Workshop Directors Committees Organisers
Welcome Venue Hotels The Royal Village of Baiona
 
 

S_p_e a k e r s _________________________

 

Following are ARWtr2007 key Speakers, who will deliver below Lectures on transformer key topics.

 

 1* Janusz TUROWSKI  (CV)

    Institute of Mechantronics (IMI)

     Technical University of Lodz-Poland

 

 2* Xose M. LOPEZ-FERNANDEZ

    Department of Electrical Engineering

     University of Vigo-Spain

 

 3* Adolf J. KACHLER  (CV)

    Ex-Director of Siemens Distribution&Power Transformers

    Transformer Consultant in Testing, QM and Diagnostics

    Nuremberg-Germany

 

 4* Ryszard MALEWSKI  (CV)

   Power Transformer Consultant

    Electrotechnical Institute of Warsaw-Poland

 

 5* Greg ANDERSON  

   Consultant, Represetative and Officer of IEEE Transformer Committee-USA

 

 6* José PENEDOS 

   Chairman of CIGRE Portugal National Committee

    Execitive Director of REN - Portugal

   (Rede Eléctrica Nacional- Responsible for the electricity transmision in Portugal)

 

 7* Miguel OLIVA  (CV)

   Chairman of CIGRE A2 Spanish National Transformer Working Group

    ABB, Asea Brown Boveri S.A.- SPAIN

 

 8* S.V. KULKARNI  (CV)

   Author of Transformer Engineering: Design and Practice book 2004

   (1990-2001) Transformer Crompton Greaves Limited- India

   Indian Institute of Technology, Bombay-India

 

 9* Hugo GAGO GARCIA  (CV)

   Responsible for IBERDROLA  Equipment Specification and Homologation, Spain

   (IBERDROLA - Group of Energetic Spanish Companies)

 

10* Donald T. ANGELL

   Director of Substation Engineering

    National Grid USA

 

11* Bernhard HEINRICH  (CV)

   WEIDMANN Electrical Technology AG, Switzerland

  

12* Stanislaw GUBANSKI  (CV)

   Research Manager of High Voltage Valley- Ludvika, Sweden

    Dept. of Materials and Manufacturing Technology

    Division of High Voltage Engineering

    Chalmers University of Technoogy, Sweden

 

13* Ernesto PEREZ  (CV)

   Quality Responsible for UNION FENOSA Power Transformers, Spain

   (UNION FENOSA - Energetic Spanish Company)

 

14* Harold MOORE

   Transformer Consultant

    Harold Moore and Associates-USA

 

15* Elzbieta LESNIEWSKA  (CV)

   Technical University of Lodz, Poland

 

16* Jeewan PURI   (CV)

   Chair of Audible Sound and Vibration IEEE Subcommitee

    Consultat at Transformers Solutions Inc., Mathews-USA

 

17* Thomas FOGELBERG 

   Corporate Executive Engineer, ABB AB, Sweden

    Former Technology Manager for ABB Power Transformers globally

 

18* Pierre BOSS    (CV)

    Chairman of CIGRE SC A2 Transformer Working Group

 

19* José C. LOPES  

    Fluidinova, Engenharia de Fluidos S.A. Porto, Portugal

 

20* Paulo GOMES  

    Fluidinova, Engenharia de Fluidos S.A. Porto, Portugal

 

21* Andrea SOTO-RODRIGUEZ  

    EFACEC Energy - Transformers Factory, Porto, Portugal

      David SOUTO-REVENGA  

    EFACEC Energy - Transformers Factory, Porto, Portugal

 

L_e_c t u r e s & Abstracts _____________

 

Following are ARWtr2007 Lectures, which will be delivered by the above Key Speakers.

 

** 50 to 60 Years of Transformers Engineering and a Perspective of the Future

 

** Presentation of CIGRE A2 “Transformers”

     Technical Developments and Inputs from Current Activities

 

** Reliability of Power Transformers - Important Aspects to

     Drive the Industry to Focus on Quality

 

 

** Reliable Diagnostics of HV Transformers Insulation for Safety Assurance

     of Power Transmission System REDIATOOL - an European Research Project.

