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.
Resistivity of transformer oil at low and medium field strengths
In certain terminal equipment used in high-voltage d.c. transmission schemes, transformer oil is subjected to a d.c. stress. The behaviour of the oil under this stress is important in the design of equipment. The volume resistivity ? of five samples of transformer oil, ranging from new oil to oil after 10 years' service in a transformer, was measured over a range of stress levels E and electrode spacings d. The resistivity increased with increasing field strength up to a critical field strength, above which it fell sharply. Below this critical value, the resistivity was found to be a function of the ratio E/d. The curve of ? against E/d consisted of two parts: (a) for low values of E/d, ? was constant, (b) for higher values, the resistivity followed the equation ? = ? ? = ? (E/d)n where ? and n are constants. Some potential-distribution measurements were also made, and the stress in the oil was found to be uniform and approximately equal to the applied stress over the bulk of the oil in the interelectrode space. The results of the resistivity measurements are interpreted in terms of electrolytic conduction due to dissociation of weak electrolytes in the oil.
기구학적 운동이 돼지 무릎 관절연골의 마찰계수 변화에 미치는 영향
In this study, the frictional behaviors of articular cartilage against a Co-Cr alloy in two types of kinematic motions were compared. Cartilage pins were punched from the femoral condyles of porcine knee joints, and Co-Cr alloy disks were machined from orthopedic-grade rods and polished to a surface roughness (Ra) of 0.002. Friction tests were conducted by using a pin-on-disk?type tribotester in phosphate buffered saline (PBS) under pressures of 0.5, 1, and 2 MPa. All tests were performed in the repeat pass rotational (ROT) and the linear reciprocal (RCP) sliding motions with the same sliding distance and speed of 50 ㎜/s. The coefficients of friction of the cartilage against the Co-Cr alloy increased with the sliding time in both kinematic motions for all contact pressures. The maximum coefficients of friction in RCP motion were 1.08, 2.82, and 1.96 times those in ROT motion for contact pressures of 0.5, 1, and 2 MPa, respectively. As the contact pressure increased, the coefficients of friction gradually increased in RCP motion, whereas they decrease and then increased in ROT motion. The interaction between the directional change of the shear stress and the orientation of collagen fiber in the superficial layer of the cartilage could affect the change in the frictional behaviors of the cartilage. A large difference in the coefficients of friction between the two kinematic motions could be interpreted as differences in the directional change of shear stress at the contact surface.