L e c t u r e s

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


Selected L e c t u r e s:



The recent CIGRE failure statistic confirms the necessity of continuous online monitoring of all major components of power transformer’s. The application of various sensors and their integration into only one monitoring system as well as correlation of all measurements and analyzed data build a so called “comprehensive monitoring system” and allow obtaining significant information about the actual as well as predictive condition of the power transformer/reactor. In the course of the ongoing extension and optimization of power systems, more and more intelligent devices are applied in order to improve the utilization of power transformers and reactors, to detect early incipient faults and avoid unexpected outages, to support operators in accurately timed maintenance activities and to ensure higher availability and reliability from energy generation to transmission and distribution.
This lecture deals with operating experiences with comprehensive online condition monitoring systems for power transformers. Examples of practical cases of bushing and active part monitoring will be presented and an intelligent alarm management for effective user information is demonstrated. Additionally an overview will be given of various applications of comprehensive online monitoring systems including advanced bushing monitoring as well as trends and new developments, like monitoring of fast transient overvoltages, ultra-high frequency partial discharge monitoring.




By 2035, the global demand for energy is expected to increase by 36%, bringing with this expansion certain challenges. In 2014 in the United States alone, there were more than 7600 generating stations. Globally, transformers sit in the background of our everyday lives transferring this power to our businesses, our homes and to every modern convenience to which we have become accustomed. Along with growth, security issues have also grown over the last few years. An April 2014 report by the United States Department of Energy, explains that large power transformers “require a long lead time, and transporting them can be challenging. . . . If several [large power transformers] were to fail at the same time, it could be challenging to quickly replace them.” In early April, 2012, the United States Department of Homeland Security announced that a successful emergency drill had been completed during five days in March to move, deploy and energize three single-phase, high-temperature, 200 MVA fast-recovery transformers that serve as prototypes for the utility industry, to dramatically reduce the recovery time associated with transformer-related outages. These power transformers were designed with a hybrid insulation system that helped to make these large capacity transformers light enough and small enough to transport by conventional low-boy trailers, within normal maximum shipping weights and height. Many obstacles lay in the path to seeking grid security and meeting growth needs. However, in this development, new materials and techniques certainly will become more important.



The use of vacuum type on-load tap-changers suggests innovations like the use of alternative insulating liquids, sealed applications, combined oil volumes and DGA. This lecture meets all these challenges by theoretical considerations which are backed up by results from laboratory tests and field experience. Solutions for the protection of the tap-changer oil against contact with ambient air (sealing) are presented and answers on how to connect the oil households of transfomer and tap-changer without influencing transformer DGA by tap-changer gases are given. Furthermore, methods for successful tap-changer DGA are depicted and limit values for acetylene are set for network service.



Dynamic resistance Measurement (DRM) has been used for circuit-breaker diagnostics for over 20 years. It is an interesting technique that can be used also to verify the switching operation of load tap changers (LTC). Existing methods and techniques for dynamic measurements on load tap-changers are based on measuring current and/or voltage on the primary side of the transformer and short-circuiting the secondary side to minimize the inductance in the circuit. Static resistance measurements per tap are performed in a separate test sequence with the secondary side open. A new technique is to combine current measurement with voltage measurements on both primary and secondary side of the transformer and then use the transformer modelling parameters to calculate inductive and resistive voltages to be able to calculate the dynamic resistance during a tap change.



The lifecycle management of a fleet of power transformers requires the definition of an adequate maintenance strategy, which should include condition assessment activities, periodic preventive maintenance, and in some cases, a deeper intervention in order to restore the condition of some parts of the transformer and to allow an extension of the expected lifetime, that may be defined as refurbishment. This work presents the results of 15 years’ experience of power transformer refurbishment activities at REN (Rede Eléctrica Nacional de Portugal) in terms of lifetime extension and transformer condition evolution. The assessment focuses on more than 30 transformers, installed in the Portuguese transmission grid, that were refurbished between 2001 and 2015, and includes oil tests, electrical tests, bushing tests, DP measurements, electrical test . The available accumulated data and experience, allowed to check that the medium-long term results desired were achieved, to identify the kind of problems that may appear, and to provide information for cost/benefit analysis that support this activity.



During the energization work of a power transformer, the system tripped and registered an unusual fault. Following tests showed that, despite of the short duration and low magnitude of the fault, the transformer accounted some internal damages. The internal inspection revealed deformations in the conductors as well as relevant damages in the insulation. Simulation studies pointed out that an unintended phase-to-phase fault in the low-voltage winding during the energization work in the high-voltage side resulted in transient overcurrents and overvoltages that ended up affecting physically the high voltage winding of the power transformer. Although the occurrence of these kind of events is very infrequent, it is too risky to ignore them since they can result not only in major damages in the transformers but also in important power outages should they happen at some strategic point of the grid. Thus, it is essential to address this problem at the design stage and to acquire the know how necessary to ensure the transformer’s security and integrity under such an event.