 

** Necessities for Power Transformer Operation and Maintenance

     for Successful Life Management

 

** Quality assurance for Power Transformers. The point of view of an Utility

 

** Stray Losses Control in Core and Shell Type Transformers.

     Upgrading of Energy-Saving and Reliability of Large Transformers

 

** Sources, Measurement and Mitigation of Sound Levels in Transformers

 

** Laminated Pressboard and Laminated Wood Products in Power Transformers

 

** Detection of Transformer Winding Displacement

     Using Frequency Response Analysis  (FRA) Method

 

** Application of Coupled Field Formulations for Analysis

     of Complex Phenomena in Power Transformers

 

** Development, Innovation and Transformer Solutions

 

** Improvement of Transient State Parameters

     in the Desing of Protective Current Transformers

 

** Infrastructure of Electrical Power Transportation Network.

 

** Improvement in Reliability of Power Transformers base on Changes in Specification

 

** Large Power Transformer Cooling – Flow Simulation and PIV analysis in an

     Experimental Prototype

 

** Studies in a Large Power Transformer – Heat Transfer and Flow Optimization using CFD

 

 

Abstracts-L_e_c t u r e s ______________

 

** 50 to 60 Years of Transformers Engineering and a Perspective of the Future

 

** Presentation of CIGRE A2 “Transformers”

     Technical Developments and Inputs from Current Activities

 

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.

 

** Reliability of Power Transformers - Important Aspects to

     Drive the Industry to Focus on Quality

 

The global demand of power transformers has been more than doubled the last 7-8 years. From about 500.000 -600.000 MVA during end of the 1990s the market has raised to roughly 1.400.000 MVA this year. Some trends are pointing on a continuous increase. The capacity of transformer production is running on 95 – 100 % which also includes the whole supply chain of transformer materials and components. A similar situation was after the Second World War, 60 years ago, when the European and American markets were covered by domestic suppliers which invested in full capacity in accordance from the need of fully state-controlled utilities and power companies working with price formula. At that time 400 kV to 800 kV AC installations were made and International Standards in IEC and ANSI were produced with huge intensity.

 

The first signals of change in the demand were shown in the beginning of 1980. After this decade the Electrical System Industry has gone through its biggest change since Edison’s and Westinghouse made their innovations.

 

The last 25 years have been characterized by a huge global consolidation on both the supply and user side of electrical equipment. A fully domestic business has changed to a fully global business with its consequences in both commercial and procurement matters.

 

The biggest change is during the last three years, 2004 to 2007, when huge regions in Asia, Middle East and South America have an immense demand of electrical energy. On top of this “the old world” has a need of re-investments of the current transformer fleet, 40 – 50 years old. Together with this energy demand, boosted by environmental concerns, the supply of some raw materials are not now more in the normal balance.

 

Those big market changes have impacted the Power Transformer industry with its suppliers, in how to design, and where to build with higher demand on Quality and Reliability.

 

This presentation will address that reliability issues for new transformers are more on the agenda than ever before. Some evidences for this statement will be brought up, taken into consideration the three times material price increase of copper and core steel, plus the very low loss evaluations caused by the high interest rates at the private utilities. The global competition drives most of the suppliers, under those conditions, to further stress the material utilisation.

 

That means that the industry must now more discuss how tenders shall be evaluated and in what way 50 years of transformer testing methodologies/accuracies still are adequate. A most relevant question is that if the next 40 years production of 300.000 power transformers will see a similar high reliability of today’s 400.000 power transformers?

 

Power Transformer industry is an “engineering to order” business where the real task is to manage all type of variances and cover up for risks. As the technical complexity is as big as fault consequences, technical weaknesses in management in both procurement and supply side can be of a very big negative impact.

