Michael Römheld

Magnetically driven high current switching arcs in vacuum and low pressure gas

The behavior of high current switching arcs in transverse magnetic fields is essential for the design of switching elements, in which arc motion is used to reduce contact erosion. In this paper magnetically driven arcs in vacuum and low pressure gases are investigated at currents of several tens of kA. Arcs driven in a low pressure environment exhibit a peak arc velocity of more than 1/spl times/10/sup 5/ m/s, exceeding those of comparable vacuum arcs by at least two orders of magnitude. The influences of current amplitude, gas pressure, and the kind of gas filling on the arc velocity are investigated. A threshold behavior concerning the minimum requirements for a nonstationary arc is found. An improved model is presented, which explains the observed peak arc velocities in vacuum and in low pressure gases.

AMF vacuum arcs at large contact separation

In order to explore the feasibility of axial magnetic field (AMF) contacts for applications requiring large contacts at large contact separation, i. e. at voltages above the typical medium voltage regime, experiments have been performed to investigate the arc behavior under these conditions. The AMF arcs were produced between AMF contacts in a synthetic test circuit; the contacts which were numerically optimized by 3- dimensional finite element modeling of the magnetic field distribution. High-speed video recording was used as a major diagnostics to investigate the arc behavior under different conditions. It was found that successful interruptions could be performed at RMS currrents of over 30 kA, at contact strokes of several tens of mm. Even at the highest currents investigated (42 kArms) the arcs are evenly distributed over most of the contact surface, indicating the suitability of AMF contacts for a current interruption capability of 40 kA of nominal short circuit current at these contact strokes.

High-efficiency high-voltage pulse generator based on a fast recovery pseudospark switch

Experiments are reported to demonstrate the ability of pseudospark switches for extremely fast recovery after forward conduction at an anode voltage of up to 25 kV and peak anode currents of 1 to 2 kA. At a pulse duration of around 5 /spl mu/s and at a rate of current rise of up to 1 kA//spl mu/s, the reverse current is blocked at current zero transition due to an extremely fast recovery rate. The maximum achievable rate of rise of anode voltage after current zero is above 50 kV//spl mu/s; the recovery time of the switch, as measured from the end of the anode current pulse to >80% of anode voltage recovery, is of the order of 0.3 /spl mu/s. Initial experiments toward reprate applications were successful in a burst mode operation, at a circuit-limited pulse repetition rate of up to 96 pps.

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.

On the motion of high current switching arcs in vacuum and low pressure gas

Magnetically driven arcs in vacuum and low pressure gases are investigated at currents of several tens of kA. Arcs driven in a low pressure environment exhibit peak arc velocity of more than 100 km/s, exceeding those of comparable vacuum arcs by at least two orders of magnitude. The influences of current amplitude, gas pressure and the kind of gas fill on the arc velocity are investigated. A threshold behaviour concerning the minimum requirements for a nonstationary arc is found. An improved model is presented, which explains the observed peak arc velocities in vacuum and in gases.

Ozone Generation in a Wire-Plate Pulsed Corona Plasma Reactor

Pulsed corona plasma (PCP) reactors offer novel solutions to problems concerning environmental issues in power generation and in a number of industrial processes. Emerging technologies like PCP conversion of noxious compounds in flue gases (nitrogen oxides, sulfur oxides, heavy metals), decomposition of (halogenated) hydrocarbons from industrial processes, or the decomposition of tar-like substances in bio-derived gaseous fuels rely on high efficiency, high throughput plasma reactors. Other applications include indoor air sterilization and odor removal in air conditioning systems, chemical synthesis in non-thermal plasmas, and plasma reforming of gaseous fuels. We report on experimental investigations of a laboratory size, wire-plate plasma reactor for pulsed corona treatment of gas flows. Results concerning experiments in ambient air without gas flow, characterizing the reactor performance with and without application of a DC bias voltage, have been published previously.

