Paul G. Slade

Electrical breakdown in atmospheric air between closely spaced (0.2 /spl mu/m-40 /spl mu/m) electrical contacts

The increasing importance of electrical contacts in air with micrometer spacing prompted recent experiments on the electrical breakdown behavior of these gaps. The electrical field between the contacts used in one of the experiments was analyzed using finite element analysis to model the electric field. The experimental data on the electrical breakdown voltage could be divided into three regions as a function of the gap spacing. First, at close gap spacing (/spl les/4 /spl mu/m) both the breakdown voltages as well as the electrical fields at the cathode were similar to values measured during the breakdown of vacuum gaps of less than 200 /spl mu/m. Second, at larger gaps (>6 /spl mu/m) the breakdown voltages followed Paschens curve for the Townsend electron avalanche process in air. Finally, in between these two regions the breakdown values were below the expected values for purely vacuum breakdown or purely Townsend breakdown. The breakdown phenomena have been discussed in terms of field emission of electrons from the cathode and their effect on initiating the observed breakdown regimes.

Advances in material development for high power, vacuum interrupter contacts

This paper reviews the development of contact materials for vacuum interrupters. During the past 15 years, chromium-copper (Cr-Cu) has gradually become the dominant contact material for use in medium-voltage, high-current vacuum interrupters. The manufacturing methods for this material are discussed. Other materials such a tungsten-copper (W-Cu), tungsten carbide-silver (WC-Ag), and copper alloys are briefly described. The performance of the vacuum interrupter contacts under operating conditions is reviewed. This review shows that Cr-Cu continues to be the best contact material for vacuum interrupters used in vacuum circuit breakers. >

Transition to the diffuse mode for high-current drawn arcs in vacuum with an axial magnetic field

The opening of electrical contacts while passing current generates a drawn arc. In vacuum, the arc begins as a bridge of molten metal connecting the contacts, which then ruptures to form a bridge column arc. Previous work observing the development of drawn arcs in vacuum with an imposed axial magnetic field (AMF) measured the time required for the bridge column to evolve into the high-current diffuse mode. Arc visualization experiments on Cu-Cr contacts with an AMF have now determined that the transition to the fully diffuse mode has a more complicated development. With high-speed photography, we characterized the appearance of the arc modes over half-cycles of power frequency short-circuit current. The opening sequence begins with the rupturing of the molten metal bridge, forming the bridge column. This column evolves into the transition mode, and then into the fully diffuse mode. This transition mode in an AMF consists of a region of concentrated cathode spots, similar to the transition mode for butt contacts at lower currents and no AMF. Over a few milliseconds, an increasing number of individual cathode spots begin to appear outside the concentrated region, until a diffuse arc forms. The transition mode produces a transient peak in the arc voltage. Increasing the AMF strength at a particular current can shorten the duration of the transition mode and reduces the arc voltage peak. Single or multiple half-cycle operations have been performed on Cu-Cr contacts to investigate the effect of the transition mode on contact melting. The melting patterns after a single half-cycle of high current are correlated with the behavior observed in the arc movies. Anode melting is confined to one or two regions of shallow melting, while individual cathode spot tracks covered most of the cathode surface. The combination of arc visualization and post-arcing contact examinations demonstrated that the transition arc mode was a significant source of contact melting.

The transition from the molten bridge to the metallic phase bridge column arc between electrical contacts opening in vacuum

The total arcing time between Ni electrical contacts opening in vacuum was controlled by interrupting a low voltage, low current circuit. The voltage across the opening contacts showed the rupture of the molten metal bridge and then its rapid increase to a value greater than the minimum arcing voltage before dropping back to a value close to the minimum arcing voltage. Using a radioactive tracer technique it was observed that at very short arcing times there was an anomalous net transfer of contact material to the cathode. As the arcing time increased this cathode gain decreased before increasing again at longer arcing times. High-speed streak photographs of Cr and W contacts opening in vacuum at higher currents showed an initial high pressure, metal vapor regime that resulted from the rupture of the molten metal bridge and before the formation of a metallic phase, columnar arc. After the rupture of the molten metal bridge, the voltage across the contacts rose to a value of a few 10psilas of volts before dropping back to a minimum value. These data will be used to discuss the rupture of the molten metal bridge, the formation of the pseudo arc in the high-pressure metal vapor resulting from the bridge rupture and the eventual development of the metallic phase, bridge column arc.

