Diego F. García

Diffusion coefficient in transformer pressboard insulation part 2: mineral oil impregnated

Moisture is one of the most influencing parameters which accelerates solid insulation degradation process in transformers. Correct estimation of the moisture within solid insulation of transformer is still a challenge due to temperature driven complex moisture dynamics between oil and paper insulation. Although some authors have proposed coefficients for Kraft paper as well as for non-impregnated pressboard much less work has been done on oil-impregnated pressboard insulations. In this paper, the determination of the diffusion coefficient of moisture in transformer pressboard impregnated with mineral oil is tackled. Studies were carried out on different samples thickness of pressboard which are typical in power transformer. The experimental conditions were simulated using a mathematical model based on the Fick¿s second law and an expression for the diffusion coefficient is proposed and validated experimentally.

Development of a Moisture-in-Solid-Insulation Sensor for Power Transformers

Moisture is an important variable that must be kept under control to guarantee a safe operation of power transformers. Because of the hydrophilic character of cellulose, water mainly remains in the solid insulation, while just a few parts per million are dissolved in oil. The distribution of moisture between paper and oil is not static, but varies depending on the insulation temperature, and thus, water migration processes take place continuously during transformers operation. In this work, a sensor is presented that allows the determination of the moisture content of the transformer solid insulation in the steady state and during the moisture migration processes. The main objective of the design is that the electrodes of the sensor should not obstruct the movement of water from the solid insulation to the oil, so the proposed prototype uses a metallic-mesh electrode to do the measurements. The measurement setup is based on the characterization of the insulation dielectric response by means of the frequency dielectric spectroscopy (FDS) method. The sensitivity of the proposed sensor has been tested on samples with a moisture content within 1% to 5%, demonstrating the good sensitivity and repeatability of the measurements.

Moisture diffusion coefficients of transformer pressboard insulation impregnated with natural esters

In previous studies moisture diffusion coefficients for transformer pressboard insulation impregnated with mineral oil have been proposed. In this paper diffusion coefficients are proposed for transformer pressboard insulation impregnated with different natural esters. The coefficients were validated with experimental data and compared with those of mineral oil. In the determination of the moisture coefficients drying experiments at different temperatures were performed on pressboard samples of several thicknesses. An optimization process based on particle swarm and a finite element model were used to derive the coefficients from experimental data.

Particle swarm optimization vs genetic algorithm, application and comparison to determine the moisture diffusion coefficients of pressboard transformer insulation

Moisture mobility inside a transformers solid insulation can be modelled by using a diffusion model based on Ficks second law. The precision of these models is related to the so-called moisture diffusion coefficient. The experimental determination of the moisture diffusion coefficient can be a difficult task. For this reason, previous studies aimed to find a more simple experimental methodology to determine the moisture diffusion coefficients of solid cellulosic insulations. This methodology uses experimental drying curves and an optimization process with genetic algorithms (GAs) working with a drying diffusion model which is solved by the finite element method. In this article, a basic particle swarm optimization (PSO) method as an alternative to the previous optimization process by GAs was implemented and evaluated. The PSO method reduces the time spent in the determination of the moisture diffusion coefficient. Additionally, optimization by particle swarm simplified the methodology to determine the moisture diffusion coefficient because a subsequent statistical analysis, as required when GAs are used, is not necessary.

Assessing the Use of Natural Esters for Transformer Field Drying

In recent years, great attention has been paid to ester fluids as an alternative to mineral oil. Since the present use of these liquids is becoming a common practice in distribution transformers, even some experiences have been published reporting their application to power transformers. One of the main differences between ester fluid and mineral oil is the much greater capability of absorbing water by esters. In this paper, the possibility of using this kind of liquid in transformer field drying is assessed. Hot oil (HO) drying with mineral oil is one of the most widely used methods to dry transformers in the field, since it is a relatively simple and well-known process and it is less aggressive for the insulation than other drying methods. Moreover, drying the oil, while it is preferably hot, is the only method available to dry transformers online. However, the water extraction rate of the process is very poor because of the highly hydrophobic character of mineral oil and, in consequence, large drying times are needed to achieve a significant reduction in the water content of the insulation. A first theoretical analysis seems to indicate that the use of a less hydrophobic liquid would significantly reduce the drying times involved in the process. This paper aims to quantify the improvement of the HO drying process that is achieved by using ester fluids instead of mineral oil. Both drying agents were compared by means of theoretical simulations as well as laboratory tests.

