Health index: The TERNA's practical approach for transformers fleet management

The asset management of any Transmission System Operator (TSO) cannot ignore the evaluation of the power transformers fleet. Even in the absence, to date, of any specific international guide or standard, every big electrical utility is adopting a home-made 'Health Index'. An HI is developed to summarize in numerical form the transformers' reliability for the purpose of evaluation, ranking and comparison. In the present paper the author will elucidate a new model that merge the evidences of periodic tests (DGA, furans, acidity, inductance, FDS, etc.) with the keraunic properties of a substation

Classification in iso-attention classes of hv transformer fleets

The largest electrical operators may have in service some hundreds of HV power transformers. It means that failures are to be expected along their operational times with possible heavy consequences in case of fire or explosion or other negative events. The traditional approach for preventing failures and their consequences is normally based on periodical evaluations of some physical, chemical and electrical parameters of the insulating oil and/or of the equipment but these data do not give any information on the overall state of health of the units when the observed properties are below the values assumed by the different existing Standards as acceptable limits. Also these data are not related to failure consequence. With this aim, a more complete risk evaluation may be an innovative powerful tool for preventing negative situation due to transformer failures and for classifying these electrical units in iso-attention classes. This risk is formed by three partial components, which are: 1) causes of the failure, 2) probability that a failure may provoke a damage and 3) type and entity of damages (magnitude). This approach may be a useful tool for supporting decisions and addressing periodical investments of electrical operators and utilities having in service transformer fleets. With this aim, approximately 700 different HV transformers of one of the biggest electrical operator have been examined, evaluating the single components forming their risks. This analysis has permitted to establish which transformers may be considered more critical with possibility to better address the successive maintenance strategies

Transformers Fleet Management Through the use of an Advanced Health Index

Power transformers represent the highest value of the equipment installed in transmission substations, comprising up the 60% of the total investment. They are expected to operate for several decades without faults and possibly without relevant unscheduled maintenance practice. The new approach is developed for reducing time based maintenance and, increasing condition based maintenance and to introduce predictive maintenance as well. The aim of predictive maintenance is first to predict when transformer failure might occur, and secondly, to prevent occurrence of the failure by performing maintenance. Diagnostic information can be evaluated individually or better by a complex algorithm which merges all the single inputs and their Rate of Increase (RoI) creating a mono-dimensional figure called Health Index (HI). This concept represents a real 'shifting of paradigm', as it deeply affects the criteria for transformers grid management and selection of the electrical utilities and grid companies. © 2018 IEEE

Oils with presence of corrosive sulphur: mitigation and collateral effects

Recent years have witnessed a relatively large number of power transformers and shunt reactors failures, which were attributed to the presence of corrosive sulfur compounds in the insulating oils and their attending reactions with copper to form electrically conductive copper sulphides. The associated problems appeared to be initially resolved by the addition to the insulating oil small amounts of a metal deactivator, a derivative of 1, 2, 3 benzotriazole (BTA). However, over the past four years additional problems, arose that were principally associated with the evolution of gases from the electrical insulating oils. These observations, recorded in the field as well as reproduced in laboratories, are examined in this paper

Effects of Metal Deactivator Concentration upon the Gassing Characteristics of Transformer Oils

Recent years have witnessed a relatively large number of power transformer and shunt reactor failures, which were attributed to the presence of corrosive sulfur compounds in the insulating oils and their attending reactions with copper to form electrically conductive copper sulphides. The associated problems appeared to be initially resolved by the addition to the insulating oil small amounts of a metal deactivator (more commonly called passivator), a derivative of 1,2,3 benzotriazole (BTA). However, over the past four years, additional problems arose that were principally associated with the evolution of gases from the electrical insulating oils, containing the metal deactivator. These observations, recorded in the field as well as reproduced in laboratories, are examined in this paper

Long Term Stability of Insultaing Mineral Oils following their Corrosive Sulfur Removal

Over the last decade dibenzyl-disulphide (DBDS) was found to be present in many mineral insulating oils. This sulfur compound tends to react with the copper of the windings, forming harmful by-products (Cu2S). Different techniques to overcome the problem such as metal deactivation, oil change and chemical treatment, have been applied. The long term effectiveness of these techniques is examined and compared

Corrosive Sulfur Induced Failures in Oil-Filled Electrical Power Transformers and Shunt Reactors

The nature and causes of corrosive sulfur induced failures are examined in oil-filled transformers and shunt reactors. Copper sulfide, which is formed when the corrosive sulfur in a mineral oil reacts with the copper conductors, is likely to diffuse into the paper tapes insulating the conductors. Since copper sulfide is partially conducting, the dielectric losses of the contaminated oil-impregnated-paper tapes are markedly increased; paper tapes in close proximity to the copper conductors are found to attain tan delta values > 1.0 even at room temperature. It is highly likely that thermal instabilities develop at those sites at operating temperatures, leading to increased loss currents and, ultimately, short circuits between the turns. This sequence of events is substantiated by evidence from the field, which indicates large areas of thermally degraded insulations and charred breakdown regions along the coils, the extent of which becomes more pronounced at higher operating temperatures (toward the top of the windings)

Transformers Surveillance Following Corrosive Sulfur Remedial Procedures

Over the last decade, dibenzyl-disulphide was found to be present in a large number of mineral insulating oils. This sulfur compound tends to react with the copper of the windings, forming harmful byproducts, such as Cu2 S. Different techniques to overcome the problem, including metal deactivation, oil change, and chemical treatment, have been applied. The long-term effectiveness of these techniques is examined and compared