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The naturally cooled panel-type radiators are widely used for transformer heat dissipation. However, their poor heat transfer performance also restricts the overall performance of the transformer. The chimney effect can improve the airflow rate due to the thermal pressure difference between the air inside and outside the flow channel, which is widely used to improve the flow and heat transfer of natural convection. In order to improve the heat transfer performance of panel-type radiators under natural convection, a novel radiator is developed, where wind deflectors are added at both lateral sides of the radiator, and a chimney structure is added on the top of the radiator to extend the chimney channel. Then, the fluid flow and heat transfer characteristics inside and outside the radiator are studied by numerical simulation. The results show that cooling air would be effectively sucked into the radiator from the bottom due to the pressure difference generated at the inlet and top of the chimney channel. As compared with the traditional panel radiator without the chimney structure, the airflow on the outer surface of the most cost-effective radiator is increased by 19.14 %, the cooling capacity is increased by 14.76 %, and the oil temperature inside the transformer is reduced to 6.72 °C. [ABSTRACT FROM AUTHOR]

Due to the anti-electromagnetic characteristics, high sensitivity and convenient installation, the Fabry-Perot sensor is commonly used in partial discharge detection of transformers. The variations of eigenfrequency and sensitivity caused by membrane temperature would have an important impact on sensor performance. In the present study, the eigenfrequency and displacement response of an intact membrane (IM) and a beam-supported membrane (BSM) under thermal expansion were numerically studied using a thin film vibration model coupled with temperature variations. The influence of the width and length for BSM's support beam on the stress softening was also analysed. It is found that the thermal expansion of the membrane reduces the stiffness and both the IM and BSM decrease linearly as temperature increases. The IM and BSM will become unstable as temperature increases to a certain value. The thermal stability of the BSM with support beam structure is lower than that of the IM. When the temperature increases, the IM amplitude increases and the BSM amplitude decreases. The eigenfrequency of the BSM is proportional to the width and inversely proportional to the length of the support beam, while the thermal stability is inversely proportional to the width and length of the support beam. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) detection is a technique widely used for high voltage equipment insulation condition assessment. In such applications, an understanding of PD mechanisms, characteristics and development processes is important. In this paper, PD characteristics of transformer oils are examined under AC conditions, using a needle-to-plane electrode system. The PD activity in transformer oil is confirmed as appearing in pulse burst form. The duration time of PD pulse bursts was found to decrease with voltage above the PD inception voltage; this decrease was accompanied by both an increase in number of discrete PD pulses and their average maximum amplitude within the pulse burst itself. In order to obtain the increase principle of pulse amplitude within PD pulse burst, mathematical fitting models for discrete pulse current peak value within PD burst are established using exponential express and validity of mathematical models is proved by fitting error. [ABSTRACT FROM AUTHOR]

The choice of the materials used as the dielectric media in large transformers continues to be a combination of pressboard and mineral oil. However, a number of failures in the field have been attributed to the surface discharge or creeping discharge over the oil/pressboard interface. This paper reports on an experiment to study surface discharge over oil/pressboard. The TJ (triple-junction) model was used to study the characteristic of partial discharge (PD) from inception to flashover. The max PD charge, pattern, frequency and current pulse were measured during the flashover process. Four oil/pressboard specimens with different aging degrees were used and their influence on the flashover process over oil/paper interface was studied in this experiment. The results indicate that the max PD charge and PD frequency increase with the increase of applied voltage. The oil aging has great influence while the pressboard aging has little influence on PD characteristic over oil/pressboard. The PD pattern and current pulse shape show great difference for various applied voltages. The results therefore suggest that the PD detection on the oil/pressboard can be used effectively to implement the early monitor of surface discharge thereby to avoid the formation of surface flashover. [ABSTRACT FROM AUTHOR]