**1. Introduction**

For modern power transmission and distribution grids, high voltage circuit breakers are among the essential elements to ensure safe power flow [1,2]. Basic technology applied therefore are self-blast circuit breakers in which a pressure build-up in a heating volume, necessary for arc quenching around current zero (CZ), is produced by the ablation of material from the nozzle wall due to intense arc radiation. Usually, gas sulfur hexafluoride (SF6) is applied as quenching and insulating gas due to its unique properties as being chemically inert, non-flammable, non-explosive, non-toxic, thermally stable, and an excellent electrical insulator and arc interrupter due to its high electronegativity (electron attachment) and density [3]. Metal-doped polytetrafluoroethylene (PTFE) is used as the nozzle material due to well-adjustable ablation, pressure built-up, and dielectric properties. A main trend of circuit breaker development is the substitution of the extremely potent greenhouse gas SF6 with a high global warming potential (of about 23,000 times that of CO2 over a 100 year period) by more environmentally-friendly gases [4]. A variety of alternative gases has been discussed and tested in the last decades, e.g., CO2, CF3I, C2F4, c-C4F8, C4F7N, and C5F10 as pure gases or in mixtures of two or three gases including components like N2, O2, and CO2. However, only a limited selection remains in the actual investigations. Gas mixtures containing fluoro–nitriles (C4F7N) or fluoro–ketones (C5F10O) as minority components (<20%) have been identified as the most promising alternatives to SF6 in high-voltage gas-insulated switchgear applications because of their low global warming potential together with their dielectric strength values being comparable to SF6 [5–7]. Due to high boiling points (at 1 bar) of the fluoro–ketones (27 ◦C) and fluoro–nitriles ( −4.7 ◦C), gas mixtures with carbon dioxide CO2 as buffer gas prevent liquefaction at temperatures below −30 ◦C. One of the most promising alternatives that has less limitations concerning temperature range and greenhouse effect but reasonable electrical insulation is CO2 [8–10]. Moreover, some of the experimental techniques and results obtained for CO2 will be relevant for other alternative gases that are usually applied with CO2 as buffer gas.

For an evaluation of the interruption performance it is mandatory to analyze and understand the time around CZ, i.e., the phase of current interruption and recovery of dielectric insulation in the electrode gap. Beside experimental investigations, computer simulations are of a high importance due to their cost-efficiency and fast adaptation to different geometries. However, such simulation tools need to be validated with experimental results and also to be provided with reliable input parameters based on experimental data. From the physical point of view, numerous transient effects can be observed such as:


Hence, the experimental investigation of the CZ phase is of high relevance for the development of high voltage switchgear. One of the main goals is the determination of physical properties as the composition, pressure, and temperature of the plasma. Ideally, this should be done with both spatial and temporal resolution as close as possible to CZ. Such investigations are usually based on optical methods and demand view ports or other access to the arc plasma that is often not available for commercial circuit breakers. A balance must be found between a conservation of geometry, functionality, and plasma conditions on the one side and a modification of the setup on the other side allowing, e.g., optical access as well as fast exchange of fill gas and components as electrodes, nozzles, windows, and ignition wires. In general, model circuit breakers are often equipped with fixed electrodes and ignited by an explosion of thin wires [11]. Experiments with moving electrodes of a pin and tulip shape may provide higher similarity with commercial circuit breaker geometries including the geometry of nozzles, heating chambers, and gas flow. However, reproducibility is often critical for experiments with moving electrodes. Furthermore, often the optical access is either fairly sophisticated and limited in time (using slits and windows [12]) or limited in spatial resolution, e.g., using optical fibers. If side-on diagnostics of the arc through quartz windows placed outside the nozzle is not possible due to non-transparent vapor, an alternative end-on arc observation can be realized through a ring electrode [13]. Alternatively, the PTFE as the standard nozzle material in commercial circuit breakers can be replaced by polymers that are transparent, e.g., PMMA [14,15].

In this work, two setups are used for investigation with optical emission spectroscopy (OES). In both setups, two pin electrodes are placed in a fixed distance and surrounded by PTFE nozzles. Ignition wires are used to initiate the arcs. The main difference between the setups is the nozzle geometry:

• The first, more simple setup is applied to generate extra-high pressure built-up and strong influence of the wall ablation. Therefore, the nozzle is made of a single, long PTFE tube. The influence of ignition wire and surrounding gas (ambient air) during the early stage of the arc discharge have been investigated in [16]. In the present work, the focus is set on the gas flow reversal as well as the detection limits for the determination of plasma temperature profiles around CZ. For these issues, no surrounding chamber is needed.

### *Energies* **2020**, *13*, 4714

• The second setup is a model self-blast circuit breaker in a CO2 atmosphere with optical access via the windows. It consists of two nozzles surrounding the electrodes and forming a heating channel which leads into the heating volume. Earlier experiments were carried out with the optical observation at the position of the heating channel and in the high-current phase [17]. Hence, the plasma emission from the central parts of the arc is influenced by the axial flow of hot gas in the heating channel, i.e., along the line of sight. The gas flow into the heating volume partly forces the plasma into the heating channel and the emission region exceeds the nozzle diameter, showing turbulence and deviation from the expected bi-convex structure that is needed for plasma temperature determination. To overcome these problems, for the experiments reported here the observation position was shifted away from the heating channel and towards the electrodes. As described below, this was realized by insertion of quartz windows into the nozzles.