 

Another aspect very often mentioned in CIGRE and IEEE discussions are the risk of a continuous deterioration of technical competence among both the supply and demand side.

 

Those risks were not so dominant in the 50s and 70s when today’s transformer fleet was built. Some negative quality signals have been given the last 10 years for some key material as transformer oil and core steel.  

 

Testing of new transformers is still the uttermost evidence of quality. Today’s designs with high material prices see more intensive material utilization with increased stresses.

 

Acceptance testing of Dielectric stresses is well covered by international Standards which have been developed during the years. To prove Thermal and Mechanical integrity of new large GSU and Intertie transformers is still a field where design and production weaknesses can pass without to be detected by traditional testing. Power transformers will see more of long distance sea transport with higher transport forces. It is evident that Short Circuit tests will be more important than earlier as the mechanical stresses now is increased. An improved thermal testing is now also under discussion in IEC . 

 

The presentation will mainly address ABB’s design, production, supply chain and testing philosophy in order to secure reliable Large Power Transformers from more Thermal and Mechanical aspects when now many old traditions and habits must be questioned.

 

** Reliable Diagnostics of HV Transformers Insulation for Safety Assurance

     of Power Transmission System REDIATOOL - an European Research Project

The lecture presents and summarises the results of the international cooperation. A general background for understanding the polarisation phenomena in electrical insulation is to be provided first, since the relations arrived are essential for understanding and processing the data obtained from dielectric measurements. This will be followed by a description of the principles of polarisation measurements and, in particular,  advices on how practical measurements should be arranged. Special emphasis will then be put on the description of how to interpret and model the results obtained. Examples of different field measurements will be described and analysed.

The knowledge presented is to be used by operators, manufacturers, service providers and scientists in their further work. Results of the investigations performed within REDIATOOL allow believing that the dielectric response measurements, when properly performed and interpreted, can provide more accurate information on moisture content in paper and pressboard in transformers that the use of conventional equilibrium curves.

REDIATOOL an European project, was initiated in 2003. The project involved collaborations among researchers and engineers from Sweden, Poland and Germany. The project in its part related to the evaluation of dielectric response methods concentrated on (i) investigations using laboratory models to improve calibration (interpretation of results) of the methods based on dielectric response measurements, on (ii) verifications performed on different types of the transformers sent for repairs, and finally, on (iii) gathering experiences from investigations of transformer insulation on-site. This work is now continued within a newly formed CIGRE TF D1.01.14 – Dielectric Response Diagnoses for Transformer Windings and aims to provide recommendations on how practical measurements should be arranged to give best possible data for conclusive diagnoses.

** Neccessities for Power Transformer Operation and Maintenance

     for Sucessful Life Management.

 

Successful Operations and Life Management of Power Transformers necessitate consequent surveillance, regular Condition Assessment (CA) and follow-ups by adequate maintenance. CA is multidimensional and reaches from Finger Printing, Trend Analysis, Statistical Data Analysis and finally also to Defect- and Failure Analysis.

 

Diagnostics and Maintenance/TLM must be based on

 

- good knowledge of the physical performance of PTs

- good knowledge of normal and abnormal behaviours

- good knowledge of the Defect and Failure symptoms/signatures

 

This lecture explains in Part 1

 

- the main ageing conditions in service

- the degradation phenomena of different temperature (pyrolysis and oxidative hydrolysis)

 

It also discuses the guidelines for qualification of defective conditions including tolerances and limiting values.

 

In Part 2 we provide ample experimental evidence for the necessities of TLM and successful field and test floor diagnostics and give guidance for acceptance criteria and for Finger Printing / Trend Analysis..

 

** Quality assurance for Power Transformers. The point of view of an Utility

 

The evolution registered by quality policies along the time are extremely important and has been turned out to be essential processes inside utilities. All tasks including in the transmission and the  distribution world must be traceable and certified even before than installations are in service. This is only possible with the introduction of skilled resources and expertise, depending on the state of the equipment, its reliability and its criticality inside the system.

