The fourth European edition of the My Trasfo Meeting , the International point of reference for the exchange of know-how and experiences on the management of fleets of transformers with insulating fluids.
The 2008 edition , to take place in Turin on 17th and 18th December, shall be rich of novelties and surprises, although the point of force of the past editions shall be maintained, such as the cooperation with lEEE Italy Section Power Engineering Society (PE31) Chapter. As for the novelties, besides a general updating session on norms, latest technologies and new frontiers of diagnostics, four theme areas shall be developed, each one of which is divided into Tutorial, Case History and Forum, all of them provided by the most important international experts…
Insulting fluids's diagnostic - risk assessment approach
Mr. Massimo Pompili, University of La Sapienza, Rome; Secretary IEC TC10, IT
Maintenance and diagnostic of transformers in service are largely done through the oil analysis. According to the actual Standard, like the IEC 60422, the associated test results may be accepted if they are comprised into some prefixed limits. Anyway it is important to note that these acceptable values for the prescribed tests for the insulating mineral oils are different and more severe with respect to the type and the importance of the electrical equipments.
In the present contribution a new possible diagnostic approach is presented, based on the risk assessment of the insulating liquids. Also some preliminary values of tolerable risks are depicted in function of the asset of the equipments, of the total entity of damages and of the conditions of installation.
Presentation of CIGRE SC A2 Transformers - Technical Developments and Inputs from Current Activities
Mr. Pierre Boss, ABB Secheron SA, CH
CIGRE (International Council on Large Electric Systems) is one of the leading worldwide Organizations on Electric Power Systems, covering their technical, economic, environmental, organisational and regulatory aspects. More specifically, issues related to planning and operation of power systems, as well as design, construction, maintenance and disposal of HV equipment and plants are at the core of CIGRE’s mission. Problems related to protection of power systems, telecontrol, telecommunication equipment and information systems are also part of CIGRE’s area of concern. The aim of SC A2 ‘Transformers’ is to facilitate and promote the progress of engineering and the international exchange of in-formation and knowledge in the field of transformers and to add value to this information and knowledge by means of synthesizing state-of-the-art practices and developing recommendations.
A.I. Systems Implementation for Advanced Diagnostics about state of Degradation
Mr. Riccardo Maina, Sea Marconi laboratory Chief, IT
Artificial Intelligence systems are used to automate decisional processes. A general overview of the most important methodologies used in the field of A.I. is given: soft modeling, fuzzy logic, neural networks, genetic algorithms.
This paper gives some general rules for the application of fuzzy logic and neural networks to the development of expert systems for diagnostic of insulating oils and transformers assessment. Some practical examples are also described.
Dissolved water in oil: from Measurement Techniques to Diagnostic Utilization
Mr. Fabio Scatiggio, Terna laboratory Chief, IT
The presence of moisture in a transformer affects the transformer insulating performances by decreasing both the electrical and mechanical strength. The mechanical life of paper, that ultimately is the transformer life, is proportionally reduced by the presence of water content. Insulating oil has a low affinity for water, anyway the solubility increase the temperature. Water in oil can exist in three different chemical states: free, suspended and dissolved. When the water content exceeds the saturation value at a given temperature, there will be free water into the oil (as little drops or suspending).
Water in Oil: Effects on Oil and Transformer
Mr. John Lapworth, Doble, UK
Moisture in power transformer: effects on oil and transformer
John Lapworth(1), Richard Heywood(1), Paul Griffin(2) and Lance Lewand(2)
(1)Doble PowerTest Ltd, UK and (2)Doble Engineering Company, US
It is generally accepted that moisture in the insulation of power transformers is undesirable because of the various immediate and long term adverse effects on dielectric integrity:reduction in dielectric strength of oil
tracking in solid insulation
bubble generation in solid insulation during overloads
accelerated ageing of insulation
However, there is less agreement on the details of these issues, in particular: how to assess moisture levels in transformers, at what level moisture becomes a problem and how these problems can best be addressed.
In order to manage moisture related problems successfully, it is necessary to understand the details of the issues involved. This paper does not claim to provide complete and final advice about how moisture related problems should be resolved. Instead, the intention is to summarise the current state of knowledge regarding moisture in transformers from a practical rather than theoretical point of view and identify those areas of uncertainty which need to be resolved before moisture related problems can be tackled with confidence, and is an update of an earlier paper on the subject .
Water in insulating system of oil filled electrical equipment: de-hydration techniques of power transformers in service
Riccardo Actis – SEA MARCONI TECHNOLOGIES
Stephane Flet – EDF CIH
The presence of moisture in oil-filled electrical equipment is dangerous for different reasons: water may lower insulation resistance and breakdown voltage of the oil and may accelerate ageing of paper insulation. The evaluation of the water content in paper insulation is not easy; the moisture content of the oil may be used to have information on the moisture level in the cellulose. Several techniques have been developed in order to reduce the moisture content of transformerboard, based upon the simultaneous effect of vacuum and temperature. The direct application of vacuum in the transformer tank is the most effective but cannot be applied keeping the transformer in operation and could damage the equipment, if it’s not vacuum proof. De-hydration by oil circulation and heating (On-Load Treatment) needs a longer time but can be applied during operation at nominal load, without loss of production.
Lifetime extension and reduced maintenance time - Low Frequency Heating to speed up drying time
Mr. Pierre Lorin, ABB Sécheron Ltd, CH
Drying of power transformer is an object that has been worked on for many years. The main problem is to achieve an optimal drying of an aged power transformer without the displacement to a workshop, as this operation is very cost intensive and sometimes not possible any more. There is a wide range of possibilities known on the market. A new possibility is now offered with the so-called Low Frequency Heating system. This system and the process have developed over the time and are used in different countries. Over 40 power transformers with a power up to 400 MVA were dried successfully on site. It has been shown by long term measuring that much better drying effect could be achieved than with the other known processes.