For the investigation of the arc plasma, OES of atomic and ionic lines is a standard method for experimental determination of temporally- and spatially-resolved profiles of composition, temperature, and partial pressures that are needed for the calculation of thermal and electrical conductivity. However, the applicability of optical emission spectroscopy is limited for low emission levels, e.g., at low currents around CZ or at positions near the nozzle wall. Usually, plasma temperatures below 8000 K cannot be measured due to low line intensity and limited sensitivity of the detector. Thus, spatial temperature profiles cannot be obtained in the close vicinity of the wall region that is of high interest due to the importance of the nozzle ablation [18,19].

An alternative to OES at lower temperatures could be optical absorption spectroscopy (OAS) of resonant lines since the majority of atoms is in the ground state and thus, detectable by absorption. However, corresponding investigations under switching-relevant conditions are rather sophisticated; most resonant lines of carbon, fluorine, and copper need an optical access in the ultraviolet (UV) wavelength region whereas others like O I at 630.03 nm are very weak. Alternatively to atomic and ionic lines, molecules might be investigated. Most molecules dissociate above critical temperatures below 10 000 K. Thus, such analysis has the potential to provide insight into the region close to the wall at lower temperature. Experiments on OAS and investigation of molecule emission and absorption are reported in an accompanying article [20].

In the present paper, several questions should be answered: What can be learned by application of different experimental techniques for instigation of ablation-dominated arcs, partly combined? How far to current zero plasma temperature profiles can be measured with OES? What happens in the low-current phase approaching CZ with these profiles when lower conductivities are needed—mainly a temperature decrease or constriction of the arc diameter?

### **2. Materials and Methods**

### *2.1. Geometry of Electrodes and Nozzles*

As described above, two setups of electrodes and nozzles are used. They are sketched in Figure 1. In both cases, two-pin tungsten-copper (W-Cu) electrodes of 10 mm diameter were placed horizontally with a fixed contact distance of 40 mm. The electrodes were surrounded by either one 126 mm long, tubular-shaped nozzle of 50 mm outer diameter (a), or two separate nozzles of about a 50 mm length and 104 mm outer diameter (b). The general structure of the tubular and separated nozzles were similar. They were made of PTFE doped with <0.5 wt% molybdenum disulfide (MoS2) as usually used in high-voltage circuit breakers (CB). In both cases, the inner diameter was about 12 mm in the central part where the arc discharge was burned and about 16 mm at the electrode side ends of the nozzle to allow an exhaust gas flow along the electrodes. In the vicinity of the electrode tips, a smooth transition was realized from the smaller to the larger diameter.

**Figure 1.** Setups (**a**) with a closed, long PTFE nozzle for experiments with strong ablation and high pressure built-up and vertical observation slits in the middle and (**b**) with two separated PTFE nozzles forming a heating channel for plasma flow into a heating volume as used for the model circuit breaker.

For the camera observation and spectroscopy, optical access was realized by vertical slits of a 2 mm width. Pairwise placement at opposite positions allowed not only an observation of the emitted radiation but also background illumination and absorption measurement. The slits were mortised directly into the nozzles, ranging over the complete nozzle diameter. Since such slits would be potential exhaust pipes of the plasma causing additional disturbances, 2 mm–thick quartz plates were applied to seal the nozzle. After each shot the sealing plates were checked visually and the transmission was measured regularly. Because of the possible fume deposition on the plates, they were exchanged after each shot but could be recycled after cleaning. With regular exchange, no melting of quartz glass was observed. Generally, it was found that the sealing was very effective and the transmission reduction was controllable. However, few cases were observed with severe, local blackening of the quartz plates indicating problems with the sealing and these shots were repeated. Breaking of the plates occurred only for ones after extremely high currents.

For setup (a) the observation slits were placed in the middle between both electrodes, i.e., about 20 mm away from both electrodes. In setup (b), the distance between the two nozzles was about 4 mm forming a heating channel. The line of sight used in [17] was along the heating channel, i.e., in a central position between the nozzles where turbulent gas flow in a radial direction cannot be avoided and may disturb the observation of radial profiles. Having the opportunity to use quartz-sealed vertical slits, the observation point was positioned in one of the nozzles (the nozzle on the left side) in Figure 1. It was at half distance to the electrode tip and the nozzle exhaust in the heating channel, i.e., about 9 mm away from both.

The arcs were operated under ambient conditions in case of setup (a), i.e., without external chambers. The apparatus shown in the left part of Figure 2 basically comprises of electrically-isolated holders for nozzle and electrodes including ceramic shielding protecting against the hot exhaust plasma. Setup (b) was part of a model circuit breaker that is shown in Figure 2. It was placed in an outer vessel of about 300 L volume that was evacuated and filled with CO2 before each shot. Additional windows in the model chamber and the vessel allowed a free view through the nozzle. Thus, in this case an observation was also possible from opposite directions as well as absorption measurements.

**Figure 2. Left**: Setup (a) was used under ambient conditions (no external chamber). **Right**: Setup (b) was part of a model circuit breaker placed inside a pressure vessel with vessel windows and the quartz plates in the observation slits. The dotted line indicates the sight through the outer vessel.

LC circuits were used to generate sine-like current waveforms of about a 100 Hz frequency and 11 kA peak current for setup (a) and 50 Hz and 5.3 kA for setup (b). The arc discharges were initiated using exploding thin Cu wires. Pressure sensors (603A from Kistler) were positioned in case of setup (a) in the middle of the nozzle, i.e., in 90◦ to the observation slits. In case of setup (b) the pressure sensor was placed in the heating volume of the model circuit breaker.