The Effect of Contact Closure in Vacuum with Fault Current on Prestrike Arcing Time, Contact Welding and the Field Enhancement Factor

Experiments were performed with vacuum interrupters containing Cu-Cr (25 wt%) and W-Cu (10 wt%) contacts. The vacuum interrupters were placed in a spring mechanism, which was placed in a tuned, capacitor bank electrical test circuit. The capacitor bank was charged to 25 kV, which allowed a symmetrical fault current of 50 kA (peak) at 30 Hz. As the vacuum interrupters contacts closed a prestrike arc occurred when the contact spacing was small enough. This contact gap was recorded. The prestrike arc initiated the ac current, which was interrupted by the test circuit after one half cycle. The contacts were then opened with no current. This process was repeated 5 times. As the experiment progressed the prestrike arcing time increased; i.e. the contact gap broke down at larger and larger gaps during the closing operation resulting in longer and longer prestrike arcing times. We explained this phenomenon by considering the effect of the prestrike arc and the subsequent contact welding on the surface structure of the contacts. The change in the contacts surface structure resulted in an increase of the field enhancement factor, which, in turn, led to the vacuum breakdown of the contacts at increasing contact gaps. For the Cu-Cr contacts the prestrike arcing time was eventually long enough that the contacts formed a weld that the mechanism could not break. Although the prestrike arcing time with the W-Cu contacts did increase, the mechanism always broke any welds that formed.

Effect of short circuit current duration on the welding of closed contacts in vacuum

High ac currents were passed through closed Cu-Cr (25 wt. %) contacts located in a vacuum ambient (pressure 10/sup -4/-10/sup -3/ Pa). The duration of the short circuit currents ranged from 1 second to 4 seconds. The contact welding was measured for a range of closed contact forces and current durations. The weld fracture strength was measured using a slowly rising opening force of 50 N/s. It was found that the contacts required a larger contact force to prevent welding if the current duration of 4 seconds was applied, than if a current of 1 second, 2 seconds and 3 seconds was applied. The data showed that there was a considerable scatter in the data for all conditions examined. It also showed that a weak weld could well follow a strong weld. A consideration of the transition to the steady state model of closed contact welding can explain the observed data.

Electrical Switching Life of Vacuum Circuit Breaker Interrupters

Experiments were performed to determine the electrical life of a vacuum interrupter as a function of current. The test currents ranged from the normal load currents of 630 A (rms) to 3150 A (rms) to higher currents of 20% to 100% of the rated short-circuit current. The two vacuum interrupter contact styles were evaluated: the transverse magnetic field (TMF) contact and the axial magnetic field structure (AMF). For the short-circuit current switching the AMF contact shows a longer electrical life, but the erosion may not be the only limitation on the life of the vacuum interrupter. The results will be discussed in terms of the expected performance of the two contact designs and on the operating life of a vacuum interrupter.

High Voltage Breakdown Performance and Circuit Isolation Capability of Vacuum Interrupters

High voltage breakdown experiments determined the ability of vacuum interrupters to provide adequate circuit isolation in the open position. The vacuum interrupters were rated for a 38kV system voltage, and contained either Cu-W (10 wt.%) or Cu-Cr contacts. Sample interrupters were voltage conditioned with a peak AC voltage of 160kV. The statistical distribution of the BIL breakdown voltage for 3 and 5 mm contact gaps were determined using the up-down method for newly conditioned contacts. A three-parameter Weibull distribution fit the breakdown data, implying the existence of a threshold value below which no breakdowns occurs. These results are discussed in terms of the high voltage capabilities of Cu-W contacts and with respect of the peak open-circuit voltages for typical 38 kV applications. The vacuum breakdown behavior is also compared to the breakdown in SF6