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The dielectric strength of dissociated binary and ternary gas mixtures containing helium, hydrogen, and nitrogen for cryogenic power applications is reported. The compositions of the dissociated gas species in the temperature range of 77–5000 K at 1.0–2.0 MPa are obtained by minimizing the Gibbs free energy assuming local chemical equilibrium. The resulting mole fractions of the dissociated gas species that vary as a function of temperature and pressure are used for calculating the density-reduced critical electric field representing the dielectric strength. The results suggest that the He-H2-N2 mixture has higher dielectric strength than the He-H2 and He-N2 mixtures, but NH3 would potentially accumulate over multiple arcing and cooling cycles and potentially cause long-term issues in cryogenic switchgear applications. On the other hand, the binary alternatives, the He-H2 and He-N2 mixtures, show lower dielectric strength than the ternary gas mixture but will maintain their original gas properties even over multiple arcing and cooling cycles. The results also show that the dielectric strength of the He-H2-N2 and He-H2 mixtures decreases substantially with increasing temperature whereas that of the He-N2 mixture stays nearly unchanged. The results of this study are useful for the fundamental understanding of gas dielectrics under arcing conditions in cryogenic switchgear applications and the development of resilient cryogenic power systems.
The dielectric strength of dissociated binary and ternary gas mixtures containing helium, hydrogen, and nitrogen for cryogenic power applications is reported. The compositions of the dissociated gas species in the temperature range of 77–5000 K at 1.0–2.0 MPa are obtained by minimizing the Gibbs free energy assuming local chemical equilibrium. The resulting mole fractions of the dissociated gas species that vary as a function of temperature and pressure are used for calculating the density-reduced critical electric field representing the dielectric strength. The results suggest that the He-H2-N2 mixture has higher dielectric strength than the He-H2 and He-N2 mixtures, but NH3 would potentially accumulate over multiple arcing and cooling cycles and potentially cause long-term issues in cryogenic switchgear applications. On the other hand, the binary alternatives, the He-H2 and He-N2 mixtures, show lower dielectric strength than the ternary gas mixture but will maintain their original gas properties even over multiple arcing and cooling cycles. The results also show that the dielectric strength of the He-H2-N2 and He-H2 mixtures decreases substantially with increasing temperature whereas that of the He-N2 mixture stays nearly unchanged. The results of this study are useful for the fundamental understanding of gas dielectrics under arcing conditions in cryogenic switchgear applications and the development of resilient cryogenic power systems.
The dielectric breakdown strength of supercritical He and supercritical Xe shows a steep decline near the critical point due to density fluctuation caused by cluster formation. Conventional gas discharge theories are limited in explaining the drastic dielectric strength variation of He and Xe near the critical point. In this study, a dielectric strength modeling approach that is based on the derived cross section data of clusters is utilized to estimate the dielectric strength decline of He and Xe near the critical point. The electron scattering cross section data of He and Xe clusters are derived from those of gaseous He and Xe. Based on the derived electron scattering cross section data, critical electric fields of various He and Xe clusters are modeled as a function of pressure by solving the Boltzmann equation. The proposed modeling approach shows close agreement with the experimentally measured breakdown electrical fields reported in the literature.
Amplified piezoelectric actuators have gained considerable attention due to their inherent advantages, including rapid response, reliability, and efficiency, making them promising candidates for Direct Current (DC) switching applications. They can operate in two distinct operational modes: Block–Free (B–F) and Free–Free (F–F) configurations. These two modes offer diverse mechanical constraints and are chosen based on the application’s specific requirements. This study aims to present a comparative assessment between the two modes to evaluate each configuration’s applicability in DC fast switching. Accordingly, the principle behind each actuation scheme was illustrated, and both designs were modeled and analyzed by the finite element method. Subsequently, two prototypes were assembled, each resembling a different operational mode. The established prototypes were then subjected to actuation and interruption tests to investigate their travel and switching performances. Comparative results revealed that while block–free could deliver a higher apparent stroke, the accumulated gap for each configuration is almost the same. Both actuators demonstrated high capability when utilized as actuation units for fast vacuum mechanical switches integrated into a hybrid circuit breaker for DC interruption. However, the free–free operation excelled in terms of fast response, as it managed to clear the mimicked fault current faster than the block–free configuration.
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