 

In the last years, we have experienced many changes in the world of electrical distribution. Liberalization of electrical markets, the increase of the importance of the environmental care and the evolution of technology has resulted in new substation designs: in large cities, the obsolete conventional air substations are been replaced by compact and optimized substations with buried cable and  GIS. In power transformers conventional bushings have changed to power and removable plugs units. As we can see, aspects not relevant in the past take now tremendous  importance.

 

** Stray Losses Control in Core and Shell Type Transformers.

     Upgrading of Energy-Saving and Reliability of Large Transformers

Electromagnetic and magnetic (shunts) screens of solid steel elements belong to most popular tools of eddy current loss and hot-spot reduction in large power transformers. However improper arrangement of them can give a contrary effect. In the lecture, on the basis of plenty years of experience, theoretical bases and results of physical and mathematical solutions for different, industrially applied constructions are presented. Unfortunately, those, who wish to resolve practical, industrial problem, can not avoid complexity.

Plenty physical, structural and mathematical details play here predominant and decisive role. Only chance to create reasonably programs is to apply rapid, expert based packages (Fig. 1), like examined already RNM-3D. Specifics of the expert system is, that the more knowledge is implemented into the knowledge-base, the simpler and faster is the program.

Most of material in this lecture consists in the relatively sophisticated Knowledge Base, whereas program and calculation process itself is very simple, chip and rapid. Fundamental for analysis was Maxwell theory, applied to elements consisted of metal and dielectric.

 

Especially within the recent research carried out in cooperation with EFACEC Transformer Works in Porto, Portugal. This work is a next development of the works from the previous Workshop ARWtr’04. (http://webs.uvigo.es/arwtr04). One of new important contributions is delivery the mathematical solution and formulae for width and thickness of stripped laminated shunts and external packages of main magnetic core , to avoid local excessive heating at the edges of windings. It was presented methods of design of stepped shunts in complicated clamping element.

 

** Sources, Measurement and Mitigation of Sound Levels in Transformers

In modern communities, due to increasing density of residential housing near substations and transformers, there is an increased prevalence of local ordinances specifying sound levels at commercial and residential property lines. Therefore it is appropriate that a good understanding of sources, measurement and the mitigation options of sound energy radiated by transformers be developed for properly specifying sound levels in transformers. A good understanding of these principles can help us minimize the environmental impact of transformer noise on neighboring communities.

 Transformer cores were recognized as major source of sound levels in transformers.  However, due to the increased demand for lower sound levels in transformer, winding noise has become a significant component of the radiated sound energy. The presence of current harmonics in modern load configurations has also become an important consideration in designing low sound level transformers.

 The demand for low sound levels has added new complexity to the measurement process of the radiated energy for transformers.

 This tutorial will discuss major sources of sound energy radiated by transformers. The influence of winding vibrations on sound levels will also be described. Approaches toward quantifying the effects of current harmonics on sound levels will also be discussed. Mitigation of sound levels in new and existing transformers will also be discussed. 

In recognition of the demand for low sound levels, IEEE and IEC Standards have added more advanced and accurate Sound Intensity measurement methods to their documents.  This tutorial will discuss the sound measurement techniques adopted by the IEEE and IEC Standards. The guidance provided by these standards toward minimizing the measurement errors and quantifying the influence of radiated sound energy on the neighboring communities will also be discussed.

** Laminated Pressboard and Laminated Wood Products in Power Transformers

 

In oil-cooled power transformers, a substantial part of the solid cellulosic insulation consists of thick solid blocks, beams, rails and clamping rings or plates. Over the years, the use of laminated wood (plywood) products as an alternative to laminated pressboard has become popular, despite some difficulties these materials may entail. Comparing laboratory tests have been conducted in order to demonstrate the differences between the insulating materials, and this paper discusses the results from investigations of dielectric, mechanical and chemical nature. The AC dielectric behaviour was studied with partial discharge and breakdown measurements on different samples immersed in transformer oil. The aging characteristics were investigated in extensive, long-term accelerated aging test series. It was found that due to the nature of manufacture, laminated wood properties are with regard to important features – as dielectric and mechanical strength, acids and sludge formation - noticeably inferior to laminated pressboard.