The latest development is the combination of the LFH process with other known processes like the hot oil spray process. With such a combination the needed drying time could be again reduced and the achievable moisture values were considerable and are laying below 1% H2O that means in range of the original values in the production. Thus the ageing velocity of the insulation material could have been reduced massively and so the lifetime prolonged.
Mr. Riccardo Maina, Sea Marconi laboratory Chief, IT
This paper describes some corrosion phenomena involving copper, insulating oil and solid insulation of oil-immersed transformers. Both copper sulfide deposition and dissolved copper formation are investigated, giving evidences derived both from field experience and laboratory tests.
A review of the available mitigation techniques to reduce the effects of corrosion phenomena is here described, with regards to their effectiveness.
Development and Evaluation of an Integrated Accelerated Aging Test for Insulating Mineral Oils and Natural Ester Fluids
Mr. Shubhender Kapila, University of Missouri Rolla, USA
An Accelerated Method for Quantitative Determination of Corrosive Sulfur in Mineral Insulating Oil is introduced. The method involves the conversion of sulfide resulting from reaction between corrosive sulfur species and metal sulfate and determination of sulfate with ion chromatography (IC). The method offers a quantitative assessment and eliminates ambiguities often encountered with the standard methods for corrosive sulfur in mineral insulating oils. Standard test methods for assessment of corrosive sulfur in mineral insulating oils invariably rely on empirical approaches that center around “eye-balling” test specimens and comparing against standard color charts. Ambiguities can and have been observed stemming from differences in color perception of analysts and oil sample condition.
Corrosion Phenomena: Brazilian Case History
Mr. Claudio Marchiori, Terna Participações, BR
The nature and causes of corrosive sulfur induced failures are examined in oil-filled shunt reactors. Corrosive sulfur compounds, which are been found to be present in some mineral oils, reacts with copper to form “semiconducting” copper sulfide. Using a combination of controlled laboratory corrosion tests and of sulfur speciation analytical techniques, it was shown that the corrosive properties of oils were caused by a single sulfurorganic specie which was identified by GC/MS (gas chromatography / mass spectrometry) as DBDS, dibenzyl disulfide. DBDS is a commodity chemical well known as a powerful extreme pressure additive in boundary lubrication conditions. DBDS is also an effective oxidation retardant additive, a property which accounts for its presence in uninhibited transformer oil. Pleads of native DBDS from crude oil are unseemly as shown by its easy removal by any known refining technology.
DBDS reacts easily with copper surfaces with complete conversion to copper sulfide, even over the temperature range of operating transformers. Its corrosiveness towards copper is reduced significantly by metal passivators, but passivators do not reduce or eliminate DBDS and their long term effectiveness in preventing DBDS related corrosion is unknown. Copper passivators, also, are unable to protect sulfidized copper as demonstrated by the unabated failure rate of reactors in Brazil following passivation.
In parallel to conventional electric test (PF, tip-up and SFRA), dielectric diagnostic based on time and frequency domain (RVM, PDC and FDS) tests are being investigated for usefulness in detecting corrosive conditions in progress and, possibly, quantifying the extent of contamination by conductive copper sulfide.
Tests were conducted both in the field and at the factory, the available ones from field are difficult to manage because the negative influence of live lines. Even results from factory seem to indicate the absence of distinctive response patterns useful to isolate contributions of sulfide particles from moisture and polar contamination.
Another approach that is being considered with a view to setting priorities relies on the decay of DBDS concentration.
The failure mechanism is connected with the increase of dielectric losses in CuS2 contaminated paper. When the heat generation will exceed the amount of heat transmission to the surrounding medium, thermal instability will result; the magnitude of the electrical current at the hot spots will increase until arcing results and short circuits will develop between the coils and cause the final breakdown.
Thermal Profile of Transformers and their relevant Papers Aging Phenomena
Mr. Tim Gradnik, Elektroinstitut Milan Vidmar, SLO
Abstract: Evolution of the theories and models for ageing estimation of power transformers with paper-in-oil based insulation system is presented in the paper. Up-to-date transformer thermal ageing model is presented and correlated to EIMV experiences in physical-chemical diagnostics of Slovene transformers. The winding hot-spot temperature, one of the most critical parameters that affect the technical lifetime of power transformers, is usually determined by calculation and indirect temperature measurements. The calculation procedure is normally based on a standard temperature model that depends on the type of transformer cooling system. Thermal profiles for commonly used transformer cooling systems and relevant temperature models are discussed as well as their deficiencies that affect the accuracy of thermal ageing models.
Keywords: Power transformer, ageing estimation, hot-spot temperature, temperature model, thermal profile, standardization.
New insulating fluids for transformers based on biodegradable high oleic vegetable oil
Mr. Pierre Boss, ABB Secheron SA, CH
This paper presents an overall view and results of a development project, fully developed by ABB Brazil to CEMIG (Companhia Energética de Minas Gerais), one of the largest Brazilian electric power utilities, dealing with the application of a new transformer insulating fluid in a high voltage power transformer. The new insulating fluid developed from vegetable oil sources is described. Its development required the selection of a suitable base oil, its purification to electrical grade, and its stabilization by antioxidants. The new fluid, designated BIOTEMP®, is fully biodegradable and performs well at elevated temperatures. It is intended as an environmentally friendly fluid for use in transformers located in areas where oil spills and contamination of the surroundings are of concern.
Power Transformer, Natural Ester, Vegetable Oil, BIOTEMP®.