Condition Monitoring of Transformer Oil and Paper

Abstract

Like other machines, transformer too has limited life. However, unlike other machines, it does not have any moving parts, except tap changers or cooling fan or pump motors. The outages, therefore, is not due to wear out. The transformers die because of deterioration of insulation over time. Generally, cellulosic paper and mineral oil form bulk of insulation in the transformer. The insulation is subjected to a variety of stresses, such as thermal, mechanical, electromagnetic, etc. Under the influence of the said stresses and in the presence of oxygen and moisture, the insulation deteriorates continuously over a period of time, eventually leading to failure. If these failures can be predicted with some degree of confidence, sudden failures can be minimized. In view of the above, efforts are being made to understand and analyze factors responsible for determining the life of a transformer. Broadly, ageing of insulating material can be looked upon as a chemical reaction, which takes place at a rate that is influenced by thermal, electrical and mechanical stresses. All chemical reactions lead to product(s), which have physical and chemical characteristics different from the parent material. Therefore, chemical and physical characterization of the reaction product(s) may assist in monitoring the health of the insulation and hence transformer. For instance, cellulose undergoes degradation under thermal stress: the Degree of Polymerization (DP) of cellulose is reduced due to thermal ageing. Besides, it leads to the formation of reaction products, such as carbon monoxide, carbon dioxide, water, furan, etc. Furans are a family of organic compounds like 2-furfuraldehyde, 2–acetylfuran, 5-methyl-2- furfuraldehyde, furfurylalcohol, 5-hydroxymethyl -2- furfuraldehyde, etc. It is therefore, possible to estimate the extent of insulation damage by monitoring reaction product(s). Moreover, these changes in cellulose, at molecular level manifest in change of physical properties. Tensile strength, dielectric breakdown voltage, tan-ä of Kraft paper are the physical properties affected by these changes. For example, breakage of cellulose chain at microscopic level may lead to microscopic voids, which may influence the partial discharge behaviour of the material. Over the years a number of studies, involving monitoring of one or more of above parameters, have been carried out to predict the impending death of transformer. However, the results are far from complete. Laboratory studies have been carried out to study ageing behaviour of cellulose paper and oil. Mostly the studies are confined to thermal ageing and thus do not simulate the real life situation. Alongside the aging studies, efforts have been directed to develop sensors and instrumentation for on-line monitoring of some of the mentioned properties. But, a holistic approach to the problem is somehow missing. In the present study an accelerated aging study involving combined application of thermal and electrical stress is carried out on cellulosic paper and mineral oil. All possible properties, which are likely to be affected by degradation i.e. such as Viscosity, Moisture, Breakdown voltage, Tan-delta, Dielectric Constant, Interfacial tension, Resistivity, Acidity and Flash point of oil and paper properties like Tensile Strength, Degree of Polymerization (DP), etc. are periodically measured. A few properties, which are related to the degradation of both paper and oil, like Partial Discharge (PD) and Dissolved Gas Analysis (DGA), have also been studied. In some cases a few reaction products are formed due to combined stresses like, different gases (CO2, H2, C2H2, C2H4, C2H6, CH4) and furanic derivatives which are dissolved in oil. The analysis of these reaction products have provided very useful information about the paper insulation system. The dissolved gas analysis (DGA) is carried out on the aged samples to predict the incipient faults. Though there is no direct method to detect these incipient faults, it is known that these faults, especially in the form of overheating, arcing or partial discharge, develop certain gaseous hydrocarbons, that are retained by the insulating oil as dissolved gases. Thus the evaluation of these gas compositions and their interpretation, known as Dissolved Gas Analysis (DGA), seems to be one of the life saving diagnostic tests on condition monitoring of transformers. A wide range of techniques is available for pattern recognition of PD signatures. The first significant system developed for analyses of PD signatures, as observed on an oscilloscope, was published by CIGRE in 1969. In the late 1980s computerized data acquisition became commonly available, facilitating acquisition, storage and digital processing of discharge patterns. Earlier the integrity and quality of insulation was checked by various tests like power frequency voltage withstand test, loss angle measurement, insulation resistance measurement, impulse voltage withstand test etc. But with above measurements, the minor insulation flows and improper/inefficient processing of insulation was not identified. Many cases were found where the material having a very high insulation resistance failed within an unexpected short duration. Many experiments were carried out to understand the phenomena and after various experiments and research on the progressive deterioration of the insulation, it was found that such a failure is only due to the phenomena named Partial Discharge. If there is an air gap or void in the solid insulation, the localized electrical discharge in the insulating media, which are restricted to a small part of the dielectric under test partially, bridges the insulation between the electrodes. Because of this, progressively large leakage current path develops in the insulation and results in an ultimate failure of the object. The early detection of partial discharge present in a transformer is very important to avoid/reduce the losses to the catastrophic failure of transformer. Also, once the PD is detected, the location of the same is very important so that the defects can be pinpointed and solved effectively. The furanic compounds analysis reveals the rate of insulating paper degradation of the equipment and it directly projects the expected life. As this degradation is not observable by any other means, it is one of the essential tests to diagnose the equipment. Cellulose insulation has a structure of long chains of molecules. The cellulosic paper contains about 90% cellulose, 6-7% of hemi-cellulose and 3-4 % of lignin. Cellulose is a natural polymer of glucose and it degrades slowly as the polymer chains breakdown during service, releasing degradation products into the oil. The paper eventually degrades to such an extent that it loses all its mechanical strength and becomes susceptible to mechanical damage, which puts the electrical integrity of the equipment at risk. Efforts have been made to identify the most sensitive properties with respect to ageing and to establish a correlation between various properties. Eventually, this knowledge data base will help in evolving a condition monitoring strategy and instrumentation for actual transformers.

In this thesis work, the accelerated thermal and electrical stress has been applied to predict the life of the liquid and solid insulation. Under these stress conditions, the physicochemical properties of oil like viscosity, moisture, breakdown voltage, resistivity, loss angle, dielectric constant, acidity, flash point have been observed. On the soil insulation i.e. kraft paper, wer have characterize the properties like tensile strength, degree of polymerization, furfural analysis, partial discharge, SEM and XRD studies.