My Transfo 2006

Turin, the 18th and 19th October 2006


Interventions and abstracts



PROF. B PAHLAVANPOUR & DR M EKLUND, Nynas Naphthenics Ltd, Wallis House, 76 North Street, Guildford Surrey UK

Insulating liquid is used for cooling and insulation in electrical oil filled equipment. The function of the oil in a transformer, a part from cooling and insulation, is also to protect the insulating paper, therefore oil quality plays an important role in service life of transformer. International standard, IEC60296, is used for purchasing unused mineral insulating oil in electrical industry. It is one of the most widely used standards for supply of oil in the electrical industry. The previous issue was revised and published in 1982 and since then quality of the oil and expectation for performance of the oil in service has improved tremendously. Although it is well established, the limits and expectations are very low, therefore many low quality oils may pass the requirements of this standard. This shortcoming was realized by both users and producers, therefore during IEC TC10 general meeting in 1998, it was decided to revise this standard. The standard was revised and published during 2003.
For reliable operation of oil-filled electrical equipment, monitoring and maintenance of insulating liquid is essential. Mineral insulating oil is the most widely used insulating liquid for cooling and insulation in oil-filled electrical equipment. The characteristics of the oil, supplied as unused, may change during service life. Therefore, the oil quality should be monitored regularly during its service life.
IEC60422 is guide for supervision and maintenance of mineral insulating oils. This standard is now revised to take account of changes in oil and equipment technology and has due regards, for the best practices currently in use, worldwide. Changes are also made to use current methodology and comply with requirements and regulations affecting safety and environmental aspects



Contamination of paper tapes by corrosive sulfur in insulating oils may cause shorting faults between turns. Typically, this occurs at higher temperature, but into the range of project specifications, in the upper portions of the windings of shunt reactors and power transformers. In many of tested oils high amount of Dibenzyl-Disulfide (DBDS) was found.


R. Maina, Sea Marconi Technologies sas

The life time of electrical equipment begins with its project and ends with its dismantling and disposal. During that long time (it is expected to be long!) the roles of the oil are manifold: used as cooler for heat dissipation and, obviously, as insulating liquid, it plays as vehicle of informations. Being a transformer a sealed machine which is very rarely opened for internal inspection, the oil acts as equipment’s «blood»: many of its parameters can be monitored to investigate the degradation of the active parts (conductor and insulators), or to detect electrical and thermal faults before they lead to a major failure.
The characteristics of oil are related to its efficacy in cooling, insulating, gas absorbance, capability to solve moisture and sludge, etc. The initial choice of the type of oil is of paramount importance for equipment’s lifetime: the oil will be in contact with all the materials, copper, papers, pressboards, gaskets. Its reactivity (which should be the lowest possible) is as important as its insulating properties, even because they are linked.

The properties of the unused oil should be carefully evaluated in relation to the application. Type of equipment (power, cooling, design, etc.), type of solid insulation, climate, expected loading are factors to be considered. The choice of inhibited oils may be the best in some cases. The corrosiveness of the oil has become a fundamental problem in last years, due to the presence of corrosive sulfur compounds.
Testing the oil is an activity that begins during the supply of the transformer. The oil used for factory tests, or the oil used for windings impregnation should be checked for undesired contaminants. The oil batches used for in-field filling may undergo a quality control.
Once the transformer is installed the oil must match specified requirements prior to energization, to ensure that its insulating properties are capable to bear the electrical load.
After energization the type and frequency of oil analysis depends on several factors: age of equipment, age of oil (they could be different!), degradation status, etc. During the first 24 months of life the probability to have a failure due to project or manufacturing gaps is higher, and the monitoring should be more frequent.
The degradation of different characteristic of the oil has different rates: sludge and acidity have normally a slow increase (also depending on the inhibitor content), moisture may increase more quickly. The frequency of control of some parameters may vary depending on the age of oil of the age of equipment (2-FAL or moisture). In some cases, when a probable fault is detected, sampling frequency may increase up to daily analysis.
Oil monitoring is a way to last equipment’s lifetime, reducing the degradation rate: maintenance on the oil should be done before its propertied become too bad, reducing the accumulation of sludge and polar compounds. The priority of maintenance actions is normally defined by comparing the degradation rate of oil and equipment. Approaching the end of life of the transformers oil’s monitoring can be a tool for evaluation of consumed thermal life. The comparison of thermal degradations on a fleet of old transformer helps in defining the priority of change. Oil analysis is an important tool even in post-failure investigation, or after the end-of-life: they may help in discovering the cause of a fault or they are an important source of information for the maintenance of other equipment with the same characteristics.


