Fixed lectures on key topics will be delivered by a group of specialists, the key speakers.

The content of the lectures intends to show general aspects on the delivered topic in Oral Session format. In this edition, some of them are also placed in a Poster Session with the intention to stimulate more deeply discussing in person.

[Updated Proposals 20.09.22]


An overview of the significant changes energy companies and especially Utilities are going through, resulting in both challenges and opportunities in different areas outside the technological one: Innovation, Planning & Regulation, Customers, Supply chain, Organizational, Operations.. and others. More from a business and strategic focus rather than specialist, and aiming to make the audience think about how to integrate both in the future.


The development of initiatives to make transformers more sustainable and to provide the industry with such solutions and or designs is nowadays one of the key targets in the transformers industry and the energy sector. This speech has as main goal to illustrate different aspects in order to reach the mentioned target, not being exhaustive: solutions for decarbonization, enhanced safety, protecting ecosystems and responsible resource use. The proposal will include also details of a real case and main conclusions.


Ester-based dielectric liquids have now been on the market for around 40 years, following the introduction of synthetic esters as PCB replacement fluids in the late 1970s. In the 1990s natural ester-based liquids were also developed as a more sustainable and environmentally friendly option. Since this time there has been a gradual increase in the use of both types of esters in transformers, up until recent years where a more rapid adoption of these alternative liquids has begun. Much work has been done over the last two decades, evaluating the properties of esters in more depth, conducting research into electrical aspects of using these liquids and gaining experience in building transformers at ever higher voltages. In the past two years the culmination of this work has seen several EHV ester liquid filled transformers built, tested, and commissioned, with operating voltages in excess of 500kV.
The lecture presents summaries of ageing tests conducted with ester liquids and kraft paper, thermally upgraded paper, aramid materials and aramid enhanced cellulosic paper. Results from several laboratory studies are cited to demonstrate that both synthetic and natural ester reduce the ageing rate of solid materials used in transformer applications. It is also shown that all types of abovementioned solid materials in combination with ester liquids can increase the thermal class by 10°C or higher. These experiments have consistently shown that replacing mineral oil with ester liquids can extend the asset life of transformers and/or operate at higher temperature.


Oil filling is a critical activity within Liquid-Immersed power transformer installation/replacement projects. Poor practice not only leads to work delays and overspending during assembly works but can also contribute to higher failure rates if unnoticed. There is currently a frame of reference within IEEE standards which lays down the main criteria to be observed when approaching oil filling. Red Electrica´s field experience reveals areas of discrepancy between manufacturer installation instructions and IEEE standards. The main areas of variation are vacuum pressure and timing, oil preconditions and flow parameters. This paper aims to shed light on this subject by presenting a comparative analysis of the evidence available and concluding with the need for having a defined Employer specification which incorporates IEEE recommendations.


Nowadays the electrical power grid is undergoing the raise of new technologies. Distributed energy resources (DERs), digitalization or the new flexibility market schemes are producing a severe change in the behaviour of the electrical grid. Most of these changes specially concern distribution power grids. Consequently, there is an urgent need to provide tools and solutions to utilities-Distribution and System Opperator (DSO) to overcome the new requirements and adapt the grid structure to them. One of such emergent proposals are Solid State-Transformers (SSTs). These power electronics-based devices are able not only to cope with the role of power transformers but also to provide additional features such as guiding the power flow. How to scale SSTs to operate in medium and high voltage power grids is one of the current issues of this technology. This lecture explains the approach of projects such as FST, TIGON and SSTAR to solve the problem of implementing a medium-frequency power stage with enough galvanic isolation levels between the primary and secondary windings.


During the tap switching operation of an on-load tap changer (OLTC) a unique vibration pattern is produced. This vibro-acoustic trace can be recorded using piezo electric sensors which are temporarily mounted to the transformer tank wall. These patterns are unique to each unit and can be regarded as fingerprint results. Thus, the evaluation of vibroacoustic measurements (VAM) can be used for a comparison based condition assessment of the OLTC. This speech at hand describes the setup and the basic principles of a VAM measurement. In a second step the recorded data is postprocessed to minimize the impact of external influencing factors and improve the comparability of the results. Finally, the effect of typical influencing factors, such as external noise sources and the sensor positioning, are discussed based on reallife measurement data.


