63 / 2024-04-08 15:47:18

Measurement of Dielectric Breakdown Properties in High Temperature CO2 Gas Heated by Tandem-Coil Inductively Coupled Thermal Plasmas

high temperature,CO2,gas insulation,thermal interruption,inductively thermal plasma

 Electrical breakdown in hot gases can occur under transient recovery voltage application after large current interruption in a gas circuit breaker. Therefore, it is important to understand the dielectric characteristics of heated insulation gases. One of the methods to verify the phenomena in gas circuit breakers during the current interruptions is to create a mockup model which indicates the use of an actual circuit breaker with observation windows and/or sensors to measure electrical signals. This can provide the practical data, but a lot of cost and time are required. Also, only statistical data can be obtained due to variations in arc discharges. As another efficient method, the dielectric properties of high temperature insulation gases have been investigated by generating high temperature gas with a carbon heater and laser induced plasma. However, these have issues, such as the temperature limit of about 1500 K at most, the impurity contamination from heater metal, the local and short lifetime of generated high temperature gas. To solve these problems, we are proposing a unique method for heating insulation gases by inductively coupled thermal plasmas (ICTP). This method enables us to increase the temperature of insulation gas above 3500 K and to maintain the high temperature gas field without any contaminations for a long time. To prevent from instability of the thermal plasma by injecting insulation gases, a tandem coil ICTP, consisting of the two coils arranged vertically in series, was adopted. The temperature and gas concentration in the downstream chamber can also be adjusted by controlling parameters of thermal plasma such as the coil current and injected gas flow rate. A pair of gap electrodes are equipped at lower part of downstream chamber to study dielectric strength of the heated gas. Since the tip of electrodes are replaceable, different electric field distribution can be realized. Moreover, the different voltage waveforms can be applied due to the independence of the voltage source.

 The objective of the present study is to experimentally verify the dependence of the dielectric property of hot CO₂ gas heated by ICTP on gas temperature. To generate and maintain ICTP, the Ar sheath gas was injected with 90 L/min and the input power from the upper coil was set at 15 kW. The CO₂ gas was used for test insulation gas, which is one candidate of alternative gases for SF₆. The CO₂ gas was injected at 10 L/min. The inside the plasma torch and downstream chamber were retained at a pressure of 0.04 MPa to maintain the thermal plasma. To vary the temperature of heated CO₂ gas, the input power of lower coil was set to 0, 5, 10 and 15 kW. The DC voltage of 1 kV/s was applied for the gas electrodes at a gap length of 1 mm to measure breakdown voltage of the hot CO₂ gas. Additionally, spectral observation was conducted to estimate the excitation temperature of Ar atom by Boltzmann plot method.

  As the result, the measured voltage was found to be lower than the prospective applied voltage of 1 kV/s. This is because the hot gas between the electrodes has enough high electrical conductivity, involving the current flow. As a gap voltage waveform measured, two types of voltage waveforms were obtained depending on the lower coil power of the ICTP. In case at the input power less than 5 kW, the gap voltage increased with time to about 600 V, then oscillated and decayed. On the other hand, the gap voltage waveform oscillated and gradually rose in case at the input power more than 5 kW. This difference in gap voltage waveform indicates that the discharge form in hot gas was different. To evaluate the dependence of breakdown voltage on the gas temperature, the excitation temperature of Ar atom was calculated based on observed Ar atom spectral lines by Boltzmann plot method. In the spectroscopic observations, Ar atom spectral lines and the spectra of dissociation products of CO₂ such as O₂⁺, C atom and O atom were detected. In addition to this, the temperature at each input power was estimated by numerical thermofluid simulation. Thus, the breakdown voltage for the gap was about 165.1 V under the gas temperature of approximately 6200 K at the input power of 15 kW, while it was about 707.2 V under the gas temperature of approximately 3800 K at the input power of 0 kW. The decrease in breakdown voltage with the increase in gas temperature can be explained by a decrease in mole fraction of CO₂ and O₂, being dominant in dissociative electron attachment cross section.

 As a conclusion, the present study can experimentally verify the decrease in dielectric properties of high temperature gas accompanying the increase in gas temperature.