PROF. ANTONIO BOSSI, Università Degli Studi Di Pavia, Chairman Of Cenelec Tc 14

The following report refers to some new tests that are carried out on large power transformers in occasion of the factory tests or as diagnostic tools during service.

The attention have been paid to:
–        measurement of winding hot-spot during temperature-rise test;
–        analysis of in oil dissolved gases during temperature-rise test;
–        transferred overvoltages on separate and regulation windings;
–        frequency response analysis (FRA) of the windings



Insulating oils represent an important component for the reliability of electrical equipment containing them; a poor quality of the oils, the loss of the original characteristics or a considerable contamination can lead to the loss of the equipment within timeframes considerably shorter than those considered during the design.
A constant inspection of the properties and characteristics of the oils is a simple ed effective instrument to monitor the health conditions of the equipment, in particular the most stressed, but inaccessible parts, such as windings, core, solid insulations (cellulose).
Thus, the maintenance and the recovery of the properties of insulating oils represent fundamental elements in the maintenance policy of electrical equipment, since from them depend in a determinant manner, the availability and the operational efficiency, as well as the extension of the life cycle of the equipment.

Transformer Economic Management : Repair vs Replacement

Pierre Boss, ABB Sécheron SA, Geneva/CH

Transformer managers are regularly expected to make this decision with respect to failed or troubled units. Besides the substantial technical and financial data specific to the transformer in question; demographics, condition, utilization and performance of the transformer population should be taken into consideration. These decisions require also intimate understanding of corporate risk tolerance, current investment strategy, prevailing business and regulatory environment.



Possibility of a transformer event?
Since 1996, more than 80 evens have been associated with large main and auxiliary, or start-up power transformers. Several, such as the Pheonix Power and light and Edison Mission events, had significant station impact as well as distribution. More than 40 reactors shut downs in nuclear plants, as well as numerous fossil plant shutdowns, and several power reductions have associated with the transformer events. Lost production based on the last six losses of 450 Mva and larger units in North America. Many resulted in lost production in excess of $102,000,000 and repair costs in excess of $39,000,000.


HUGO OSCAR PISANI, Electrical engineer, Transba s.a., Argentina


Every time we decide to intervene on a critical equipment (transformer filled with mineral dielectric insulating oil) we must examine the situation within the contest in which it operates, the operational conditions, the operational limitations or other conditions, the residual life.
Under this point of view, the treatment acquires an added value not only for the perspective related to the decontamination, per-se, but also fro a substantial improvement of the dielectric and physical conditions of the oil and the concomitant de-sludging of the contaminants present in the active part, obtained by exploiting the conditions of circulation of the oil, acting as a natural solvent, inside the transformer.
The possibility of performing the treatment with the transformer in operation, makes this process exceptionally effective, since the frequency proper of alternate current applied on the windings and the core, generates small amplitude vibrations, maintained through time, favouring in an evident manner the exchange and the migration of all contaminants, such as PCBs, sludges and water.
The efficiency of the decontamination process has been demonstrated by periodic tests and the traceability of the same, performed “a posteriori” on each equipment treaded over two years of work.

Nº protocol at 360 dd
Date Sampling
Cegelec 13717
1.9 ppm
Ansaldo 25632
1.9 ppm
TTE 36911
No quant.
Cegelec 9805
No quant.
Cegelec 11183
No quant.

