Wilfried Haas

Investigation of arc roots of constricted high current vacuum arcs

The anodic and cathodic arc roots of constricted high current vacuum arcs were investigated with a fast framing charge-coupled device camera of 1 /spl mu/s exposure time. The experiments were performed with cup-shaped contacts, with sinusoidal currents of amplitudes between 20 and 100 kA, and a sine halfwave duration of 10-12 ms. The arcs were drawn by contact separation and accelerated by the Lorentz force between the arc current and the transverse magnetic field generated by the contrate contact. The anode and cathode arc roots behave reproducibility and arc scaleable within the range of currents investigated. Both types of arc roots are elliptical, with a major to minor axis ratio of 1.4. The major axis points are in the direction of arc propagation. Anodic and cathodic arc root cross-sectional areas as a function of current can both be described by a potential law with a common exponent of 0.76. For currents of 20-100 kA, mean current densities of 81-121 and 41-60 kA/cm/sup 2/ were found in anode and cathode arc roots, respectively. Estimations of their temperature and vapor densities were performed. For the investigated current range T/sub A//spl ap/3300-3600 K, n/sub A//spl ap/1.6*10/sup 19/-2.2*10/sup 19/cm/sup -3/ and T/sub C//spl ap/3200-3400 K, n/sub C//spl ap/0.8*10/sup 19/-1.2*10/sup 19/ cm/sup -3/ were found for anode and cathode, respectively.

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.

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.

Thermodynamic Model of Moving Vacuum Arcs on Contrate Contacts

The behavior of constricted vacuum arcs on radial magnetic field (RMF) contacts in vacuum tubes of medium voltage circuit breakers strongly determines the breaking capacity of these contact systems. The movement of these arcs under the influence of the RMF is investigated with the help of a one-dimensional (1D) thermodynamic model of the arc and its interaction with the contact surface. Heat conduction and evaporation are taken care of analytically, while the size of the arc root and, hence, its current density has been determined experimentally. The energy balance is solved by using empirical values for the respective sheath voltage drops. It is complemented by a momentum balance incorporating neutral vapor loss from the arc boundaries, which is found to be essential for the quantitative understanding of the arc physics. A time-dependent analysis of the arc properties like the arc velocity during a sine half-wave produces a good agreement with experimental values measured for different sizes of contrate contacts. The model is explained and compared to experimental results of arc velocity and experimentally determined switching capacity, respectively