Dielectric Recovery Strength Research Articles

Recovery of Dielectric Strength after Current Interruption in Vacuum

Recovery of Dielectric Strength after Current Interruption in Vacuum

Dielectric Recovery and Predicted AC Performance of Blown SF6 ARCS

The rates of recovery of dielectric strength of gasblasted SF6 arc channels following current interruption have been measured by using the free recovery method. The experiments were performed with upstream arc chamber pressures of 35 psia and 60 psia, with a 1 cm electrode spacing, and 400 μs duration square current pulses whose amplitude was varied between 20 A and 1000 A. These arc chamber pressures were sufficient to ensure that critical pressure and sonic velocity occurred near the nozzle throat. An empirical linearization of the recovery characteristics has enabled these dc results to be translated into terms of ac circuits. This has enabled a prediction of ac interruption capability to be determined, assuming that the arc power input does not approach the energy clogging regime.

Dielectric Recovery and Shield-Currents in Vacuum-Arc Interrupters

Dielectric recovery data is presented following forced current interruption of 930 A dc copper vapor arcs in a vacuum interrupter. At given free recovery periods during the first 6 to 25 , us following interruption, the instantaneous dielectric strength is determined by the application of high voltage step function pulses. This instantaneous dielectric strength is defined as the maximum voltage which can be consistently reapplied without causing reignition. The influence of electrode spacing (0.6 or 1.3 cm), shield potential and the polarity of the reapplied voltage on the recovery of dielectric strength have been determined. The more rapid rate of recovery initially observed at long spacings is attributed to the lower mean inter-electrode vapor density immediately following interruption. Recovery is also more rapid for reverse rather than same polarity reapplication indicating surface effects, in particular the roughening of the cathode surface during arcing. Post arc currents lead to an initial reduction of dielectric strength for reverse reapplication with the vapor shield tied to the arcing-anode. The mechanism of these post arc currents has been investigated by observations of the high ion currents which flow to a negatively biased shield during and immediately following arcing.

Enthalpy-flow limitation of gas-blasted SF6 arcs

Experimental measurements of arc voltage and dielectric-recovery strength after current zero have been made for gas-blasted SF6 arcs in a small polycarbonate (insulating) nozzle, to determine the arc power input which exceeds the enthalpy-removal capability of the gas flow. The experiments were conducted with a 1.9cm electrode spacing, 400μs duration square current pulses of 100 A and 350 A, and an upstream arc-chamber pressure of 4atm, which ensured that critical pressure and sonic velocity occurred near the throat. The results have been analysed using an enthalpy-flow arc theory, which incorporates the concept of an expanding arc in the direction of the gas flow, as arc energy transported radially heats, and thus indirectly accelerates, the cooler gas surrounding the arc. The maximum enthalpy flow that can be cleared through the nozzle by the SF6 gas flow is about 200 kW/cm2 atm, which corresponds to an axial arc temperature in the nozzle throat of about 25000K.

Dynamic response of arcs in various gases

THE ELECTRICAL characteristics of an arc drawn upon separating the contacts of a circuit interrupter can be classified as the static, or steady-state, and the dynamic characteristic. Both are involved in the circuit interrupting process, but of the two the dynamic characteristic is generally most important. The latter is essentially involved in the transition of the arc from a conductor to an insulator which must precede the recovery of high dielectric strength between the switch contacts. This transition, which depends on changes in energy content associated with the arc conductance, can most conveniently be described and analyzed in terms of differential equations which serve as approximate mathematical “models” of the arc. By means of such models, the arc can be treated as an element in the connected circuit. Usefully simple mathematical models have been devised by several authors by this method, all of which include a characteristic “time constant” which will be designated as θ. This quantity is a measure of the time required for the arc conductance to reach a new steady state when its original state is disturbed.

Dielectric Recovery of a.c. Arcs in Turbulent Gases

It is pointed out that arc extinction in modern high voltage alternating-current circuit interrupters is actually achieved by subjecting the confined arc to a turbulence-producing gas blast, thus accelerating the arc's dielectric recovery near current zero. Experiments to investigate the effects of gas velocity, gas composition, arc current magnitude, and constriction of the arc space upon the rate of recovery of dielectric strength by the arc near current zero are described. Results of these experiments show that most of the dielectric recovery occurring in a turbulent arc space within a hundred or so micro-seconds of current zero takes place before the actual moment of zero current. With a 600-ampere confined arc in carbon dioxide, it was found that the dielectric strength at current zero could be increased from 100 to 215 volts per centimeter by increasing the gas velocity from 35 to 110 meters per second. Dielectric recovery continued after current zero at about 25 volts per centimeter per hundred micro-seconds, and was nearly independent of gas velocity. A reduction in distance between confining walls from 1.9 cm to 0.635 cm resulted in a one-third increase of dielectric strength at current zero and a 100 percent increase in rate of recovery subsequent to current zero. Decreasing the arc current from 600 to 200 amperes resulted in a 120 percent increase in dielectric strength at current zero and an eight-fold increase in subsequent rate of recovery. In various gases, the comparative results for a 600-ampere arc are given in the table. These results are analyzed from the standpoint of recent theory.

Extinction of A-C. Arcs in Turbulent Gases

A recent theory of a-c. arc extinction in gas-blast circuit interrupters is reviewed. It is predicted on the basis of this theory that the interrupting ability of an a-c. arc should increase very considerably with the degree of turbulence to which the arc gases are subjected, and also that the recovery of dielectric strength by a turbulent arc space during a period of zero current should be very rapid for the first few microseconds and then comparatively slow for a considerable time thereafter. Experimental results obtained with an arc in atmospheres of various common gases verify these predictions, and also reveal the importance of the gas medium. A curve has also been obtained showing recovery of dielectric strength with time during the current zero period by an arc in an expulsion fuse tube. This curve has the characteristic shape predicted by the turbulence theory, thus proving the validity of the theory as applied to the behavior of a practical gas-blast device.

