Roman Renz

Future trends in vacuum technology applications

In the medium voltage range the vacuum switching principle is well established. Today vacuum circuit breakers are available up to 52 kV rated voltage and 80 kA short-circuit current. Based on long term experiences and knowledge from basic research development there is a clear progress into lower and higher voltage ranges and highest interrupting capabilities. In addition to expansion of ratings also new applications for the vacuum technology have been attained. For example loadbreakers and recloser as well as transformer tap changers make more and more use of vacuum tubes. Field experiences reveal high reliability of the vacuum chambers with mean time to failure (MTTF) of several tenthousands of tubeyears. Due to high-tech production processes and modem tube designs vacuum switching devices today are maintenance-free.

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

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.

Development of a FEM simulation of axial magnetic field vacuum arcs

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.

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

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.

On criteria of optimized application of AMF- and RMF-contact systems in vacuum interrupters

In case of interrupting high AC-currents in vacuum, a metal vapour arc burns between the separating contacts, till the next current-zero. However, if the current exceeds the range of approximately 10 kA the arc will pinch due to its own magnetic field. In order to avoid a local contact overheating there are two possibilities: movement of the constricted arc on the contact surface by a radial magnetic field (RMF); and prevention of arc constriction by an axial magnetic field (AMF). In this short overview the functional principles and physics of RMF and AMF are given. Pros and cons of RMF versus AMF with regard to interrupting capability, electrical resistance, tubes dimension and production costs are compared for the low, medium and high voltage range.

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

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.

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

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.

High Voltage Vacuum Interrupters; Technical and Physical Feasibility versus Economical Efficiency

Tn the medium voltage range the vacuum switching principle is well established (Fink and Renz, 2002). Today vacuum circuit breakers are available up to 52 kV rated voltage and 80 kA short-circuit current. By using a series arrangement of two or more vacuum interrupters it is basically possible to double or multiply the dielectric strength without increasing the operating energy substantially. Applications with two 24 kV or 36 kV standard vacuum interrupters in series for rated voltages of 52kV or 72kV are well known. In principle there is no restriction on the medium voltage range for single vacuum interrupters. Tubes for 72kV and even 145kV applications are described by several manufacturers. However due to a nearly square-root dependence between the dielectric strength and the contact stroke it is not trivial to find acceptable solutions. Concerning environmentally aspects also the external insulation of the interrupter causes problems. SF6 is well established in compact gas-insulated switchgears (GTS). Of course designs for air-insulated switchgears (AIS) are possible. But adequate solutions have to be found. Another issue to be considered is the interrupting capability. The dependence on contact stroke for conventional contact systems may limit short circuit current interrupting capability. The higher the stroke the lower the maximum current, which can be proper, interrupted. Increasing of the contact diameter ad infinitum is useless. Switching capacitors is the next technical challenge. On one side a dielectric sufficient stroke is necessary, on the other side increasing contact gap reduces the interrupting capability. Overvoltages produced by switching inductances also deserve closer attentions. Common contact materials for medium voltage vacuum interrupters are maybe not suited for low surge behaviour in high voltage devices. Together with the electrical items the mechanical stability of the interrupter has not to be sneezed at. The endurance of the bellows and housing as well as the bending and upsetting of the conduction bolts demand high sophisticated engineering. In spite of all the technical problems proper solutions up to 72kV were developed in the past and 145kV-bottles with a single interrupter gap are possible. The main handicap against the introduction of vacuum technology into the high voltage domain is obviously the economical efficiency. Well established technologies and production facilities enable cost-effective products today. A decision pro vacuum is possible for high voltage applications as recently as technical and commercial solutions are available

Thermodynamic Models for RMF- and AMF- Vacuum Arcs

In the medium voltage range the vacuum switching principle is well established (Fink and Renz, 2002). Today vacuum circuit breakers are available up to 52 kV rated voltage and 80 kA short -circuit current. Based on long term experiences and knowledge from basic research thermodynamic models for the RMF- and AMF-arc behaviour were developed and are presented in this paper. In the case of RMF-driven arcs the equation of motion caused by the LORENTZ-force describes various experimental experiences rather well with a minimum of assumptions. The model-approach is that the arc mass is equal to the spot-lake mass. Hence the linear contact diameter-dependence of the interrupting capability and the current-dependence of the arc radius can be described in a rather good agreement to the experimental data. In the case of the diffuse burning AMF-arc the boundary condition for the effective contact area (Renz, 2000) is given by a minimum value of the specific axial-magnetic induction Gamma. This area is related to the molten surface and is a well defined part eta of the geometrical contact area. Treating the arc as a magneto-hydrodynamic medium both eta and Gamma can be estimated in a self-consistent calculation

Permissible X - ray radiation emitted by vacuum - interrupters / - devices at rated operating conditions

In the medium voltage range, the vacuum interruption (VI) principle and vacuum devices (VD) are well established. Today, vacuum devices are available in the medium voltage range up to 52 kV rated voltage, as well as vacuum circuit breakers up to 80 kA short circuit current interruption ability. Besides the excellent interrupting performance, the vacuum interrupters and vacuum devices have to withstand at normal service condition and between the open contact system, while rated voltages and the corresponding test voltages applied according to the national and international standards. These are the power frequency voltages UPF and the lightening impulse voltages (Basic Insulation Level, (BIL)).