The effect of temperature and field strength on the resistivity of transformer oil
Safety operation of total ultra-HVDC transmission system could be directly affected by the operation reliability of converter transformer which is one of the most important equipments in the system. The transformer oil is the main liquid dielectric medium in the converter transformer. Thus, it is significant to study the dielectric properties of the transformer oil. In this paper, the dc resistivity of transformer oil under different conditions has been systematically investigated, and then the temperature and field strength were the test parameter variables. The variation range of temperature was 25~80°C, and under each temperature point the field strength changed ranging from 0.5V/mm to 10kV/mm. Meanwhile, 15 min is selected as the duration of testing resistivity. In this paper, the authors obtained the systematical data points and analysis curves of resistivity under different temperature and field strength, and obtained a empirical equation which describe the relationship between resistivity, temperature and field strength. It enhanced the understanding of the resistivity for transformer oil and provided the theoretical and test basis to expand the engineering application of this transformer oil.
Investigations of Temperature Effects on the Dielectric Response Measurements of Transformer Oil-Paper Insulation System
Dielectric testing techniques, in both time and frequency domains, are currently widely used by power utilities for assessment of the condition of transformer oil-paper insulation systems. However, it has been reported that results of these tests are highly influenced by the operating temperature during measurements. The distribution, migration and equilibrium of moisture between oil and paper in a complicated insulation system is highly temperature dependent. It requires adequate experience and proper understanding to interpret the dielectric response results in the presence of temperature variations and thermal instability. Proper analysis of the dielectric test result is only possible with an understanding of the physical behavior of the insulation system in response to temperature. A circuit model, which describes the dielectric behavior of the transformers main insulation system, has been investigated in this paper. The values of the parameters of the model have been identified from the dielectric tests. A correlation has been observed between the operating temperature and the equivalent model parameters that can be used as additional information for better interpretation of the dielectric test results. This paper thus reports a detailed study on the effects of temperature on dielectric measurements of a transformer under controlled laboratory conditions. Some results of practical on-site testing are also presented to demonstrate the possibility of errors that may be introduced in dielectric test results analysis unless temperature effects are taken into consideration.
DESIGN CONSIDERATION TO PPM KLYSTRONS FOR INDUSTRIAL LINAC
Toshiba has been developing the C-band and X-band Periodic Permanent Magnet (PPM) focused klystron in collaboration with KEK and manufactured prototypes for the JLC project. [1] The PPM klystron has an advantage to enhance the total efficiency as a result of eliminating the electric power for the focusing magnet. From experience on the development the PPM focused klystrons for the JLC project, we plan to apply the PPM focused klystron to the medical and industrial linac. The periodic magnet focusing scheme has some technical difficulties such as stop band voltage. To design the PPM klystron for the medical and industrial linac, it is necessary to give heed to electron beam instability; so that the tube parameters of the PPM focused klystrons have some constraints. The size of the accelerator system is substantially reduced if a PPM focused klystron is used. The majority of medical and industrial linacs use frequencies of S-band range. We discuss the availability of the S-band PPM focused klystron in this paper.