Summary:
Dissolved Gas Analysis, prediction, purification and replacement Power systems and transformers are essential equipment, therefore their reliability and safe operation are important in determining their operating conditions, and the industry uses quality control tests in the design of oil-filled transformers. The use of dielectric strength testers or spark gap testers is therefore essential. The effect of ageing on transformer oil properties (physical, chemical and electrical) was studied using the international analysis methods for the assessment of transformer oil quality. The study was carried out on six transformers operating in the field, with monitoring periods of more than twenty years. The properties that are strongly time-dependent were specified, and those having a major impact on transformer oil acidity, breakdown voltage and dissolved gas analysis were defined. Several tests on transformer oil were studied in order to determine the purification or replacement time and, moreover, the prediction of the characteristics under different operating conditions.
I. INTRODUCTION
Power transformers are very important, high-cost items of equipment used in the transmission and distribution of electricity. Their optimum performance is important for electrical operating systems, since the loss of a critical unit can have a major impact on the safety, reliability and cost of the electrical power supply. Power transformers are used to step the voltage up or down and are a key component in any efficient power distribution network. A typical transformer incorporates coils of conducting wire wound around a core and covered with insulating paper support. An essential part of the operation of this equipment are the transformer oils, which serve two functions: electrical insulation and heat dissipation.
Unfortunately, there are occasions on which transformers fail, which means high costs for the power supplier and, in extreme cases, possible explosions, with the resulting threat of serious injury to workers and significant environmental impacts. Power transformers are among the most valuable and important elements of electrical power systems. The ageing of the insulation system reduces both the mechanical strength and the dielectric strength of the transformer. An aged transformer is subject to faults that result in high radial and compressive forces.
In a fault on an aged transformer, the conductor insulation normally deteriorates to the point where it can no longer withstand the mechanical stresses caused by the fault. The life/age of the transformer is commonly related to the degradation of the insulation, caused mainly by the thermal stress on the paper insulation, together with its electrochemical decomposition. The known life of a transformer is based on a parameter designed with respect to normal operation and climatic conditions. Ageing does not only depend on the load, but also on the type of paper, the composition of the pulp, the moisture and oxygen content, as well as the acidity level of the insulating liquid.
The insulation is the main component, and it plays an important role in the life expectancy of the transformer. The oil undergoes continuous deterioration and degradation due to the sustained application of the thermal stresses of electricity, the thermal load cycle and climatic conditions. This can be dangerous for the electrical equipment and the installation. Continuous monitoring of the insulating characteristics of the oil has become an important task in order to prevent the oil from deteriorating under working conditions. Several efforts have been made in recent years to study the electrical, physical and chemical properties of insulating oils. The daily preventive maintenance of an oil transformer consists of recording oil level readings, temperature readings, the annual average of oil samples and taking samples to check the physical, chemical and electrical properties. Oil analyses include specific gravity, kinematic viscosity, flash point, total acidity, moisture, breakdown voltage and dissolved gases. That is transformer oil condition monitoring.
Among those efforts there is a characterisation of fresh and aged natural oils. This area is still open to the study of the effect of the service period on transformer oil properties. The various oil tests on the transformers studied allow us to know when to purify, when to replace and to predict ageing under the different operating conditions.
II. EXPERIMENTATION
The reliable performance of insulating mineral oil in an insulation system depends on certain characteristics of the base oil, which can affect the overall performance of the electrical equipment. In order to carry out its multiple dielectric, heat transfer and arc-quenching functions, the oil must possess the basic properties required. Oil in service varies widely in its degree of degradation and its degree of contamination. Mineral oil in service is subject to alterations due to the conditions of use. In many applications, the insulating oil is in contact with the air and therefore subject to oxidation reactions accelerated by high temperatures and the presence of metals, metallic compounds, organometallic compounds, or both acting as oxidising agents.
There are a large number of tests that can be applied to the oil delivered together with the equipment or to the oil of apparatus in service, but the following tests are believed to be sufficient to determine whether the condition of the oil is suitable for continued operation and to suggest the type of corrective action required. In general there is no single examination as the sole criterion of the condition of the oil sample.
