This research focuses on empirical observations of horizontal series arc faults. These faults differ from ground faults, for series faults encompass the electromagnetic transient effects of arc formations in series with sustained current flow when there is a break in the circuit. This may happen intentionally (as in a breaker) or unintentionally (as in a loose, damaged, or severed cable). This paper studies some of those transient effects during arc ignition, propagation, and cessation. Emphasis is on dc systems, for series faults present some of the more challenging safety concerns relating to widespread dc micro-grid acceptance and proliferation. However, arc behavior is also compared to that of ac systems under “quasi-equivalent”, passive circuit parameters. Variables of study primarily include arc voltage, current, and their relationship to electrode spacing under dynamic conditions. Results indicate that interruptions in dc current, while appearing more chaotic from a localized standpoint, do not produce the fast-acting transients associated with ac disturbances. Additionally, if dc arcs propagate over a slowly increasing distance of separation, they can be modeled as quasi-static in nature. An equation model is developed and curve-fitting parameters match well with historically tabulated constants. / text

A vacuum interrupter is a switching device used in vacuum circuit breakers, which are widely employed in medium voltage circuits for interrupting the short-circuit fault currents. The vacuum interrupter is the chamber in which the arc extinction and hence the current interruption takes place. On the occurrence of a fault, the breaker mechanism separates the contacts of the vacuum interrupter. As the contacts separate, an arc is established between the contacts. The arc evolves in the contact space and extinguishes at or near the current zero, thus interrupting the current. The processes of arc ignition, evolution and extinction are very complex. These processes are fundamental to the design and the performance of the vacuum interrupter and hence the circuit breaker. The evolution of the arc predominantly depends on the short-circuit current, the design and metallurgy of the contacts. The evolution of the vacuum arc has been the focal point of considerable research activity. Significant effort has been concentrated to understand the various modes of the arc, the transition between the modes, the arc movement and the dependency on the contact design and finally the effect of the arc evolution on the current interruption performance of the vacuum interrupter. The voltage across the contacts during the arcing, termed as the arc voltage, has been a focal point of several research projects. Research has shown that the arc voltage depends strongly on the mode and the evolution process of the arc. The dependency is observed with respect to the magnitude and the nature of the arc voltage. This dependency has been established through the comparison of the arc voltage trace and the actual arc photographs. The arc voltage is thus an important parameter in understanding the arcing process in the interrupter. Arc voltage could also be utilised to compare the arcing behaviour in vacuum interrupters with different contact geometries and metallurgies. Having understood how the arc voltage depends on the arc modes and how it can be used to analyse the arcing performance of the interrupter, this work aims to establish experimentally the dependency of the arc voltage on fundamental parameters of the short-circuit current and the contact design. The variation of the arc voltage is studied with respect to the magnitude of the short-circuit current. It is seen that the magnitude of the arc voltage is higher, for a higher short-circuit current. This dependency is also reflected in the nature of the arc voltage waveform. The effect of cumulative short- circuit operations has been understood through the study of arc voltage variation with respect to the accumulated arcing time. It has been found that the arc voltage consistently decreases as the accumulated arcing time increases. The effect of the contact diameter on the arc evolution has been studied by comparing the arc voltage variations for contacts of different diameters for the same short-circuit current. It is observed that the variation of arc voltage with respect to the contact diameter depends on the type of contact. In the case of radial magnetic field contacts, it has been observed that the arc voltage is lower for a contact with lower diameter. Whereas in the case of axial magnetic field contacts there is an inverse relation between the contact diameter and the arc voltage. Finally, the effect of the type and distribution of the magnetic field on the arc voltage variation as well as the contact erosion has been studied. In general, the observations show that the arc voltage magnitude for the radial magnetic field geometry is higher than the axial magnetic field geometry. Also, there is a significant difference in the appearance of the arc voltage waveforms for the arcs under the two types of magnetic fields. Finite element simulations and short-circuit evaluations have shown that the axial magnetic field contact system with 90 deg coil orientations yield a more uniform distribution of the flux density and hence lower erosion of the contacts. These results show a clear dependence of the arc voltage on the various above mentioned parameters. Thus the arc voltage could be utilised as a diagnostic parameter during the evaluation of the vacuum interrupter. In the present scenario, significant research is being done to increase the breaking capacity of the interrupters. This calls for optimization of design of the existing contacts and the design of novel contact geometries. The arc voltage would be used as an important diagnostic tool in this process. Also, the utilization of vacuum interrupter in high voltage and extra high voltage circuits is being explored. This application requires increase in the contact gap or series connection of gaps. The arc behaviour in longer gaps and gaps connected in series would be an important research area. Again the arc voltage could be used to study the arc evolution in these specialised conditions. The experiments in this research work have been performed on commercial vacuum interrupters. For a dedicated research on vacuum arcs and vacuum interrupter contacts, development of a vacuum arc research facility has also been attempted as a part of this research work.

Switching of shunt reactors and capacitor banks is known to cause a very high rate of rise of transient recovery voltage across the circuit breaker contacts. With improvements in circuit breaker technology, modern SF6 puffer circuits have been designed with less interrupter per pole than previous generations of SF6 circuit breakers. This has caused modern circuit breakers to operate with higher voltage stress in the dielectric recovery region after current interruption. Catastrophic failures of modern SF6 circuit breakers have been reported during shunt reactor and capacitor bank de-energisation. In those cases, evidence of cumulative re-strikes has been found to be the main cause of interrupter failure. Monitoring of voltage waveforms during switching would provide information about the magnitude and frequency of small re-ignitions and re-strikes. However, measuring waveforms at a moderately high frequency require plant outages to connect equipment. In recent years, there have been increasing interests in using RF measurements in condition monitoring of switchgear. The RF measurement technique used for measuring circuit breaker inter-pole switching time during capacitor bank closing is of particular interest. In this thesis, research has been carried out to investigate switching transients produced during circuit breaker switching capacitor banks and shunt reactors using a non-intrusive measurement technique. The proposed technique measures the high frequency and low frequency voltage waveforms during switching operations without the need of an outage. The principles of this measurement technique are discussed and field measurements were carried out at shunt rector and capacitor bank installation in two 275 kV air insulated substations. Results of the measurements are presented and discussed in this thesis. The proposed technique shows that it is relatively easy to monitor circuit breaker switching transients and useful information on switching instances can be extracted from the measured waveforms. Further research works are discussed to realise the full potential of the measuring technique.