 

 

** Detection of Transformer Winding Displacement

     Using Frequency Response Analysis  (FRA) Method

Ageing power transformers create an increasing risk of in-service failure due to winding displacement by a dynamic force due to short-circuit current in the network. Cellulose looses its elasticity after an extended period of operation at a high temperature, and the initial pressing force applied to the winding disappears gradually with the transformer age. Windings are then prone to displacement or deformation, since the dynamic force may exceed the winding withstand strength initially assumed by the designers.

This force tends to lift coils close to the winding end, and to produce hoop-stress that bends coils in radial direction. In consequence, the insulating system dielectric-strength is reduced, since the dimension of oil gaps has changed in unpredictable way. In many cases the transformer does not explode immediately after such displacement, but keeps operating at the rated voltage, owing to a generous safety margin applied by the designers of winding insulation. However, a lightning or switching overvoltages may break the weakened insulation and trigger a major transformer failure in service.

Such failure can be predicted in statistical terms only, since the transient overvoltages have random character. However, an accumulated experience shows that transformers with thermally aged insulation fail more frequently. Utilities tend to operate older transformers as long as it is possible, but would like to avoid unexpected failures in service. There is growing interest in development of an early-warning method that would reveal the actual condition of the transformer insulating system, and provide an alarm when the risk of failure becomes too high.

A deformed winding can be detected by measuring its frequency response, and checking for a change of the winding natural resonance-frequencies with respect to the response records taken on a new unit in the factory. Unfortunately, such reference frequency-response has not been measured on older transformers that are prone to winding displacements. In absence of the reference, “finger print” frequency response it is possible to compare response of three phase windings, or the respective windings of twin transformers.

Specialized manufacturers offer winding frequency-response recording-instruments, and CIGRE working group A2.26 has initiated an activity to standardize the instrument characteristics, as well as the measuring technique. The frequency range required to detect winding deformations, shorts between parallel conductors, and displacement of the winding leads, extends from power frequency to 1 MHz, and even a few MHz in physically smaller transformers. The dynamic range required to span peaks and troughs of winding frequency-response ranges from 100 to 120 dB. Measuring cables have to be attached with low-inductance jumpers to the HV bushing top, and an appropriate grounding of the instrument is of paramount importance to take reproducible winding-response records.

A comparison of a few makes of the winding frequency-response measuring instrument has revealed two main types: sweep-frequency and low-voltage impulse recorders. The first type applies to the examined winding a sine voltage of a fixed magnitude, and frequency controlled over five or six decades. The applied voltage, as well as the winding neutral-terminal current, is recorded, and their quotient is calculated and displayed. The low voltage impulse is applied to the winding under test, and recorded simultaneously with the resulting transient-current at the winding neutral-terminal. Frequency spectrum of these two signals is then calculated and the winding response is derived from these two spectra. Theoretically these two measuring methods are equivalent, but in reality the signal to noise ratio of sweep-frequency instruments is more favourable.

 Utilities of industrialized countries have included the winding frequency-response analysis (FRA) in the transformer maintenance and commissioning tests. An initial “finger print” response is recorded on each new transformer during acceptance tests, and repeated after commissioning in substation. Comparison of such records may reveal winding displacements caused by shocks during rail or road transportation. Subsequent measurements taken at the occasion of periodic tests in service provide important information on the winding mechanical integrity, or indicate necessity of an internal inspection of the transformer.