The condition assessment should preferably be based on the evaluation of significant characteristic compounds determined in properly equipped laboratories. The experimental tests are carried out on transformer oils in order to determine their electrical, physical and chemical properties experimentally. The tests described were carried out at Central Laboratories, Egyptian Electricity Holding Company, Ministry of Electricity and Energy. Six transformers with test oil samples were used to study the characteristics of the service life of the oil. Oil samples were taken from the different transformers. These transformers operated at the Cairo South power station. The samples were taken from transformers operating for several years, with different loads and operating conditions.
The tests carried out on the transformer oil included: breakdown voltage, total acidity, flash point, density and kinematic viscosity. The different transformer oil cases used in the tests were:
- The transformer oil of transformers (1 and 5) is new (fresh).
- The transformer oil of transformers (3 and 6) is purified.
- The transformer oil of transformers (2 and 4) is replaced
The ratings of these transformers are as shown in table (1). The effects on the physical, chemical and electrical properties of the transformer oil were studied using the international analysis methods for the assessment of transformer oil quality.
The determination of the breakdown voltage of each transformer oil sample was carried out in accordance with test procedure IEC 156. The total acidity for an oil sample, given in (mg KOH / g of oil), was determined in accordance with the procedure set out in IP 139/64. The flash point of the transformer oil sample was carried out in accordance with the ASTM D92 standard. The analysis procedure to determine the viscosity of the transformer oil (mm ² / s) is given in the reference, ASTM D445.
The specific gravity was determined by the ASTM 1298 standard. There are several methods of interpreting dissolved gas analysis (DGA) in transformers in service, which are provided in standard IEC 60599, [IEEE Guide C57.104].
III. EXPERIMENTAL RESULTS AND DISCUSSION
Transformer oil ages rapidly at high temperatures and moisture acts as a catalyst for its ageing. There are also other catalysts present in a transformer that are responsible for the degradation of the oil. These include copper, paint, varnish and oxygen. The main ageing mechanism of transformer oil is oxidation, which gives rise to acids and the formation of other polar compounds. These oxidation products will have a detrimental effect on the study of the degradation processes. Transformer oil, when subjected to thermal and electrical stresses in an oxidising atmosphere, gradually loses its stability and breaks down, oxidises, increases in acidity and, finally, begins to produce sludge. This is the oil degradation mechanism. In fact, the ageing mechanisms of the oil are complicated.
In general, oxygen reacts with certain hydrocarbons through a free radical process, which generates hydroperoxides. Hydroperoxides are not stable and break down to form ketones and water. The presence of hydroxyl groups will result in the production of alcohols and phenols. Most of the oxidation products will have a negative effect on the electrical properties of the oil. The carboxylic acids that are produced will be dissolved in the oil or evaporated. Dissolved acids can cause damage to the paper and to the copper windings, while evaporated acids corrode the top part of the unit. As a result, the conditions required for the degradation of the oil are present. An important part of the degradation of the oil comes more from the air in contact with the hot oil in the apparatus than from the results of oxidation in the degradation of the oil. Hot cellulose is also a source of oxygen.
The experimental tests are performed on transformer oil in order to clarify its electrical, physical and chemical properties experimentally. The tests carried out on the transformer oil included: breakdown voltage, total acidity, flash point, density and kinematic viscosity.
The breakdown voltage increased in the first period and then decreased over long periods of time. Under normal operating conditions, the breakdown voltage will be at a minimum because of oxidation and contamination. Contamination is commonly found in transformer oils containing water and particles, these contaminants reduce the insulating qualities of the transformer oil.
The decrease in the breakdown voltage due to long periods in service increases some impure particles, this will increase the moisture and the oil will become non-homogeneous, consequently the strength of the transformer oil decreases, which will reduce the maximum value of the breakdown voltage of the transformer oil. Silent discharges and concentrated current flows lead to the formation of water, acids and hydrogen. Arc discharges and liquids encourage the production of carbon particles, wax and gases such as carbon monoxide, carbon dioxide, acetylene and acids, due to oxidation and discharges, iron and copper in the liquid, which lead to the reduction of the electrical strength.
More information in the section Transformer test equipment / Dielectric strength tester - Portatest Spark Gap Tester