Numerical investigation of positive and negative streamers in CO2/O2 mixtures

SF6 is commonly used as gaseous insulation media in high voltage equipment, but it has high global warming potential and urgently needs to be replaced with eco-friendly alternatives. CO2-based mixtures have shown promising results as SF6 alternatives. Streamers that form the initial phase of electrical breakdown have become important for the reliable design of high-voltage equipment based on gaseous insulation. In this paper, the influence of concentration ratio (90% CO2/10% O2, 80% CO2/20% O2, and 70% CO2/30% O2), applied voltage (±8 kV, ±10 kV, and ±12 kV), pressure (1.5, 2.0, and 2.5 bars), and gap distance (4, 4.5, and 5 mm) on positive and negative streamer formation and propagation is investigated in detail using a 2D axis-symmetric simulation model. Considering the low probability in gas mixtures with higher concentrations of CO2, photoionization is excluded, and background ionization is used for generating free electrons along with the Townsend ionization equation and the Gaussian approximation for the initial electron density distribution. The simulation results show that by increasing the O2 concentration in CO2, the electron density, electric field, and streamer velocity increase under positive and negative polarities. The σ (collision cross section) value of O2 is greater than CO2 at the specific ionization energy, indicating that O2 molecules in a gas mixture have a higher reactivity and a lower molecular stability. The negative streamer has an overall high electron density as compared to the positive streamer. To understand the effect of applied voltage, pressure, and gap distance, only 80% CO2/20% O2 concentration ratio was chosen. By increasing the applied voltage, the electron density, electric field, and streamer velocity increase. Furthermore, a decreasing trend of electron density, electric field, and streamer velocity was observed by increasing the gas pressure and the electrode distance.