Verification of a model-based diagnosis system for on-line detection of the moisture content of power transformer insulations using finite element calculations

It is a well known fact that the moisture content of a power transformer insulation system is a key parameter for the estimation of its aging condition and operation reliability. Therefore, detection of the moisture content is a very essential task within power transformer diagnostics. While conventional methods for estimation of the water content of the insulation system require a transformer shutdown of several hours, the proposed method calculates the profiles of water concentration in the cellulosic part of the insulation system, as well as the concentration of water in transformer oil from operational parameters. An one-dimensional model for the moisture diffusion in paper estimates the temperature dependent moisture distributions in the paper. It includes temperature dependent diffusion coefficients and acts as a non-linear control loop for the water concentration in transformer oil. Beside experimental work, two-dimensional finite element (FE) calculations of the temperature dependent water concentration distribution in the transformer insulation system have been performed for model verification. The calculations are based on an opensource Whitney-Element solver. The computed results show the validity of the diffusion model and give important guidelines for the required complexity of thermal models.

I. Introduction

Power transformers are one of the most expensive investments in an electric power system. The deregulation of the electricity market forces the power utilities to reduce maintenance costs. This leads to a lack of transformer specific knowledge inside a power utility and increases the risk of a transformer failure. Moreover, the changed market situation increases the demand for controlled transformer life-time management.