No contamination even after one year from the treatment– Not quantifiable at 1 ppm according to ASTM 4059

In summary, a method for the circulation of the oil in a continuous mode has been developed ensuring the normal operation of the transformer to be treated, keeping it energised and o-load, during the treatment.

The process is effective and is traceable through the monitoring chain provided by homologated laboratories complying with the legislation in effect in Argentina.

Modern On-line DGA Monitoring

D. Bidwell, USA, Serveron Corp

It is worth considering some of the high level issues that plague this industry as they relate to on-line DGA – not only in the US but also in Europe and elsewhere as liberalization becomes the norm (Fig. 1). We are at a time with increasing demand for energy, but we are also in a time where the infrastructure is aging and this is especially so with transformers. We will see later that average transformer life is well in excess of 30 years and is rapidly approaching the 40 year mark. This obviously means that there are transformers on the network which are at the end of their life expectancy.
The industry generally has suffered from a lack of capital investment, although we do see that trend reversing in certain countries and networks. With liberalization and privatization of many markets so comes the intense pressure to reduce budgets for operation and maintenance purposes. These issues are creating a strain on all grid assets and are a real worry. Added to this is the obvious brain drain which is occurring as expertise which has been so long in developing is now leaving the industry. Existing personnel is being stretched.


ROBERTO CAMPI, F L   Selenia   SpA, Divisione Rondine, Pero – Milano (Italia)

This paper tries to provide an updated landscape concerning the various kinds of products suitable to be used as high-temperature insulating liquids in the oil-immersed transformers.

From the chemical point of view these liquids belong to very different kinds of families, but till now all types can be listed as organic products.

The most relevant characteristic of all these chemical families is the possibility to be used at temperatures even significantly higher than those allowed for the standard mineral oils. From the different chemical composition also depends the possibility to choose the most suitable product for any specific application. The following paragraphs will underline the peculiar characteristics useful to suggest the best choice.
In any case it is necessary to consider that these products cover, all together, a small percentage of the total market demand of insulating liquids for oil-immersed transformers. However the market share of these products is growing in the high-temperature application.
This paper recalls the main characteristics of silicones, special hydrocarbons, organic synthetic esters and vegetable esters and gives some more details about the performance possibilities of the new class of hydrocracked hydrocarbons


V. TUMIATTI VANDER (*Ass. Secretary IEC TC 10), M. TUMIATTI, R. ACTIS, C. ROGGERO, S. MAINA, Sea Marconi Technologies s.a.s.

S. KAPILA – University of Missouri Rolla – USA

Corresponding author to:
The “Corrosive Sulfur Risk” and Countermeasures for Transformers and Oils: the “DBDS & Corrosion Free” Program and IFED (Integrated Fingerprinting and Elemental Diagnostics) applications

The “Corrosive Sulfur Risk” involves directly all transformers and equipment in mineral insulating oil (reactors, load tap changers, bushes etc.) used by generation, transportation, distribution and use of electricity.
The cases of damages occurred at international level, caused by “Corrosive Sulfur” indicate a considerable increment of the phenomenon starting from 1992-1995. These failures are correlated with some typologies of oil that, since the same years were subject to radical changes in the refining and/or blending process.
The new IFED (Integrated Fingerprinting & Elemental Diagnostics) techniques developed mainly in Italy by the co-operation between Sea Marconi, Terna and University of Missouri Rolla USA, provide the identification and quantification of specific compounds and/or corrosive contaminants responsible for these failures.
The discovery of the main corrosive contaminant in the oils, designated as DBDS (DiBenzilDiSulfide), in typical concentrations of hundreds of mg/kg, allowed a better study of the countermeasures and the interaction mechanisms involved by the failures (Copper + DBDS) leading to the formation of copper sulfides on the naked copper conductors, on the paper covered conductors and on the contacts.
A new innovative process for Selected Depolarisation (Chedcos® by Sea Marconi) has been developed and validated at industrial level, provided by a multi-function mobile unit DMU-D5 on-site, in continuous mode and closed circuit on the equipment, without requiring even a partial draining of the oil charge (with the option of operating under energised load) capable of eliminating for good corrosive contaminants from the oil and equipment, classifying them “Non Corrosive” and reinstating the functional features prescribed by Standard IEC 60422 Ed.3 2005-10.
This paper presents the Countermeasures for Transformer in Oil ”DBDS & Corrosion Free” program to be implemented to prevent and/or mitigate the Corrosive Sulfur Risk on fleets of equipment in oil (new and in operation) in accordance with the “Best Available Techniques (BAT)” for their most effective Life Cycle Management (LCM).


