High-Current Anode Phenomena in Vacuum Arcs

The emerging interest on the application of vacuum interrupter (VI) in medium and even high-voltage application has led to increase of comprehensive and fundamental research on VIs. Application of these interrupters is however hindered by limitation of interrupting capability, when high-current anode phenomena occur.

The goal of the work is to investigate the high-current anode phenomena using spectroscopic methods se well as electrical measurements. Different current waveforms, i.e. AC and DC pulses are applied. Moreover, the impact of vacuum interrupter properties, e.g. electrode diameter and geometries, contact materials, opening time and opening speed on the formation of high-current anode phenomena are investigated.

The results reveal that the threshold current of the high-current anode formation is dominated by the current in case of high-frequency pulsed, whereas in case of pulsed DC, it is controlled by transferred charge.

Along with typical and already known high-current modes, i.e. footpoint, anode spot, and intense modes, a new type of high current anode mode, anode spot type 2, is introduced. Anode spot type 2 is similar to the intense mode but it appears at larger gap length. In contrast to anode spot (anode spot type 1), both anode and cathode are active in case of anode spot type 2. The line emission intensities of all species increase near the anode and the cathode in case of anode spot type 2. The anode spot type 2 is examined by determining radiating density, ground state density and electron density which is about two times higher compared to anode spot type 1.

Optical emission spectroscopy combined with high-speed camera imaging (video spectroscopy) is used to investigate the temporal and spatial distribution of Cu I, Cu II, and Cu III during discharge modes.

The results show different patterns during discharge modes. The formation of anode plume, is investigated using video spectroscopy. Existence diagrams confirm that the anode plume appears always after the extinction of anode spot type 2 in case of CuCr electrodes before current zero. The results unfold that the emission from the inner part of anode plume is covered by atomic lines, whereas the outer part is dominated by ionic line radiation. Anode plume formation which is a result of shock effect in front of the anode shows that anode plume contracts first and then it expands. Radiating densities of atomic and ionic Cu lines, and temporal evaluation of temperatures inside the anode plume are determined during the formation of anode plume.

The ground state density of Cr I before and after current zero is calculated for two cases; with and without anode plume using broadband absorption spectroscopy. The results point out that the ground state density after current zero is higher in case of anode plume. Moreover, during active phase the ground state density is higher in case of anode spot type 2 compared to anode spot type 1. Furthermore, the temporal evolution of ground state density of Cr I is calculated during high-current anode phenomena.

A new electric arc model based on the existence diagram is proposed in this work. In contrast to conventional electric arc model, e.g. Mayr, Cassie, Habedank, Schavemaker, etc., this model can track or detect the arc voltage of vacuum arc even during anode spot type 2 and abrupt change in the arc voltage. Moreover, this model based on the existence diagram has a predictability characteristic. By providing the existence diagram to the model, the arc voltage can be traced in quite different interrupting current ranges.