Field experience, as well as failure statistics,confirm the necessity of continuous online monitoring of all essential transformer components since it is in fact one of the most critical assets in the substation.
The application of different sensors and monitoring systems dedicated for transformer components is the standard approach, however, only the combination of the devices and their integration into one system to build a so called “comprehensive monitoring system” allows the correlation of all measurements and analyzed data to observe critical assets, thus gain greater insight into the operational status of power grid equipment.
This lecture addresses the importance and advantages of deployment of comprehensive monitoring systems, consisting of sensors which obtain significant information about the operational status of various power transformer components.


In this lecture, the authors focus on a highly practical testing procedure bringing two main benefits to all power industry operators: a) higher confidence on the evaluation of dielectric testing, and; b) better utilization of technical and human resources. In the field, the application of dissipation factor measurements at (typically) 10 kV and at line-frequency (50 or 60 Hz) has been in use for many years without significant changes to the method. Nevertheless, multifrequency measurement of dielectric losses has opened a wider spectrum of analysis for power, distribution transformers and bushings. It is missing though, in the literature, a clear and properly supported discussion about how and when a single frequency measurement is validated with a multifrequency measurement or completely challenged.Important to recognize the link between line-frequency measurement, narrowband measurement (1 – 505 Hz) and full spectrum dielectric response (typically 1 kHz – 1mHz). Limited information obtained in the frequency domain may identify a faulty unit but may not be used as a preventative test to minimize risk of failure. Preliminary research does address the need for limits of narrowband measurements. This is only possible when a correlation is established between temperature and frequency as it has been done using the well-known Arrhenius correlation. Therefore, in this speech, the analysis of the dielectric response at frequencies beyond line-frequency leads to the undeniable benefit of using a multi-frequency dielectric loss procedure to establish higher confidence at the time to state not only technical but also financial decisions.


Power transformer manufacturers and asset owners are increasingly focused on sustainability and environmental aspects. Alternative liquids have gained increasing interest over recent years; however, it is important that the different aspects of the various alternative liquids are fully appreciated and that the transformer application in question is appropriate. Low Eco-toxicity and biodegradability are desirable properties of insulating liquids in some applications. Carbon footprint reduction can be achieved by using bio-based feedstocks. Moreover, re-refining techniques can yield a sustainably manufactured mineral insulating oil. A number of case studies used with bio-based hydrocarbon insulating liquid are presented.


The intermittent nature of renewable energy sources (RESs) hamper their integration to the grid. The stochastic and rapid-changing operation of RES technologies impact on transformer reliability. This talk will describe a hybrid transformer prognostics framework for probabilistic predictions in RES applications. Physics-based transient thermal models and probabilistic forecasting models are integrated using an error-correction configuration. The thermal prediction model is then embedded within a probabilistic prognostics framework to integrate forecasting estimates within the lifetime model, propagate associated uncertainties and predict the transformer remaining useful life with prediction intervals. The proposed approach is tested and validated with a floating solar power plant case study. The talk will also cover a discussion about the extension of lifetime models to include electrical ageing factors such as overvoltage transients.


Due to the large short-circuit radial electromagnetic force, the inner winding of the transformers may fail due to the buckling phenomenon. To avoid the buckling of the inner winding, the value of the critical buckling stress of the winding should be more than the applied stress during the short-circuit condition. There are many factors on which the critical buckling stress depends viz. proof stress of the conductor material, number of axial supporting spacers and their dimensions, the flexibility of the axial supporting spacers, mean radius of the winding, the radial thickness of the conductor, number of conductors radially stacked, epoxy bonded conductors, clamping pressure, and many more which are to be pointed out and investigated. In this speech, the effects of these factors on the value of critical buckling stress of the inner winding will be discussed through some case studies.


This lecture presents an investigation of partial discharge inception for synthetic ester and gas-to-liquid insulation liquids under positive lightning impulse voltage using a conductor-plate setup with gap or creepage path. From the electrical point of view, synthetic ester insulation liquids are assumed to behave differently to the better known and still used gas-to-liquid. High voltage tests with lightning impulses of positive polarity are applied to a common insulation arrangement of power transformers: A paper-insulated U-shaped copper winding rod vis-à-vis a copper plane in insulation liquid with a gap between them. In a second test setup for creepage paths, a circular pressboard disk replaces the gap. With increasing impulse peak voltage, partial discharges are detected via the occurrence of streamers and optical measurement thereof using fluorescent fibre optics, a photomultiplier tube and an oscilloscope. The results are evaluated by electrical field strength using conversion factors obtained by a finite element method simulation. For the investigated creepage paths, the study reveals that the existing design curves cannot be applied to the tested insulation arrangement.


