Search

Short circuit (SC) strength of a power transformer is influenced by its winding clamping pressure. The required level of clamping pressure is selected at the design stage based on the maximum allowable SC current level of the transformer. Progressive loss of clamping pressure is a common problem in power transformers. Shrinkage and fading of the stiffness of solid insulation due to thermal degradation are the most common causes of this problem. It has also been identified that changes in the moisture content and repetitive compression cycles on pressboard during high current faults tend to change the winding clamping pressure. This study aims to investigate the effects of moisture and compression cycles on winding clamping pressure through a set of laboratory experiments and finite element analyses. Thickness change and compressive stress-strain curves of pressboard were measured under different moisture conditions. The results were then used to calculate changes in winding geometry and clamping pressure of a 100 kVA three phase, disc type transformer winding. [ABSTRACT FROM PUBLISHER]

This study investigates the influence of moisture, temperature and diverse oil properties on the frequency domain response of pressboard insulation samples, which are impregnated with four different insulating liquids including two commercially available natural esters, one synthetic ester and mineral oil. Measurements are performed over a wide frequency range from 0.1 mHz to 1 kHz at variable moisture contents and temperatures (35, 55 and 75°C). It has been found that dielectric response is extremely sensitive to moisture content regardless of the type of impregnated liquid. Furthermore, dielectric response behaviour of ester impregnated pressboard insulation is qualitatively resembles to that of mineral oil impregnated pressboard which contains two distinct relaxation processes including Quasi-DC (q-dc) conduction features at low frequencies, and loss peak behaviour at mid frequency range. This paper proposes that the loss peak arises due to cooperative microscopic motion in water cellulose mixed phase dipole clusters together with the microscopic level interfacial polarization occurring at the interface of oil filled cavities. It is also discussed the q-dc process in pressboard insulation due to the charge transport between water and oil cavity based charge clusters through a percolation network of water molecules and hydroxyl groups along the cellulose fiber wall. In order to provide better understanding on charge transport and polarization phenomena in oil impregnated pressboard insulation, this paper also proposes an equivalent circuit, which is mainly derived based on Dissado- Hill (DH) cluster framework theory. Furthermore, this study has used activation energy to characterize the temperature dependent dielectric response of pressboard impregnated in mineral oil, natural esters and synthetic esters. [ABSTRACT FROM AUTHOR]

Cellulose based insulation materials and mineral oils have widely been used in liquid filled transformers. Environmental and fire-safety concerns have accelerated the use of Natural Esters (NE) as a sustainable alternative to mineral oil. There is a rapid growth of their application for distribution level transformers. To ensure a safe and long-term operation of the vegetable oil filled transformers, it is necessary to clarify the knowledge gaps with respect to the ageing of vegetable oil impregnated cellulose and insulation diagnostic methods. The condition monitoring of mineral oil filled transformers has been well characterised, due to the numerous laboratory and field studies conducted during the past several decades. The application of these available condition monitoring techniques for NE filled transformers is yet to be validated as the use of NE for power transformers is in the evaluation stage. In this paper the performance of NE as insulation in transformers by evaluating the ageing of NE impregnated cellulose pressboard is presented. For this purpose, an accelerated ageing experiment has been carried out in sealed tubes at 120 °C. The ageing of oil impregnated pressboard has been characterised by decrease in the degree of polymerisation (DP) of pressboard, and the formation of both 2-furfuraldehyde and dissolved carbon dioxide (CO2) in the oil. The structural changes of cellulose have been supported by Fourier Transform Infrared (FTIR) spectra. Moreover, the results have been compared with cellulose insulation samples aged under similar conditions in conventional mineral oil. Acidity value, dissipation factor and viscosity measurements have been used to compare the stability of NE in a transformer operating environment to mineral oil. [ABSTRACT FROM PUBLISHER]