Lifetime considerations of high voltage semiconductor diodes for pulsed power applications

A novel pulse generator scheme using a fast recovery pseudospark switch and a stack of high-power semiconductor diodes was tested. The prototype pulse generator is able to drive capacitive loads of over 150 nF, at peak voltages of up to 40 kV, pulse duration of 6 to 15 /spl mu/s (FWHM, full width at half maximum), and repetition rates of up to 80 pps. Nominal pulse current is between one and 1.5 kA peak. The main limitation in lifetime is caused by the high peak current load in the semiconductor diodes during flashover in the ESP. Diode current can reach up to 8 kA in some cases. A variety of different types of diodes has been investigated, with different physical constructions, i.e. fast high-power press-pack types as well as smaller type, fast, stud-mount diodes. Although the larger press-pack diodes experienced a considerably longer absolute lifetime in these experiments as expected, the comparison of lifetime versus current/charge density on the chip reveals advantages of the stud-mount design (with the diode chip soldered to the substrate) over the press-pack design. The experimental results are discussed in terms of an optimization strategy to achieve the highest power density at minimum cost and volume.

Compact, high-current radial pseudospark switch

A compact, high-current pseudospark switch with radial discharge gap and annular trigger slit has been developed, which is suitable for pulse durations of tens of microseconds and peak currents of up to 100 kA. Using homogeneously distributed plasma ignition, improved insulator shielding and a permanent magnetic field in addition to an azimuthal self-magnetic field, encouraging results with respect to lifetime have been obtained.

Design of a high current pulse generator for magnetoforming

Magneto forming of tube or sheet metal parts can significantly extend the range of geometries conceivable with state-of-the-art forming methods. Major advantage is the considerably higher forming speed of the process achievable by using a magnetic piston without inertia. Key for this technology is the development of reliable, long-lifetime, high current pulse power generators able to deliver tens to hundreds of kiloamps of peak current to a mainly inductive load which is highly variable in time during the forming process. We report on the design of a pulse generator for peak currents of up to 200 kA at a pulse width of typically 80 mus, depending on the load parameters. In order to meet lifetime requirements suitable for industrial applications, the short circuit handling capability of peak currents of up to 450 kA is a major issue in the pulse generator design. A modular, 3-branch design has been adopted to achieve the requirements concerning reliability, lifetime, and short circuit handling. The prototype design is based upon off-the-shelf devices, including high-current discharge switches based on semiconductor devices. The basic concept of the pulse generator is presented, including results from extensive circuit modeling concerning nominal performance as well as short circuit behavior

All-Solid-State Power Modulator for Pulsed Corona Plasma Reactors

Pulsed corona plasma (PCP) reactors of wire-plate design offer novel solutions to problems concerning environmental issues and in a number of industrial processes. Emerging applications include indoor air sterilization and odor removal in air conditioning systems, chemical synthesis in non-thermal plasmas, and plasma reforming of gaseous fuels. We previously reported on experimental investigations of a laboratory sized, wire-plate plasma reactor for pulsed corona treatment of gas flows. Operation with gas flow, at pulse repetition frequencies of between 10 pps and 200 pps, has been achieved at pulse voltage amplitudes of between 10 and >30 kV, at pulse durations of around 0.3 mus (FWHM). High efficiencies of up to 70 g/kWh have been reported using an all-solid-state pulse generator. In this work, we report on the optimization of an all-solid-state power modulator for use with nonlinear loads like pulse corona plasmas. The pulse generator is based on a fast thyristor switch (IGCT) discharging a pulse capacitor, a pulse step-up transformer, and a single-stage magnetic pulse compression. At pulse repetition rates of up to 200 pps, amplitudes of >30 kV into a resistive-capacitive load (1 kohms/200 pF) have been achieved, at risetimes of about 80 ns and a pulse width of 0.3 mus. The pulse generator is insensitive to load variations, in particular to sparking in the reactor. The modulator and its performance concerning experimental results will be described in detail when driving a pulsed corona reactor