Evaluation of the moisture content in power transformer

Abstract: An online method for the assessment of moisture in a power transformer is presented in this paper. It is experimentally tested on simple insulating materials as well as on a sample bushing insulation. It is finally tested during the drying of a 110 kV transformer. The results are presented and show that the method is clearly able to distinguish between levels of moisture in the range of interest.

Abstract: Cellulosic paper and oil insulation in a transformer degrade at higher operating temperatures. Degradation is accelerated in the presence of oxygen and moisture. Power transformers being expensive items need to be carefully monitored throughout their operation. Well established time-based maintenance and conservative replacement planning is not feasible in a current market driven electricity industry. Condition based maintenance and online monitoring are now gaining importance. Currently there are varieties of chemical and electrical diagnostic techniques available for insulation condition monitoring of power transformers. This paper presents a description of commonly used chemical diagnostics techniques along with their interpretation schemes. A number of new chemical techniques are also described in this paper. A number of electrical diagnostic techniques have gained exceptional importance to the utility professionals. Among these techniques polarisation/depolarisation current measurement, return voltage measurement and frequency domain dielectric spectroscopy at low frequencies are the most widely used. This paper describes analyses and interpretation of these techniques for transformer insulation condition assessment.

Abstract: Dielectric response is a non-invasive diagnostic method with a potential to give information about the status of the solid insulation in a power transformer. To investigate what conditions the method can and cannot reveal, frequency domain spectroscopy has been applied to oil-impregnated paper and pressboard with and without moisture and/or acids added, and to paper at various stages of aging. Water and carboxylic acids are important aging indicators. It is found that moisture is clearly detectable. Also low molecular weight carboxylic acids will affect the dielectric response in a way similar to moisture, but high concentrations are needed for a contribution comparable to that of moisture. It is difficult to tell moisture and acids apart from the responses obtained. High molecular weight carboxylic acids make little or no contribution to the dielectric response of the solid insulation in a paper/pressboard - oil system. It was also observed that keeping paper or pressboard at elevated temperatures had a conditioning effect on the response

Abstract: Eddy current effects are included in a model of a power transformer for the study of electromagnetic transients. Existing analytical formulae for the calculation of losses in the windings are evaluated. Various equivalent circuits are fitted to represent in the time domain the damping produced by eddy currents in the windings. A frequency dependent model is derived for the iron core, based on the physical distribution of losses and magnetization effects. The parameters of this model are obtained by optimal discretization of the laminations. Simulation show the effects of eddy current in the damping of transients. >