II — Various factors aid extinction of arcs

EXPERIMENTS to determine the influence of such parameters as electrode material, current strength, and arc length on the characteristics of the more common type of stationary arcs with molten electrodes, have given results not previously expected. Simplified calculations made by Slepian a few years ago based on the theory that ions are lost from the arc space only by direct ion recombination, predicted a manner of recovery of dielectric strength substantially the same as that actually revealed by experiments made on cold electrode arcs at about the same time. The rate of recovery of voltage across the arc space following a current zero was controlled by shunting the arc with resistance, and the maximum allowable rate of rise of voltage was determined from the maximum value of shunting resistance at which the arc would just be extinguished after one-half cycle.

Extinction of Short A-C. Arcs

This paper presents the results of a study made with short, stationary a-c. arcs between metallic electrodes, in order to determine the effect of arc length, electrode material, and current magnitude on the rate of recovery of dielectric strength of the arc space following a current zero. The experimental results show (1) that the arc space recovers the ability to withstand a hundred volts or more within a few microseconds after a current zero, as predicted by Slepian's theory, (2) that the maximum allowable rate of rise of voltage across the arc space which will just permit arc extinction is much greater for very short arcs (1 cm. or less) than for longer arcs, (3) that the critical rate of rise of voltage depends largely on the electrode material used and tends to vary as a decreasing linear function of the boiling point, and (4) that the higher current arcs have a much lower critical rate of rise of voltage than the lower current arcs.

Abridgment of extinction of a long A-C. arc

The extinction of an a-c. arc is analyzed as depending on two factors — the rate of recovery of dielectric strength of the arc space after current zero, and the rate at which voltage tending to re-ignite the arc is applied by the external circuit. In the short arc, most of the recovered dielectric strength resides in a deionized layer next to the cathode, but in the long arc, the rest of the arc space contributes largely to the dielectric strength. The breakdown gradient of the still ionized arc space is defined, and using a thermal ionization theory, a formula for growth of breakdown gradient is derived. The extinction of long a-c. arcs in the open is greatly influenced by the sectional area which the arc stream has at current zero. By confining arcs to slots and holes, the rate of deionization at current zero is greatly increased, and so large voltages per cm. of arc can be interrupted. A gas blast passing turbulently through an arc stream greatly accelerates deionization at current zero and so is effective in increasing the capacity of the a-c. arc to interrupt high-voltage circuits. The expulsion fuse is an example of a gas blast circuit interrupter, the gas blast resulting from the decomposition of the fiber fuse case. The oil circuit breaker is also a gas blast circuit interrupter, the blast arising from the gases produced by the decomposition of the oil. Means which increase the rate of oil decomposition improve the operation of the breaker. The magnetic blow-out in oil breakers is effective by causing an increased rate of oil decomposition. Electrostatic unbalance may lower the volts per cm. which a long arc can interrupt. The use of static balancing devices may then become advisable.

Extinction of a Long A-C. Arc

The extinction of an a-c. arc is analyzed as depending on two factors, the rate of recovery of dielectric strength of the arc space after current zero, and the rate at which voltage tending to reignite the arc is applied by the external circuit. In the short arc, most of the recovered dielectric strength resides in a deionized layer next to the cathode but in the long arc the rest of the arc space contributes largely to the dielectric strength. The breakdown gradient of the still ionized arc space is defined, and using a thermal ionization theory, a formula for growth of breakdown gradient is derived. The extinction of long a-c. arcs in the open is greatly influenced by the sectional area which the arc stream has at current zero. By confining arcs to slots and holes, the rate of deionization at current zero is greatly increased, and so large voltages per cm. of arc can be interrupted. A gas blast passing turbulently through an arc stream greatly accelerates deionization at current zero and so is effective in increasing the capacity of the a-c. arc to interrupt high-voltage circuits. The expulsion fuse is an example of a gas blast circuit interrupter, the gas blast resulting from the decomposition of the fiber fuse case. The oil circuit breaker is also a gas blast circuit interrupter, the blast arising from the gases produced by the decomposition of the oil.

Flames from Electric Arcs

The origin of flames from arcs is considered. Their low dielectric strength is attributed to ionization, and their rate of recovery of normal dielectric strength is computed. The large influence of temperature is brought out. Computations are given and experiments described which show how flames can be reduced by passing the arc gases through narrow channels.

Flames from electric arcs

The origin of flames from arcs is considered. Their low dielectric strength is attributed to ionization, and their rate of recovery of normal dielectric strength is computed. The large influence of temperature is brought out. Computations are given and experiments described which show how flames can be reduced by passing the arc gases through narrow channels.

Abridgment of extinction of an A-C. arc

The transition from high conductivity to high resistivity which an a-c. arc undergoes on extinction is studied. Theory and approximate calculations are given for the rate of recovery of dielectric strength of the arc space for short arcs, and results are given of experiments on short arcs, and arcs in holes and slots in insulating material and insulating plates. The influence of chemical activity in arc gases is discussed. Factors contributing to the success of the a-c. oil circuit breaker are suggested.

Extinction of an A-C. Arc

The transition from high conductivity to high resistivity which an a-c. arc undergoes on extinction is studied. Theory and approximate calculations are given for the rate of recovery of dielectric strength of the arc space for short arcs, and results are given of experiments on short arcs, and arcs in holes and slots in insulating material and insulating plates. The influence of chemical activity in arc gases is discussed. Factors contributing to the success of the a-c. oil circuit breaker are suggested.