ower transformers are one of the most expensive elements in a power system, and their failure is a very costly event. Power transformers are mainly involved in energy transmission and distribution. Unplanned power transformer outages have considerable economic impact on the operation of electric power networks. To have reliable operation of transformers, it is necessary to identify problems at an early stage before a catastrophic failure occurs. In spite of corrective and predictive maintenance, preventive maintenance is gaining due importance in the modern era, and it must be taken into account to obtain the highest reliability of power apparatus such as power transformers. The well-known preventive maintenance techniques such as dissolved gas analysis (DGA), thermal monitoring, partial discharge measurement, capacitance and tan delta measurements, frequency response analysis, etc. are performed on transformers for a specific type of problem. DGA is a very efficient and reliable tool for the earliest detection of inception faults in transformer and other electrical equipment using insulating oil. Condition monitoring of electrical equipment, such as transformers, helps users in many ways such as planning of maintenance schedule, obtaining knowledge of the health of equipment, estimating the remaining service life of equipment, finding areas of further improvement, refining product specification, etc. Kraft paper (cellulose) immersed in mineral oil is used as the insulation system for the copper windings in large power transformers. As the system ages under load, the paper and oil can degrade, potentially leading to catastrophic failure. Partial discharge (PD) is an important tool for improving the reliability of HV insulation systems. It is a very sensitive and nondestructive method of evaluation of the health of the insulation of any HV equipment. PD always is associated with the degradation of insulation systems in HV equipment. Therefore, PDs need to be detected, measured, located, and reduced to a safe value so that the quality of the insulation system is not affected.

Condition Monitoring of Power Transformers - Bibliography Survey

A different approach to the study of surface tracking reveals a new view of the oil-pressboard interface and suggests a link between the electric double layer and the boundary layer.

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Tracking and surface discharge at the oilߝpressboard interface

The condition of the internal cellulosic paper and oil insulation are of concern for the performance of power transformers. Over the years, a number of methods have been developed to diagnose and monitor the degradation/aging of the transformer internal insulation system. Some of this degradation/aging can be assessed from electrical responses. Currently there are a variety of electrical-based diagnostic techniques available for insulation condition monitoring of power transformers. In most cases, the electrical signals being monitored are due to mechanical or electric changes caused by physical changes in resistivity, inductance or capacitance, moisture, contamination or aging by-products in the insulation. This paper presents a description of commonly used and modern electrical-based diagnostic techniques along with their interpretation schemes.

Electrical-Based Diagnostic Techniques for Assessing Insulation Condition in Aged Transformers

In many items of high voltage plant, a mineral or synthetic oil is used in conjunction with paper as the dielectric medium. However, increasing awareness of the environmental impact of human activity and increasing disposal costs have encouraged researchers to direct their attention to renewable and biodegradable alternatives. Originally used in capacitors, vegetable oils are now finding widespread use in some transformer installations, particularly in the United States. Therefore, it seems prudent to begin systematic investigations of the aging behavior of a number of vegetable based oils and assess their potential for application in high voltage systems. A total of five food grade vegetable oils, an oil specifically formulated for high voltage applications (Envirotemp FR3) and dodecylbenzene (DDB) were aged at various temperatures in air. Their aging behavior was assessed by ultra-violet/visible, infrared and dielectric spectroscopies supplemented with measurements of viscosity. It was found that olive oil offered excellent resistance to aging (comparable to Envirotemp oil), rapeseed oil offered intermediate properties whereas corn and sunflower oil oxidized appreciably after aging. Despite being more prone to oxidation, all the vegetable oils offered dielectric properties which were better than dodecylbenzene and similar to mineral oil.

Aging of biodegradable oils and assessment of their suitability for high voltage applications

Physicists, chemists, and electrical engineers are all interested in the properties of dielectric materials, though with different purposes in mind. This book is intended to give a systematic account of the theory of dielectric properties, and thus to provide a background against which applications can be made. It seldom requires the reader to have a knowledge of mathematics greater than is implied by an acquaintance with calculus, and the reader is assumed to have a certain elementary knowledge of atomic and molecular physics, statistical mechanics, and electrostatics.

