The number of making operations as well as the amplitude of the inrush current is varied. A clear relation between inrush current during closing and field emission current after interruption was established.

Modeling Spots on Composite Copper–Chromium Contacts of Vacuum Arcs and their Stability

The structures of cathode roots on axial magnetic field (AMF)-stabilized contacts at gap distances of a few millimeters are investigated by observation of the arc behavior with a high-speed high-resolution electronic camera. While the current that is carried by a single cathode spot is almost independent of the arc current, the number of cathode spots and the voltage drop between the contacts linearly increase with the total current. At the limit of the breaking capability of the contacts, the average current density is about 1.8 (effective current), which conforms well to the experience with commercial vacuum interrupters. At this point, the CuCr contact surface homogeneously melts over several tenths of square centimeters. Although the molten surface layer thickness is assumed to be a few tens to hundreds of micrometers at most, millimeter-sized protrusions rapidly grow, providing the impression of a “boiling” surface. The growth and dynamics of these surface structures are discussed and compared with the details of spatial-temporal measurements, and an explanation is given for the high rates of acceleration of droplets in the radial direction.

Cathode spots and arc structure in a dense, axial magnetic field-stabilised vacuum arc

In this paper the ampacity of cathode spots and structures of cathode spots on an AMF contact at short gap distance are investigated by observation of the arc behavior with a high-speed, high resolution CCD video camera. While the current carried by a single cathode spot is independent of the short circuit current, the number of the cathode spots and the voltage drop between the contacts increase linearly with the total current. Further the dynamical structure of the arc is studied and the variation of the density of the cathode spots over the current is measured. At the limit of the breaking capability of the contacts the maximum average current density is about 1.8 kA/cm2 (RMS), which conforms well to the experience with commercial vacuum interrupters. Above this point the CuCr contact surface melts homogeneously over several square centimetres. Although the molten surface layer thickness is assumed to be a few tens to hundreds of microns only, millimetre-sized protrusions grow rapidly, providing a rough and quickly changing cathode surface. We discuss the growth and dynamics of these surface structures in terms of hydrodynamic instabilities and provide a model for the formation process.