Jörg Teichmann

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.

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.

Switching behavior of vacuum interrupters under practical system conditions

The vacuum interrupter is generally accepted as a very reliable circuit breaker in the medium-voltage power system. Therefore, this technology becomes increasingly important for further applications such as load breakers. Each new range of application results in different requirements on the contacts. For instance, copper-chrome in different compositions and fractions has been established as the best material for circuit-breaker applications in all distribution voltage levels. But the chopping currents may cause high switching overvoltages in case of inductive loads such as transformers. Concerning the measurement of chopping current, no standardized test circuits exist. In the literature, mostly synthetic test circuits are regarded, where the current is generated by discharging a capacitor. As the interrupter under test in this paper shall be optimized for load breaking, the used test circuit is adapted towards load breaking standards. The influences of the test circuit parameters are deeply examined in this paper. Current interruption at and before natural zero as well as the switching overvoltages for different small inductive currents (30 to 650 A) are evaluated. In addition, the influence of contact material (CuCr and Cu) is analyzed. From the result of this analysis and of further investigations recommendations on the applicability of these contacts shall be developed.

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.

Influence of supply and load circuit parameters on the chopping phenomena of vacuum interrupters

Vacuum interrupters are the state of the art technology for breaking short-circuit currents and load currents in medium-voltage systems. Consequently, further applications may be established for vacuum switchgear technology. For each new application, the characteristics of vacuum interrupters have to be observed. One of these is current chopping that results in high overvoltages when switching off inductive loads. It is well known from literature that this behavior is strongly dependent on the contact material of the interrupter contacts. For example, copper-chromium has been established for the most circuit breaker applications in medium and high voltage levels. To compare the different materials and their properties, a test circuit is necessary which helps to measure the chopping current strictly according to the test circuit parameters. In this research project, it was possible to show that the measured chopping currents are influenced by inductivities and capacities both in the load and in the supply circuit. This result is of high importance for the interpretation of phenomena in practical applications. Also, the impact of the supply voltage level was investigated to validate the measurements that were performed at lower voltages.

Investigations on contact erosion in vacuum circuit breakers by arc rotation measurements with external magnetic field sensors

This research project focuses on developing a measurement system which is able to detect the arc motion in a RMF (radial magnetic field) contact system. The arc movement is measured by analyzing the magnetic field signals of several external induction coils detecting the magnetic field changes from the rotating arc. In this way it is possible to analyze the dynamic characteristics of constricted vacuum arcs rotating in a commercial vacuum circuit breaker. The main benefit of this approach is that commercial vacuum interrupters can be tested. The working principle of this measurement system is discussed, as well as the additional effects that need to be taken into account to correctly analyze and interpret the measured signals. The contact erosion can cause two or more spiral arms melt together, thereby eliminating the RMF and decreasing the performance of the contact system. In this new approach, measurements of the rotation behavior of a constricted arc made it possible to detect when this event happened. The rotation behavior of constricted arcs was measured during the entire lifetime of a circuit breaker to observe the erosion phenomena step by step. With this method, the performance limit of the circuit breaker can be detected before the end of life is reached.

Low-pressure switching plasmas with axial magnetic field stabilization

Low pressure gas discharge switches of the usual pseudospark design are known to have rather large electrode erosion rates when operated at high current levels and long switching times. In order to improve the plasma confinement and to reduce the electrode erosion in high-current applications, an axial magnetic field has been used in a low-pressure pulsed switching device. For a pulse duration of 28 /spl mu/s and current amplitudes of up to 50 kA the plasma expands quickly, leading to a diffuse discharge mode. The radial expansion velocity increases with current amplitude and reaches up to 10 km/s at a peak current of 53 kA. With a further increase in current, a constricted plasma is superimposed onto the expanded diffuse part. These findings are compared with the case in which no magnetic field is applied.

Low-pressure switching plasmas stabilized by an axial magnetic field

An axial magnetic field has been used in conjunction with a low-pressure pulsed switching plasma in order to reduce contact erosion in high-current applications. For a pulse duration of 28 /spl mu/s and current amplitudes of up to 50 kA the plasma expands quickly into a diffuse mode. Radial expansion velocity increases with current and reaches up to 10 km/s at 53 kA. With increasing current, a constricted plasma is superimposed onto the expanded diffuse part.

Cold-cathode high-power thyratron for high-Coulomb switching applications

Industrial pulsed power applications like magnetoforming, concrete recycling and rock fracturing require high-current switches for the power conditioning system which are not yet available with sufficient lifetime and reliability. In particular, the simultaneous handling of pulse currents of the order of 100 kA in amplitude and tens of microseconds duration, at a voltage approaching 20 kV or more, is not yet possible with a standard product at a reasonable lifetime on the order of hundreds of kilo-Coulombs. Investigation into a cold-cathode gas discharge switch is presented, which promises high-coulomb transfer switching capability at high peak current and long lifetime. It is based on an axial magnetic field superimposed to a conventional pseudospark discharge; the low-pressure switching arc diffuses to a diameter up to 40 mm at currents above 30 kA, whereas the arc column tends to constrict to a diameter on the order of 5 mm without the application of an axial magnetic field.