K. Anderson 1, R. Seemamahannop 1, P. Nam 1, S. Kapila 1; V. Tumiatti 2, C. Roggero 2, S. Maina 2, M. Tumiatti 2
1Center of Environmental Science and Technology, University of Missouri-Rolla, MO 65409
2Sea Marconi Technologies, Turin, Italy

Presence of sulfur in mineral insulating oils and its detrimental effects on electric power equipment such as transformers have been a matter of concern ever since the introduction of mineral oils as dielectric coolants. Over the years a general consensus has emerged between equipment manufacturers, mineral oil suppliers and the electric power utilities that to avoid premature failure of electric equipment sulfur content in mineral insulating oils must be maintained at very low levels.

A variety of techniques have been used for speciation and quantification of sulfur in mineral insulating oil. However, due to complex nature of mineral oils and sulfur species present in petroleum oils quantification of “corrosive “sulfur remains elusive. Official methods described by the standard setting bodies still rely on rather empirical approach of “Eye – balling“. This article provides an overview of current techniques for finger printing sulfur species in mineral insulating oil and presents an Ion Chromatography approach for quantification of cuprous sulfide (Cu2S) – the major by-product of corrosive interaction between sulfur and metallic copper.


MASSIMO POMPILI, IEC TC 10 Secretary – Presidente CEI CT10, Dipartimento Di Ingegneria Elettrica Università Di Roma “La Sapienza” – (Italy)

I Policlorobifenili (PCB), miscela di sostanze di sintesi nota in Italia con i nomi commerciali di Apirolio o Askarel, è stato largamente utilizzato in tutto il mondo per circa 50 anni come isolante nei trasformatori e in altri apparati elettrici. All’atto della sua prima comparsa sul mercato, rappresentò infatti una valida alternativa agli oli minerali, essendo stato definito come una sostanza ininfiammabile……..


Victor Sokolov,

Many transformer manufacturers have been considered useable life of a large transformer 25-30 years. Relevant calculations and life-tests models based on simulation rated stresses and reasonable short circuit and overvoltages stresses supported anticipated life span. However a huge amount of equipment shows no symptoms of significant degradation and a low failure rate after 30 -40 and even 50 years. The question comes: why they live such a long life. One explanation is that that in fact a few transformers operate continuously under rated stresses. If e.g. average load doesn’t exceed 70% (50 % losses) one can reasonable expect doubling of calculated life.

However it might be another scenario: process of degradation is progressing but only by different manner that just thermal deterioration of cellulose insulation, and diagnostic tools available are not effective to some aging –mode defects. Eventually accumulated changes would explode all of a sudden in cascade-mode failures.
Factual information about real changes in transformer one can obtained only my means of direct study of the condition of aged material, what unfortunately possible only after disassembling of transformer active part. Thus, any or post mortem inspection is becoming a vital procedure.
A failure is usually a “tuning fork” of Life Management procedures.
Failure analysis delivers a key information allowing to determine “what happened” and “what to do” in terms of managing network reliability , risk assessment, maintenance optimization, estimation end of life, and on the other hand improving design and manufacturing of an equipment.
Failed transformer brings lots of problems associated with direct and indirect costs.
However it may return significant amount of money back if its failure mode and failure cause learned properly. It could become a powerful teaching aid, which would save a number of sister units and suggest an efficient strategical program for life assessment and life extension of operating population. Proper postmortem inspection may bring multilateral benefit and first of all that permits performance of full-scale design review.