This lecture goes on to describe gas generation in windfarm transformers. (WTTs – Wind Turbine Transformers). Local transformers, those directly linked to the turbine of a wind farm, generate abnormally high amounts of dissolved gases. This is virtually independent of the type of transformer (pad mount or distribution) and the type of fluid (mineral oil or synthetic ester) contained in the transformer.
The causes of this anomalous gas formation are not fully understood yet, although some possibilities have been proposed, such as a breakage or delamination of the core, harmonics, low voltage fault ride through, over voltage size limitation, variable loading cycle or switching surges and transient overvoltages.
Nor can it be ruled out that the cause of these gases could be the interaction of the transformer with the electrical system: windfarm design. These causes are discussed in the speech.


Utilities face enormous challenges to reinforce, expand an enable power grids to meet the requirements of the next years. Voltage regulating distribution transformers (VRDT) are an effective and sustainable tool to answer these challenges in the distribution network. MR's latest OLTC for VRDT enables voltage regulation to become a standard function of distribution transformers. In this speech we will explore how the use of a VRDT can support distribution grid in their current transformation. How it helps operators maintaining voltage in a defined and stable band in public, industrial and private distribution grids. How this allows higher penetration of decentralised Energy resources in the grid with a reduced stakeholder impact, investment and operating cost compared to other solutions, while increasing it resiliency. The VRDT is today one essential asset for the distribution grid of the energy transition.


In a context of resource scarcity, network asset management is a promising way to operate electrical systems with a certain level of risk while optimizing the life of the assets.
Transport network operators and owners inform their decision based on physics-based nominal models or monitor valuable assets to build digital twins entirely based on data. Both approaches have pros and cons. Physical-based models are often complex to give accurate predictions and full data-based models must be learned from the scratch, necessitating a large amount of data.
To overcome the cons and take benefit of the pros of both approaches, an emerging trend is to use a hybrid twin to support the decisions by using the data for enriching the state-of-the-art models and at the same time using the existing model to set-up the data collection protocols and procedures.
This speach aims to show how physic-based thermal generic models of power transformers can be supplemented with machine learning functionalities to obtain individual conditions at a reasonable cost in order to research future applications on maintenance optimization or dynamic dimensioning of transformers.


Present transmission networks extend for hundreds of kilometers, transferring significant amounts of electrical energy. For every country there are sort of “back-bone” lines, where a massive failure of transformers would create serious problems to continue with reliable energy supply. Natural phenomena and human driven actions may be triggers of such events. For large utilities, or utility groups, it would be highly convenient to count on flexible-transformers which could be used as replacement for different voltage levels and with impedance adjustable to optimum local requirements of impacted substations. Currently applied emergency transformers are commonly fixed-impedance devices. A new concept of Flexible, Grid-Ready type of power transformers specially devised for transmission networks is presented in this contribution. The concept has been proven to be functional and stable in the factory test lab and in the field for a prototype of 100 MVA, 161 kV.


The clamping pressure is essential for the integrity of a transformer during short circuit events. The introduction of new loading patterns, increased dynamic loading, may accelerate cellulose degradation and reduced clamping pressure. The clamping pressure of transformer windings can be directly measured by novel pressure sensors. Measurement of clamping pressure and transformer temperatures will, together with physical models, facilitate condition-based maintenance and digital twins. This lecture explains how the sensors work, and then presents data from the heat run test as well as in-service conditions. It is demonstrated how the clamping pressure changes with the transformer loading and temperature.


The asset management methodologies implemented at REN allowed to have a more integrated approach to all of the power transformer lifecycle phases. These can be defined as (i) identification of the needs, (ii) specification, (iii) procurement, (iv) installation and commissioning, (v) operation, (vi) additional investment (special maintenance / life extension measures) and (vii) end of life (replacement / disposal). The main challenges for asset managers are dealing with the ageing population, keeping a significant expansion of the asset base to cope with new demands from the grid, the need to adapt to regulatory regime and its evolution, new procurement processes and integration of new suppliers, while keeping the asset health and risk under control, optimizing total expenditures. In this lecture we will focus on some of the steps taken at REN and the tools used to face these challenges, covering all of the above mentioned lifecycle phases.


Different companies for the same asset apply different inputs that determine how the transformers are specified, operated, maintained … i-DE intelligent networks is the electricity distribution company of the Iberdrola group which operates in Spain. Different inputs that determine the rules of the regulated sector are evaluated in contrast to those applied by companies operating in a liberalized sector. Aspects such as standardization, quality assurance, load level, customer service, how and under what conditions investment and maintenance are remunerated, morphology, accessories, materials, degree of resilience, monitoring, and design rules are analyzed in order to justify the decisions taken with respect to the above aspects.

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