The measuring technique and instrumentation has approached the stage of standardization, but interpretation of the difference observed between compared records represents still a challenge to the engineers in charge of transformer maintenance. Development of transformer electromagnetic-model that covers the frequency range from e.g. 100 Hz to 1 MHz has been initiated by a few research centers, but till now no one can claim an acceptable accuracy of the simulated winding response. Initial attempts to represent three-phase transformer using two-dimension model failed to take into account influence of delta-connected windings. A more complex, three-dimension models are fairly complex, and calculation of winding-section parameters: series and parallel capacitance, frequency-dependent self and mutual inductance and resistance requires specialized programs and high-performance computers.

At present the practical experience gained by identification of deformed windings in transformers in service provides utilities with valuable clues how to read and interpret the winding response. However, the final assessment depends on experience and skills of the expert analysing the frequency response recorded in different configuration of the measuring circuit.

** Application of Coupled Field Formulations for Analysis

     of Complex Phenomena in Power Transformers

 

Aim / Objective: The lecture explains certain aspects of complex transformer behavior using advanced field-circuit coupled formulations.

 

Starting Point: Current research trends indicate increasing use of coupled field formulations for complex design problems. Although the principle of operation of transformer is quite straightforward to understand, some of the phenomena need the application of coupled field formulations. Advancements in computational algorithms and facilities provided the necessary infrastructure to tackle these problems.

 

Purpose: Some practical and very relevant problems have been solved by using coupled field formulations. The problems and issues thereof are related to important design aspects of the transformers, viz. losses, short circuit forces, inrush currents and temperature rise.

 

Methodology / Approach: Nonlinear, transient, field-circuit coupled approach is used to solve the problems. Special attention is given to computational economy whenever required. Some geometrical simplifications are used, so that 2-D formulations can be used with reasonable accuracy for a particular type of problem.

 

Findings:

1. Methodology and results of theoretical and experimental investigation of core loss during the load loss measurement test of large power transformer, which takes place when winding leads are taken out from different sides of the core, are presented . The phenomenon is called as half-turn effect.

 2. Sympathetic inrush currents, in the case of series and parallel connected transformers, are investigated. The parameters affecting the magnitude and duration of the sympathetic inrush current are also studied in detail.

3. Currents, magnetic fields and electromagnetic forces in split-winding transformers under short circuit test conditions are analyzed. The short circuit forces are compared for two cases, viz. one winding short-circuited and both the windings short-circuited.

 

Originality: All the considered problems are solved using the field-circuit technique for the first time to the best of author’s knowledge.

 

Conclusions: The lecture will analyze in depth some complicated problems in transformers for which coupled field computations are essential. The results presented would be quite useful and relevant to transformer researchers and designers.

 

** Development, Innovation and Transformer Solutions

Energy efficiency, environmental care, reliability and secure supply of electricity are some of the main areas of interest in the energy industry. The transformer industry needs to address those topics considering new developments and innovations and providing practical solutions. 

The lecture will present some particular ABB innovations and developments in the transformers field to improve efficiency by reducing losses, to reduce noise levels and to produce dry type transformers of higher voltages with environmental, safety and efficiency advantages. The presentation will also address different transformer solutions related to reliability and the secure supply of electricity including polytransformers and universal transformers, on-field repair and upgrading of power transformers and high voltage testing on site. 

Some examples of transformer innovations and developments will be presented:

- Load losses reduction by using 3D magnetic field analysis tools.

- Noise reduction techniques.

- Dry type transformer technology to increase the voltage levels and applications of dry type transformers.

 Different transformer solutions related to reliability and to secure the supply of electricity will be also presented:

- Polytransformers and universal generator step up transformers including main characteristics, applications and advantages of these types of transformers.

- The advantages of on field repair and upgrading of power transformers along with some practical examples of those activities both in core and shell form technology including high voltage testing on site.

