Recovery of dielectric strength and post-arc currents after diffuse and constricted vacuum arcs were measured for filat OFHC-Cu contacts (D = 25 mm, d = 7.5 mm) enclosed in a bakable UHV chamber. The arc current pulse had a trapezoidal shape of 5.5-ms duration with peak values up to 11 kA. In comparison with the fast recovery of diffuse arcs, the recovery of constricted arcs with gross melting is considerably retarded. Post-arc currents are simulated using the Andrews-Varey model extended to include the effects of secondary electron emission due to ion bombardment of the cathode and loss of the plasma due to thermal motion. The flow of charge carriers to the anode and the shield, which is at the anode's potential, are registered separately. The amount and decay of the residual plasma is evaluated from the measurements of post-arc current. The decay times of a few tens of a microsecond give evidence of ions with energies below 1 eV. The origin and effect of slow ions on recovery is discussed.

Dielectric Recovery of Vacuum Arcs after Strong Anode Spot Activity

With rectangular power current waves recovery times of a vacuum interrupter with copper-based button contacts were measured. They were found to increase from 7 , ?at 250 A to 630 ?s at 12 kAp, where a stationary anode spot had formed. With a stationary anode spot present recovery time was also approximately proportional to current pulse duration. An attempt was made to explain the long recovery times by evaporation from the highly heated anode after the current had ceased. In addition, some high-speed photographic observations were made on the formation of stationary anode spots.

Recovery Times of Vaccum Interrupters Which have Stationary Anode Spots

The physical processes occurring during recovery of a vacuum interrupter after high-current interruption are surveyed. New results have been obtained by the application of modern diagnostic techniques like laser shadow imaging, laser-induced fluorescence of atoms and of metal vapor ions, Mie scattering of droplets, and high-resolution recording of voltage and current on digital oscilloscopes. The density of metal vapor produced by the arc determines the kind of breakdown processes occurring after current zero. Liquid droplets and residual plasma play a minor role. The commonly used contact material, i.e., copper-chromium, is compared with pure copper, and its advantage is explained by the surface structure of the melt. >

Investigation of high-current interruption of vacuum circuit breakers

A statistical model of a medium voltage vacuum circuit breaker was developed in EMTP by using new powerful TACS MODELS. The model includes the arc voltage characteristic, calculation of mean chopping current value with known di/dt, the dielectric breakdown voltage characteristic, contacts separation dynamics, probability of high frequency arc quenching capability and probability of high frequency (HF) zero current passings. The influence of a switching-off time is studied and a statistical estimation is given of the transient overvoltages expected for a specific network configuration. The relation between the switching-off times and the overvoltages is also presented. In addition to these results the paper includes explanation of the modeling technique in the EMTP for all elements constituting the distribution network and validation of the vacuum breaker model behavior. This approach provides valuable information to engineers constructing metal-enclosed switchgear, electric utilities and other users who are going to install vacuum breakers in medium voltage networks. >

A statistical vacuum circuit breaker model for simulation of transient overvoltages

Hybrid direct current circuit breakers (DCCBs) have great prospects for the isolation of dc faults in multi-terminal dc grids Current commutation from the mechanical switch branch to the static DCCB branch is the precondition to interrupt fault currents successfully for the hybrid DCCBs A current commutation drive circuit (CCDC) is presented in this study, and it has features of low cost, low operating losses and free maintenance Experiments have been carried out on a unidirectional 44 kV hybrid DCCB prototype with CCDC The results show the current of 34 kA can be commutated by CCDC within 130 μs, and be interrupted successfully within 2 ms For a bidirectional 80 kV hybrid DCCB cell, the effectiveness of CCDC is validated with simulations carried out in PACAD Then, an index is defined to represent the current commutation capability of CCDC and a mathematical model for CCDC is established to calculate the index To reduce the cost of CCDC with certain current commutation capability, an optimisation method based on genetic algorithm is proposed in this study According to the optimisation results of CCDC suitable for bidirectional 80 kV hybrid DCCB cell, the cost of CCDC has been reduced by optimisation

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-gtd.2015.0840

Research on a current commutation drive circuit for hybrid dc circuit breaker and its optimisation design

Typically a vacuum interruptor will be used in series with a powerline which is supplying power from an AC source. In the event of a fault on that system, relaying devices provide signals to the trip mechanism of the interruptor to cause the contacts to separate, an event that ultimately causes the circuit current to be interrupted. A fault on such a system can occur at any time. The parting of the interruptor contacts is, therefore, random with respect to any given point within the alternating current half-cycle. The actual interruption of circuit current, however, always occurs near a sinusoidal zero of current. As the arc current in a vacuum interrupting device approaches a sinusoidal zero the arc cathode spots, which provide the emission to sustain the arc, becomes unstable and abruptly extinguish. The current rapidly drops to zero. Although the emission processes have ceased to function at this time, there remains within the gap volume a residue of ions, electrons, vapors and particles which require a finite length of time to disperse. Until this residue is dissipated, the ability of the electrodes and gap to withstand high voltage is diminished. The effect is especially important in circuit interrupting devices where rapidly rising voltage transients may immediately appear across the gap after current interruption. If the gap withstands these transients and the ensuing open circuit voltage, the interruption function will have been complete. If, on the other hand, these voltages produce breakdown, a power arc may be restarted. The interrupting device will then have failed to fulfill its function. The study of the processes by which a gap recovers its electric strength after arcing, therefore, helps to answer important practical questions. This paper is devoted to discussions of the various processes that tend to limit the voltage withstanding capability of the vacuum gap during the decay of arc residue. Our comments will deal principally with recovery under somewhat special low current conditions. One might question the realism of applying low current studies to phenomena which are of particular interest at the high currents appropriate to power circuit interruption. At such high currents, however, there are numerous physical processes acting simultaneously. Therefore, failure to interrupt may occur for a variety of reasons and the true cause often goes undetected. Low current experiments, on the other hand, can be designed to investigate certain specific processes which are known to occur at higher levels without introducing a number of concurrent effects which can cloud the interpretation of the result. Such studies also help to reveal fundamental information about the discharge. The following discussions fall roughly into three parts. In the first, we consider a few selected recovery strength measurements to illustrate the parameters which are likely to be generally important. These data are then used as a basis for further discussion in the second and third parts where we explore in greater detail the influence of neutral vapor and of residual ionization during recovery. Our objective in these sections is to indicate the processes and material properties that are significant to the recovery of dielectric strength in vacuum.

G. A. Farrall

Papers

Recovery of Dielectric Strength after Current Interruption in Vacuum