Characteristics of interface charges on oil-pressboard insulation and their influence on flashover under polarity reversal voltage
Converter transformer is one of the important apparatus of high voltage direct current (HVDC) transmission system. Because of the complex structures and operating conditions, the winding of valve side of converter transformer will withstand DC voltage, combined AC-DC voltage and even polarity reversal voltage, and the insulating properties of which under polarity reversal voltage has drawn great attention. Under the special condition, the interface charge will be accumulated on the oil-pressboard interface, which can directly affect the electric field distribution of the converter transformer dielectric and even decrease the flashover voltage. Therefore, it is necessary to study the characteristics of interface charges and their influence on flashover voltage. The oil-pressboard insulation test platform is designed in this paper, including the interface charge of oil-pressboard insulation measurement platform and polarity reversal test platform. The minimum spatial resolution of interface charge measurement platform is about 1mm2, and its measuring sensitivity can reach 0.129pC/mm2. This paper adopts a needle-plate test model for measurement. The reversal time of the polarity reversal test apparatus is on the level of millisecond that conforms to the actual operation of the converter transformer. The experimental results under polarity reversal voltage indicated that the interfacial charge could greatly reduce the flashover voltage of polarity reversal, and the surface potential produced by charges can be up to 40% of polarization voltage. The changes of oil-pressboard interface charge density before and after the polarity reversal have been measured. It is found that the electric field produced by interface charge can strengthen the maximum synthesis field intensity of the needle electrode after polarity reversal, thus greatly decreases the flashover voltage.
Partial discharge characteristics of oil-immersed insulation systems under DC, combined AC-DC and DC reversed polarity voltage
Breakdown characteristics of insulating oil, under combined ac-dc voltage and dc reversed polarity voltage, and partial discharge characteristics of oil-immersed insulation, under combined ac-dc voltage and polarity revesal of dc voltage Were made clear by elementary experiments. After that, a numerical dc field mapping method for electric field analysis of two-dimensional configurations which consist of composite insulation and field mapping methods under combined ac-dc voltage and dc reversed polarity voltage were proposed. Futhermore, partial discharge characteristics of transformer insulation under dc, combined ac-dc voltage were experimentally made clear by practical trial models.
Resonant Auger decay study of C1s -> pi* core-excited OCS
The present work aims at characterizing short-lived C1s((-1)) pi*((1)) core-excited states of the OCS molecule based on the analysis of the vibrational fine structure and lineshape profiles of the high-resolution resonant Auger decay spectra recorded at the excitation energies along the C1s -> pi* resonance in the binding energy region 15-19eV. Very different behavior in terms of lineshape and resonant enhancement is observed for the (A) over tilde, (B) over tilde and (C) over tilde final states. This is explained by (1) the variation in the C-O bond lengths for the states involved in the electronic relaxation and (2) different contributions in terms of Mulliken population to the molecular orbitals determining the electronic character of the corresponding states. Since the final-state geometries are known from a number of previous experiments and ab initio calculations, the geometry of the C1s((-1)) pi*((1)) intermediate states can be predicted in analogy with e.g. the N-2, CO2 and N2O molecules. (C) 2009 Elsevier B.V. All rights reserved. (Less)