**1. Introduction**

Gaseous insulation is used in many high voltage applications as, for example, gas-insulated switchgear (GIS) and circuit breakers (CB) [1–6]. For compact insulation compressed gas in the pressure range of 0.13–1 MPa is used. In most applications today, SF6 is used; however, it is a strong greenhouse gas (e.g., [7,8]). Therefore, the search for alternative insulation gases has significantly increased during the last years. The most promising gas in HV switchgear applications for replacing SF6 is CO2, which is used in mixtures with low-additive concentrations of, e.g., O2, Perfluoroketones (PFK) or Perfluoronitriles (PFN) [8–12]. The additive concentrations are typically in the range of a few percent due to boiling point requirements. This will be referred to as CO2-based insulation.

Gaseous insulation strength depends on the critical field of the gas [2,5,6], the field at which ionization leads to multiplication of electrons within an avalanche. This is determined by the zero crossing of the effective ionization coefficient, which is specific for a gas. The slope of the effective ionization coefficient determines the sensitivity of the insulation system to the surface imperfections [13–24]. Thus, depending on the properties of the gases, the insulation performance is determined not only by the critical field of the gas but also by imperfections like surface roughness or particulate contamination. This is especially more pronounced at higher pressures of 0.7 to 1 MPa, which is the preferred pressure range for CO2-based insulation [25–33]. Since CO2 is the base gas in these mixtures, a good understanding of the insulation characteristics of CO2 at conditions of practical applications is needed.

Several investigations addressed the effect of electrode surface roughness and representations of those by single or multiple protrusions in SF6—see [14,16–18,21]. Single-protrusion models have shown reasonable agreemen<sup>t</sup> with measurements when taking into account the availability of a start electron for an avalanche, streamer inception and leader propagation [34–38]. Surface roughness and the e ffect of insulating coatings in SF6 have been investigated experimentally by [23,24].

CO2-based insulation is still less investigated than SF6, and investigations with fluorinated additives to CO2 just recently appeared. The e ffect of surface roughness and protrusions in pure CO2 was investigated by [29–33,36,37], respectively. Breakdown at surfaces in CO2-PFK and CO2-PFN mixtures is reviewed in [8–10]. The e ffect of surface roughness at pressures above 1 MPa was investigated, e.g., by [27,36]. Breakdown at single protrusions in CO2 was investigated by [37], showing that streamer propagation, in contrast to SF6, also plays an important role for the breakdown in practical configurations, like in the presence of particulate contamination.

Several investigations addressed the e ffect of enlargement laws, i.e., the reduction of the breakdown field by the size of the electrode area or volume and time laws—see [5,6,13,39]. A theoretical investigation on approximation of surface roughness by an array of multiple protrusions was done by [40]. In this investigation, the surface roughness was approximated by multiple protrusions of given length and with hemispherical tip. The influence of the distance between protrusions was also addressed. Application of discharge inception and breakdown criteria based on start electron, streamer inception and leader inception were used to estimate breakdown fields for such arrays.

The previous investigations have shown that there is still only limited understanding how to describe the surface roughness induced breakdown fields by single and multiple protrusions, especially when considering the surface area and time scaling e ffects. Therefore, measurements are still needed to characterize breakdown fields in practical applications as, e.g., shown in [5,27,29,34]. It would be desirable to have models for surface roughness or protrusion/particle induced breakdown which can, for arbitrary gases, electrode size and number of protrusions, describe the breakdown fields su fficiently reliable. The present investigation addresses this in a first step by measuring breakdown fields at single and multiple protrusions for short duration voltage waveform, which is similar to Lightning Impulse (LI) in standard testing according to IEC [6]. This procedure is similar to the theoretical study of [40]. Investigation of di fferent gases in this study allows for understanding the di fferences between strongly attaching (SF6) and weakly attaching (CO2) gases. A novel test setup with a multiple protrusion array in a uniform background field is used and fill pressures of 0.4 MPa SF6 and 0.6 MPa CO2 are tested. The protrusion lengths are varied between 50 μm and 2 mm. The number of protrusions is varied between 1 and 100. Protrusions of small lengths of a few 10 to few 100 μm correspond to the surface roughness, whereas long protrusions of more than 500 μm correspond to particulate contamination or severe surface damages [35]. The e ffect of the number of protrusions, i.e., the statistical enlargement law, is addressed by scaling of the measured breakdown probability distributions using Weibull approximations—see [6,39,41,42]. This allows the validity of enlargement laws for such well-defined situations to be checked. Further insight is obtained by comparison to theoretical discharge inception and breakdown models based on start electron, streamer inception and streamer and leader propagation from [34–38]. Additionally, the surface area scaling is investigated experimentally on rough electrodes of similar surface structure. Results are interpreted with the findings from the protrusion arrays.

Section 2 shows the experimental setups and Section 3 presents the results. Discussion and conclusion are given in Section 4.
