DIELECTRIC RESPONSE OF PRESSBOARD IMPREGNATED IN MINERAL OIL AND SYNTHETIC ESTER F. Pratomosiwi 1* , N. Pattanadech 1 , A.A. Kemma 2,4 , G. Pukel 3 , R. Schwarz 3 , Suwarno 4 , M. Muhr 1 1 Institute of High Voltage Engineering and System Management, Graz University of Technology, Graz, Austria 2 PT.PLN (Persero), Indonesia 3 Siemens AG Österreich Transformers Weiz, Weiz, Austria 4 School of Electrical Engineering and Informatics, Bandung Institute of Technology, Indonesia *Email: <fpratomosiwi@student.tugraz.at> Abstract: Currently, the widely used fluid insulation for electrical insulation and heat transfer in transformers is mineral oil. However, some of other type of fluid insulation, such as ester fluids, attracts interest as a potential substitute for mineral oil. To employ synthetic ester in a power transformer requires a good understanding of the behaviour of dielectric properties of not only the fluid insulation but also the fluid impregnated pressboard (FIB). Therefore, it is important to have a proper insight on the dielectric properties behaviour of pressboard impregnated with synthetic ester. In this paper, we analysed the dielectric properties behaviour of fluid impregnated pressboard including the relative permittivity, dielectric dissipation factor and conductivity over frequency range of 1 mHz – 5 kHz. We measured dielectric properties of pressboard impregnated in mineral oil and pressboard impregnated in synthetic ester. The experiment was done at three temperatures, 25°C, 60°C and 90°C. From our measurement, it was found that 1. Cellulose based pressboard impregnated in different fluid has different dielectric characteristic depends on the type of dielectric fluid for impregnation, 2. The observed relaxation peak for FIB is shifted to higher frequency as the temperature is increased, 3. Tan delta value measured at several temperatures could be used to calculate the activation energy. The activation energy obtained could be used for developing master curve of dielectric properties which might be useful for design practice, 4.The shift in frequency of dielectric properties with temperature depends on the same activation energy value. Therefore, all dielectric properties have the same sensitivity of reaction rate to temperature, 5. FIB in synthetic ester has lower activation energy. Therefore, it is less influenced by the change of temperature compared to FIB in mineral oil. 1 INTRODUCTION In most power transformers, the insulation systems consist of oil-impregnated paper on the copper windings and several oil-impregnated pressboard barriers between the high- and low-voltage windings. The whole construction is then soaked in the transformer oil which acted as both insulation and coolant. Therefore, efficient utilization of fluid insulation and oil-impregnated pressboard in a transformer requires knowledge of the manner in which their dielectric properties behave as functions of temperature, voltage, and frequency. Consequently even though a given insulating fluid and pressboard may be used only at a power frequency of 50 or 60Hz, a proper insight into its electrical behaviour can only be obtained by observing its dielectric response as a function of frequency with the temperature as a parameter. For this reason, the dielectric behaviour of oil- impregnated pressboard over a wide range of frequencies will provide much of the required insight to comprehend more fully the intricacies of the dielectric behaviour [1]. Currently, the most widely used insulating fluid for electrical insulation and heat transfer in transformers is mineral oil. However, some of other type of insulating fluid, such as ester fluids, attracts interest as a potential substitute for mineral oil. It is mainly due to their biodegradability and high flash point properties. To employ synthetic ester in a power transformer requires a good understanding of the behaviour of dielectric properties of fluid insulation i.e. synthetic ester and pressboard impregnated in synthetic ester under different temperature as it affects the dielectric properties to different extent [2]. Hence, in this paper we analysed the dielectric properties including relative permittivity, tan delta and imaginary part of the complex relative permittivity (loss part) of pressboard impregnated in mineral and synthetic ester over a wide range of frequencies i.e. 1mHz – 5kHz and at three different temperatures i.e. 25°C, 60°C and 90°C.