Juan Carlos Burgos

Modeling Power Transformer Field Drying Processes

Power transformer field drying is becoming a habitual practice because water is damaging for transformer insulation, and its presence increases the probability of unexpected failures. Different drying methods are currently being used by electrical companies but sometimes without a profound knowledge of the drying processes involved, and consequently drying is not as effective as would be desirable. Physical models have been developed by the authors in order to study power transformer drying processes. The use of the models will help to plan more effective drying processes, tailoring the drying times and drying conditions for each particular case. The models have been tested in a test transformer fitted with sensors. In order to monitor the process, insulation samples were extracted from the transformer before and after the process. In this article, several transformer drying procedures are described. Theoretical models and their experimental validation processes are reported. Finally, some general recommendations about transformer drying in the field are provided.

Studying the loss of life of natural-ester-filled transformer insulation: impact of moisture on the aging rate of paper

Moisture is one of the most harmful agents for transformer solid insulation. The catalyzing effect of moisture on the degradation process of paper and pressboard has been studied by several authors, who developed models which they used to estimate how the aging process would be accelerated in the presence of water [1]-[5]. In [2] and [3] Lelekakis et al. reported the results of an experimental study in which samples with different moisture contents were aged in the presence of mineral oil (MO). In [3] an equation to determine the reduction of service lifetime of insulation subjected to increased temperature and with increased moisture content is proposed.

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.

Statistical study on the reference values of furanic compounds in power transformers

This article presents a statistical study devoted to obtaining the reference values of 2FAL in power transformers.

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.

Comparison of methods for measuring zero sequence impedances in 3-phase core-type transformers

This paper deals with two topics about the measurement of zero sequence impedances in 3-phase core-type transformers. The first topic is a comparison of 3 methods to perform the measurement in a 3-phase winding: a) parallel connection of windings; b) series connection of windings; and c) measurement of elements of the phase impedance matrix. A brief theoretical discussion about these methods is included. The second topic is a comparison of 2 methods for measuring the zero-sequence impedance for the Dyn connection: a) with all the delta branches short-circuited; and b) without short-circuiting the delta branches. The results for the Dyn connection are compared with the obtained ones for YNyn connection, by using the standardized method for this test (parallel connection of star windings). A discussion about the results is included.

Analysing the impact of Moisture on the AC Breakdown Voltage of Fe 3 O 4 Based Nanodielectric Fluids

The interest in the development of new nanodielectric fluids suitable for electrotechnical applications has increased significantly in the last decade. Several authors have reported experiences on fluids manufactured using different base fluids and nanoparticles. Most of these studies are focused on the comparison of the thermal and dielectric properties of the nanofluids and the base liquids and most authors have found that the addition of nanoparticles can lead to an improvement of the properties of insulating liquids. The main objective of this paper is to study the enhancement of the AC Breakdown Voltage (AC BV) at 50 Hz achieved when dispersing nanoparticles (NP) in transformer oil. In particular, transformer mineral oil was used while the nanoparticles used were Fe3O4 in hexane base. In order to obtain a good dispersion of the nanoparticles an ultrasonic agitator was used. The impact of moisture in the dielectric strength of fluids prepared with different concentrations of NP was evaluated and compared with that of the base liquid. The experimental results show a certain improvement of the dielectric strength of the nanodielectric fluids compared to that of the Mineral Oil.

Applicability of nanodielectric fluids to the improvement of transformer insulation properties

The interest in the development of new nanodielectric fluids suitable for electrotechnical applications has increased significantly in the last decade. Several authors have reported experiences on fluids manufactured using different base fluids and nanoparticles. Most of these studies are focused on the comparison of the thermal and dielectric properties of the nanofluids and the base liquids and most authors have found that the addition of nanoparticles can lead to an improvement of the properties of insulating liquids. In this paper, a literature survey on this field is presented.

Transformer field drying improvement by applying low-frequency-heating

Water is damaging for transformer insulation, and its presence increases the probability of unexpected failures. Field drying is currently being employed by utilities to reduce the transformer water content when it reaches an excessive value. One of the key aspects to achieve an adequate drying level, is the temperature attained by the solid insulation during the process. Low-frequency heating (LFH) consists of heating the transformer active part using the heat generated in the windings when they are fed with current at low frequency. This method is becoming popular and some companies are using trying to improve the efficiency of the drying process. In this paper, the improvement in drying obtained when LFH is applied is analyzed. Simulations have been conducted using theoretical models to predict the real response of the transformer when LFH is applied. Experimental tests have been also performed in the laboratory to quantify the improvement achieved when LFH is applied. Finally an experimental study was carried out on a real transformer dried under controlled conditions with and without LFH to compare the effectiveness of both processes.