Theory of Dielectrics. Dielectric Constant and Dielectric Loss (E. Justi)

The materials used as the dielectric media of choice in large transformers continue to be a combination of pressboard and mineral oil. These materials offer a cost effective solution and are backed by a historical knowledge base of performance data and lifetime characteristics. A proven method to prevent inter-phase flashover is to use successive layers of pressboard and oil gaps. However, a number of failures in the field have been attributed to surface discharge or creeping discharge over the pressboard surface. This paper reports on an experiment to study surface discharge on pressboard in oil. A new technique for the generation of repeatable surface discharge is outlined. The paper explains the differences between surface tracking, and surface flash-over. The results indicate that the tracking is the result of surface discharge and that surface discharge and surface flash-over are two distinct phenomena.

A new approach to the study of surface discharge on the oil-pressboard interface

Surface discharge on the oil-pressboard interface is considered to be serious fault condition. This phenomenon leads to the formation of a conducting path on the surface of an insulation material as a result of surface currents. The surface current is effectively the drift of charge under the action of an electric field which may be either the general electric field or a locally enhanced field developed around a space charge. Sustained surface discharge can eventually result in equipment failure. A needle-bar electrode configuration has been used to investigate the surface discharge phenomenon at the oil pressboard interface. This paper presents the partial discharge (PD) patterns corresponding to the surface discharge events.

Investigation on the surface discharge phenomenon at the oil-pressboard interface

Creeping discharge at the oil-pressboard interface is a serious fault condition because it can lead to catastrophic failure under normal operating conditions of large transformers. Creeping discharge leads to the development of a conducting path that is generally accepted as white marks on a board surface due to localised heating. The visible white marks indicate a drying out process of the pressboard through moisture evaporation and the breaking of oil molecules to generate gases in the pressboard pores. The processes can continue from minutes to months or even years until failure. Characterising this type of faulty condition in a condition monitoring programme is important for transformer life management. This paper explains the behaviour of surface discharge observed during a surface discharge experiment at the oil-pressboard interface using a needle-bar electrode configuration. Correlation between partial discharge (PD) activities and measured data in terms of phase resolved partial discharge (PRPD) patterns have been undertaken. The experimental results also show a decreasing trend of the average discharge magnitude during the formation of the white marks that propagate towards the earth bar. This finding is qualitatively comparable to on-line problems reported previously, i.e. there was a falling trend and no obvious evidence of imminent failure from the monitoring data of PD activity of a large power transformer that eventually suffered from an unexpected fault.

Partial discharge characteristics of surface tracking on oil-impregnated pressboard under AC voltages

High voltage equipment (e.g. transformers and switchgear), use mineral oil and paper as a cost effective and well tried insulation medium. However, this insulation is known to age over time and usage. An obvious sign of ageing is the change in the colour of the oil which changes from pale yellow to reddish brown. This colour change can be measured by the technique of ultraviolet/visual (UV/Vis) spectroscopy. Other techniques such as acid number measurement and infrared (IR) spectroscopy can also be used as indicators of the age of the oil. In general, decisions can be taken by the operator, depending upon the results of these and other tests, as to when and how to maintain the equipment. Maintenance solutions include refurbishment of the transformer or the replacement of the mineral oil. However, these treatments are expensive and a more cost effective option is to use either oil purification or reclamation in order to cleanse the system before refurbishment becomes necessary. The question then becomes: which of these two processes is the more effective? There is little up-to-data research published on the effects of fuller's earth on transformer oil. This paper considers an experiment where aged oil is processed using fuller's earth and contrasts the results of selected ageing indicators against the known results of the oil purification process.

P. M. Mitchinson

Papers

Tracking and surface discharge at the oilߝpressboard interface

A new approach to the study of surface discharge on the oil-pressboard interface

Investigation on the surface discharge phenomenon at the oil-pressboard interface

Partial discharge characteristics of surface tracking on oil-impregnated pressboard under AC voltages

Oil reclamation - just a question of moisture?