72 / 2024-04-10 11:44:30

Numerical Investigation on Influence of Oxygen Ratio in CO2/O2/C4F7N on Thermal Interruption Performance of Arc Extinction

CO2/O2/C4F7N gas mixture, SF6 alternative gas, oxygen ratios, arc decay process, thermal interruption, transient recovery voltage

Gas circuit breakers (GCB) play a crucial role in protecting high-voltage electric power transmission systems by interrupting large current resulting from short circuit accidents. When the electrodes in the GCB open to interrupt such current, it generates an arc plasma characterized by high temperature and high electrical conductivity between the electrodes. To successfully interrupt large current, a high-pressure arc quenching gas medium is forcefully directed towards the arc discharge. In the conventional GCB, SF6 gas has been widely used. However, SF₆ gas is regulated for a very high global warming potential (GWP) of 25200. Recently, CO₂/O₂/C₄F₇N gas mixtures have been proposed as alternative gases for SF₆ gas. This study specifically focuses on the ratio of O₂ content to CO₂ in CO₂/O₂/C₄F₇N gas mixtures, as it significantly influences the temperature distribution of arc plasma in a decaying state, concerning the thermal interruption ability. Oxygen gas addition to CO₂/C₄F₇N is expected to prevent from solid carbon appearance and to improve thermal interruption ability.

We conducted numerical thermofluid simulation on arcs in gas flows. Calculation space was taken for our arc experimental device with with electrodes and a nozzle. Between the electrodes the arc plasma is formed. To this arc, the alternative gas mixture was blown at 100 L/min. First, the arc current was set for DC 50 A. This current value was then changed stepwise from 50 A to 0 A at timing t = 0 µs to obtain decaying arcs in free recovery state. The free recovery state is the most basic state of arc decay, where the current and voltage applied between the electrodes are 0 A and 0 V. In the calculation, quasi-transient recovery voltage (TRV) was applied between the electrodes to investigate a re-ignition characteristics. Following a given delay time t_d, the quasi-TRV was linearly applied between the electrodes with a RRRV=1.7 kV/µs. The gas species treated here were 100%CO₂, CO₂/O₂/C₄F₇N gas mixtures and 100%C₄F₇N. The CO₂/O₂/C₄F₇N mixture was characterized by varying the ratio of O₂ to CO₂ between 0.1 (9% of total) and 0.9 (81% of total) while C₄F₇N content was fixed at 10%. The minimum t_d for successful thermal interruption was found for each of gas species condition. The successful thermal interruption was judged for post arc current was converged to 0 A even after quasi-TRV application. Otherwise, the thermal interruption failure occurs with arc reignition flowing a large current.

As a result, at a delay time t_d=60 µs for quasi-TRV application, lower oxygen content ratios from 10% to 30% end up with failure interruption, while higher oxygen content ratios from 40% to 90% lead to successful interruption. This means that the shorter t_d indicates higher thermal interruption ability. Increasing O₂ content in CO₂/O₂/C₄F₇N enhances the cooling effect on the arc plasma, decreasing the temperature faster during arc decay process. This is attributed to an increase in reactive thermal conductivity and specific heat of the gas mixture by O₂ dissociation around 3100 K.