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Improving the online water content measurement of transformer insulation paper using low pass filters

Water inside a transformer tank is undesirable because this water causes the following problems. Firstly, on overload bubbles of water may form in the oil weakening its dielectric strength and increasing the risk of catastrophic flashovers across the oil. Secondly, it shortens the life expectancy of the Kraft paper insulation, which is one of the main factors limiting the functional life of transformers. It is therefore necessary to measure the water content of paper insulation so that a utility can take corrective action when required. Cellulose adsorption isotherms have been used to determine the water content of paper using online water activity probes. A disadvantage however is that the application of this method requires that the insulation media is in thermodynamic equilibrium, which due to load cycling usually is not the case. Practically, this problem is resolved by taking a long-term average of the measurements. This solution is not optimal because actual changes in water content will also be smoothed out. The migration of water between paper and oil is a very slow process. Therefore, a low-pass mathematical filter was developed which would retain this process but reject higher frequency components. This filter was also investigated using measurement from operating power transformers.