Effect of the Thickness on the Water Mobility Inside Transformer Cellulosic Insulation

Traditionally, water mobility inside the cellulosic insulation, as those used in power transformers, has been modeled using the so-called Ficks diffusion theory, considering that the water movement in this porous material only depends on local conditions, such as temperature and moisture concentration. However, recent experimental works have demonstrated that the moisture movement inside the cellulose insulation could be also influenced by a global condition, that is, the insulations thickness. In this paper, the influence of the insulation thickness in the mobility of moisture inside the cellulosic materials, such as Kraft paper and pressboard, is analyzed. In addition, the improvement in moisture dynamics estimation obtained when this variable is included into the diffusion models has been proved.

Diffusion coefficient in transformer pressboard insulation part 1: non impregnated pressboard

Water is harmful for transformers; the dynamic behavior of moisture influences the dielectric integrity and insulation aging in a transformer on operation. In this paper, the determination of the diffusion coefficient of moisture in transformer pressboard is tackled. This work has been focused in analyzing non oil-impregnated pressboard, so the obtained coefficient is useful for studying the processes of transformer drying in factory. Drying experiments were developed in a thermogravimetric analyzer on nonimpregnated samples of pressboard subjected to different drying temperatures. The experimental conditions were simulated by means of a diffusion model implemented by means of Finite Element Method. The comparison between estimated and measured values allows drawing conclusions about the accuracy of the proposed coefficient.

Moisture sensor to characterize transformer solid insulations in transformers under non-equilibrium conditions

Moisture is an important variable that must be kept under control to guarantee a safe operation of power transformers. Because of the hydrophilic character of cellulose, water mainly remains in the solid insulation while just a few parts per million are dissolved in oil. The distribution of moisture between paper and oil is not static but varies depending on the insulation temperature and thus, water migration processes take place continuously during transformers operation [1]. In this work, a sensor is presented to determine the moisture content in the solid insulation of transformers during moisture migration processes. The main objective of the design is that the electrodes of the sensor should not obstruct the movement of water from the solid insulation to the oil, so the proposed prototype uses a metallic mesh-electrode. The measurement scheme is based on the characterization of the insulation dielectric response by means of the Frequency Dielectric Spectroscopy (FDS) method.

Experimental study on moisture dynamics in transformers insulated with natural-esters

In the last years great attention has been paid to ester fluids as an alternative to mineral oil. At the present the use of these liquids is becoming an habitual practice in distribution transformers and some experiences are also starting to be reported about their application to power transformers. One of the main differences between ester fluids and mineral oils is that those are able to absorb much greater amounts of water when they are in contact with cellulosic insulation. Moisture is one of the key variables that must be kept under control for a safe transformer operation. As is well known the distribution of moisture between transformer solid and liquid insulation depends on transformer temperature, and thus a continuous migration of moisture between both materials takes place during transformer normal operation. In the case of transformers isolated with ester fluids, the equilibrium points, and probably the time constants of the migration processes, would be affected by the different properties of these materials with regard to moisture. In this work the moisture dynamics in ester fluids is compared with that in mineral oils. Drying processes of pressboard has been experimentally simulated using mineral oil and a natural ester as insulating fluid. The results of these experiments will be used in the future to develop dynamic models that will allow to simulate the moisture migration processes in transformer insulation during normal operation as well as transformer drying processes.

Modelling the dielectric response of a water-in-solid-insulation sensor for power transformer monitoring

A significant proportion of power transformers installed across the world approaching the end of their designed life. One reason for a power transformer to reach the end of its life is that its paper insulation has degraded to the point where it has lost its tensile strength. The rate at which paper degrades is mainly related to the temperatures within the transformer and the water content of its solid insulation. Being able to monitor the water content of transformer solid insulation would be very desirable to improve the maintenance of the equipment. In a previous work, a sensor to monitor this variable was proposed by the authors. In this paper the characterization of Sensors dielectric response is presented through a multiphysics model under inhomogeneous moisture distribution using Frequency Dielectric Spectroscopy (FDS) method.