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.
Kerr electro-optic field measurement and charge dynamics in transformer-oil/solid composite insulation systems
We measured the electric field in transformer oil/solid composite insulation system under dc voltage application using a Kerr electro-optic measurement system. When a pressboard was inserted at the center between two parallel-plane electrodes, the electric field at the mid-point between the pressboard and the cathode decreased with time. On the other hand, the electric field at the mid-point between the pressboard and the anode increased initially with time, reached a maximum value and then fell down. We measured also the electric field before and after polarity reversal of the dc voltage. The results revealed that the electric field after polarity reversal reached a magnitude 2.7/spl times/ as much as the average dc applied field. We interpreted the above results on the time transition of the electric field in terms of a charge dynamic model, considering the difference in charge accumulation speed on the pressboard between positive and negative charges. This model was verified to be applicable to the time transition of the electric field in oil/solid composite system and BTA added oil/solid composite system as well.
Developmental aphasia: The perception of brief vowels and extended stop consonants
Abstract Developmental dysphasics and matched controls were examined for their ability to discriminate (a) two synthesized vowel-vowel syllables and (b) two synthesized consonant- vowel syllables. For both vowels and consonants, dysphasics were impaired when the discriminable components of the two stimuli were brief (43 msec) but unimpaired when these components were 95 msec or longer. It is concluded that developmental dysphasics have no difficulty in discriminating transitional auditory information as such and that their impaired discrimination of synthesized stop consonants is attributable solely to the brief duration of the discriminable components.