 

 

** Improvement of Transient State Parameters

     in the Desing of Protective Current Transformers

 

There are two kinds of protective current transformers: the class P CT’s for protection at steady state and class TP CT’s for protection at transient state. The multi-core type current transformer is composed of a number of cores with individual secondary windings and a common primary bar in the same casing. During a transmission line short circuit, the primary current takes on an exponential component resulting in core saturation and deformation of the secondary current. The core of measuring current transformer should be saturated during a transmission line short circuit for the protection of measurement equipment. Therefore, it has a core without air gaps. The class P protective current transformers do not have air gaps either, but their design gives the core saturation for higher overcurrent. The TPZ class protective current transformer has a core with air gaps which guarantee a linearity of magnetic characteristic of the core at an assumed value of primary short circuit current.

The behaviour of protective current transformer at a transient state is very important because it influences the proper functioning of the protection system.

The requirements set for TPZ class protective current transformers concern the transformation of currents, with high accuracy especially at transient states. IEC standard obliges designers to determine the instantaneous error current vs. time.

In this research was estimated the influence of the type of the core construction, especially the distribution  of air gaps, on transformation errors as well as limitation of the mutual influence between secondary windings of neighbouring current transformers, which can occur through the magnetic field

 

** Infrastructure of Electrical Power Transportation Network.

 

** Improvement in Reliability of Power Transformers base on Changes in Specification

 

This lecture will explain how Iberdrola has carried out the research of aspects relating to the improvement in reliability of its Power Transformers, based on changes in the specification of equipment and accessories. These changes have motivated new maintenance needs. The lecture will describe the relationship at organizational level of the personal involved in labors of standarization and in labors of maintenance.

 

An important aspect in this change of specification has been the important change in philosophy in the substations, due to  the necessity to reduce occupied flour space , which has lead to, at least in the large urban areas, the need to design indoor substations with GIS breakers. This has resulted in Iberdrola an important change in the specification of its power transformers to adapt these to the new requirements of installation inside of buildings, with special requirements concerning dimensions, refrigeration, type of bushing, etc., taken as first consideration aspects that minimize the possibility or reduce the effects in case of catastrophic failure due to the repercussion that one incident can have (fire, blackout,..).

 

On the other hand, within the maintenance activities of power transformers, exists a multitude of aspects that  should be taken into account and solved at the specification phase of the equipment. This lecture will explain how Iberdrola has solved this dual Specification -Maintenance within its organizational structure.

 

In the next items we have put together some constructive aspects taken into consideration in the specification phase of the power transformers in order to improve reliability and achieve efficient labours maintenance in the future:

 

1.- Specification of bushing:

- Specification of a Resin Impregnated Lecture (RIP) bushing for all high voltage levels. This improves reliability.

- Definition of an unique specification of bushing per type of transformer, this allow us to install it in any of Iberdrola´s power transfomer in case of failure.

- Specification of bushing oil-SF6 at 220kV level. Avoid fire risk.

- Interchargeable bushings: specification of a 100% interchangeable bushing that permits its exchange in case of change of function: e.i. Oil  - air to oil – auto plug in medium voltage.

 

            2.- On load Tap Changers (OLTC):

- Specify On Load Tap Changers (OLPC) with low maintenance requirements, in short or medium time.

 

            3.- Redesign of some constructive aspects:

- Location of the conservator to improve future accessibility to the top of the transformer for assembly, inspection and measurements, in the case bushing of the different type from oil - air.

- Valves normalization for installation of oil monitoring equipment.

- Cooling  system through coolers: specification of new buildings and fulfilment of the philosophy of N-1.

 

   4.- Protection and control systems: new strategy of Iberdrola of including within the existing protection, logic and control maintenance and supervision activities. Example of new protection and control system: new thermal image protection and residual life evaluation of a transformer based on load guides.

 

            5.- Definition of safety systems for workers in height (above the transformer).

            - …

 

On the other hand some of this change in the design of the power transformers has made it necessary to redefine certain maintenance procedures, thus for example it has been necessary to anticipate the way we carry out field tests ( preventive maintenance tests) during the life time of the transformer in case of  bushing oil - SF6 where the accessibility to the terminals of the transformer for the tests is difficult. One example  of electrical measurements will be provided (winding resistance, winding turns ratio, power factor and capacitance, etc.), measurements carried out before put into service the transformer and that will serve as future fingerprint. This case is one transformer 220/20kV 50MVA with oil -SF6 bushing at High Voltage and plug-in at Medium Voltage, all the tests were made from the position GIS  without disconnecting the machine to the GIS.

 

** Large Power Transformer Cooling – Flow Simulation and PIV analysis in an

     Experimental Prototype

 

Power transformers dissipate an enormous quantity of energy when promoting the variation of electric current from high voltage and low intensity to high intensity and low voltage, according to Joule effect and Foucault currents [1].

To remove the heat generated, an oil flow is forced to flow between the cellulose slabs and copper galetes, acting as a heat exchanger. Several flow problems may arise, namely the non-uniformity of oil velocity (stagnation and high velocity zones), vortex formation and pressure drop, all of which decrease the effectiveness of heat transfer. This effect is undesirable since a power transformer operating at higher temperature than its limit will have a shorter life time.

 Fluidinova has developed together with EFACEC and in partnership with FEUP (Engineering Faculty of Oporto University) a model using Computational Fluid Dynamics (CFD) that simulated the oil flow inside a power transformer. The simulation model was developed using the CFD software Fluent (version 6.3.26) where the continuum and mass balance equations are solved for each finite element, in all geometric domain of interest. The finite element grid was developed with AutoCAD 2006 and Gambit (version 2.3.16). The model permitted the study and detection of flow problems on the currently used power transformers configuration such as: stagnating zones, high velocity zones, vortex formation and pressure drop that occur for the oil flow between the cellulose slab and copper galette, where the heat transfer takes place. After this analysis, a new power transformer configuration design was developed to diminish the problems detected.

To validate the simulation model, the main purpose of this work, two acrylic prototypes were created for both studied designs (scale 1:1) and were used for Particle Image Velocimetry (PIV) tests, with TSI laser system. Comparing PIV and CFD analysis it can be concluded that the simulation model described correctly the oil flow. Furthermore, the new design proposed shows better results for the oil flow.

** Studies in a Large Power Transformer – Heat Transfer and Flow Optimization using CFD

Computational Fluid Dynamics (CFD) has proven to be a good tool for fluid studies in several geometries and physical conditions where heat transfer takes place.

Power transformers, due to their complex geometry and heat exchanging rates between solid and coolant fluids (such as oil) represent a very difficult domain to obtain experimental data without destroying or modifying their structure and internal conditions, where the power transform and heat exchange takes place.

Previous studies carried out by Fluidinova, comparing experimental results from Particle Imaging Velocimetry (PIV) tests with those obtained by CFD simulations have proven to be in accordance; the flow problems detected in both cases were the same: non-uniformity of oil velocity (stagnation and high velocity zones), vortex formation and pressure drop.

 In this work, Fluidinova together with EFACEC and in partnership with FEUP (Engineering Faculty of Oporto University) developed a simulation model for power transformers taking into account several physical parameters: heat exchange between solid and liquid phases; conductivity of copper from the galettes, its curved geometry and insulation with cellulose; variation of fluid viscosity with temperature by means of an User Defined Function (UDF) obtained from experimental rheology tests in agreement with theoretical laws for Newtonian fluids. In the model, the fluid density also varied with temperature. The CFD software used was Fluent (version 6.3.26) where the momentum, mass and energy balance equations are solved for each finite element, in all geometric domain of interest, for solid and liquid phases. The finite element grid was developed with AutoCAD 2006 and Gambit (version 2.3.16).

After analysing the oil flow and heat exchange problems for the currently used geometry, a new power transformer configuration design was developed, optimizing the oil velocity field, its pressure drop and temperature ranges and heat transfer rates – the main purposed of this work.

A software was also developed using Visual Basic for Applications (VBA) where new configuration designs for the power transformers where created and evaluated in terms of distances and geometric restrictions for mass production.

 

 

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