Power transformers are the most important and expensive equipment of power systems They represent a major portion of the investment electrical utilities make in their power delivery systems. So, their acquisition process must be classified as strategic for electrical utilities. In this context, the design review has become one of the most important processes in the power transformer procurement. It should be recognized that it is important that the transformer users effectively participate in the design review process to improve their transformers performance.
This lecture deals with the design review, key step in the power transformer acquisition process. The objectives of the design review according to the CIGRE Technical Brochure “Guide for Conducting Design Reviews for Power Transformer” are presented. The main components of the transformer procurement process, i.e., the user specification review, the design review and the manufacturing review processes are addressed. Technical specifications, methodologies and tools of the more critical aspects of the transformer design are pointed out and enumered.




Over the past decade, there has been a rising awareness of use of renewable energy sources (RES) and the extension of such matters as reduction of CO2 emissions and energy consumption becoming worldwide issues. Low carbon technologies (LCT), such as electric heat pumps (EHP), electric vehicles (EV) and distributed generators (DG), can contribute to fulfil the objectives of limiting global warming. The development of a new compact smart distribution transformer with an On-Load Tap Changer (OLTC) keeps the voltage stable in distribution grids by compensating voltages instabilities in the medium voltage (MV) and can help the integration of LCT by regulating the voltage automatically to cope with the voltage fluctuations generated by new loads and DG in the low voltage (LV) side of the electric network. The smart transformer operation has been assessed in a new ‘laboratory’ called Demonstration and Experimentation Unit (UDEX), a real grid designed as a platform for the research, development and verification of new technologies, products, services and systems. It permits the reproduction of normal conditions and anomalous situations such as voltage instabilities or fluctuations.



New challenges in Smart Electical Grids, such as integration of Electric Vehicles chargers and Distributed Energy Resources, require advanced functionalities, both at power and communication levels to be provided in distribution electrical grids. The Smart Transformer is intended to replace conventional power transformers providing such advanced features but the challenge is replacing a tested and proven technology with power electronics based one. This lecture provides an overview to ST technologies and current develpments, comparing the characteristics of the available prototypes, and gives a glimpse of future trends and required funtionalities.



In this lecture applications and particularities of the Power Voltage Transformers (PVTs) used for direct conversion of power from high to low voltage are reviewed. PVTs are a power source that effectively supply to auxiliary services in substations or serving as the main transformer in a compact substation for isolated loads. Therefore, state of PVTs technology that covers the international standards and some design and testing aspects are presented. Finally, the technical features for both oil and SF6 insulated PVT models are addressed.



This lecture is devoted to toroidal transformer dedicated for DC spot welding system. The transfomer is based on amorphous magnetic core with very high magnetic permeability, shaped as a toroid. Primary winding, classical one, is made of copper wire. The secondary is made of copper strips shaped in proper way. The secondary consists of two parts, each is separate winding with one common lead. Such arrangement is required for cooperation with full wave, two diode rectifier. Very high permeability of magnetic core and proper arrangement between windings results with high coupling ratio, which is required by supplying inverter. The transformer operates with high turn-to-turn ratio of 70:1 and the output current at the frequency of 10 kHz is about 9 kA. The output power and efficiency obtained by FEM calculations are 43 kW and 92%, respectively.



It is well known that windmill technology for both onshore and offshore applications is under continual development, leading the size and power rating of each turbine to higher values. Because of this increase, the ratings of the transformers located either inside or outside the turbine’s nacelle are also increasing. Considering the technical complexity and the high cost of each square meter built on the windmill, the importance of having compact/reduced footprint solutions is a key driver for transformer selection. Another key driver for transformer design is the level of environmental protection that is dependent on the geographical location of the windmills themselves. The degree of the ambient corrosivity will determine the amount of environmental protection from humidity, salt and pollution when considering life expectancy and maintenance activities. Solving this problem from a technical and economical viewpoint is a major challenge due to the location of most of today’s wind farms.
Based on these previous reasons, the compactness and robust environmental protection of transformers are two key technical challenges for design. This is especially the case for higher voltage rated wind transformers (36 kV) where the combination of reduced ambient dielectric withstand and high electrical stresses need to be very well evaluated. Transformer cooling implications will also need to be addressed.
This paper will start presenting the main technical challenges and the innovative solutions developed to design, using the adequate state-of-the-art tools, 36 kV compact dry-type transformers robust enough for different corrosive environmental conditions. An innovative concept to achieve reduced size solutions for these type of protections will be described also. The paper will continue describing the manufacturing of several 36 kV and testing prototypes performed at world-class laboratories. Results of the tests will also be presented. The document will end explaining the conclusions of the development and proposing some recommendations that may be considered of interest; taking into account that the IEC standard of transformers for windmill turbine applications are currently under revision.




An important function of insulating liquid in a transformer, apart from insulation, is cooling. In this lecture heat transfer calculations are done based on a simplified model of a transformer radiator system, simulating the type used on larger transformers, where forced convection is used to increase heat transfer. The results from calculations show that high end naphthenic transformer oil and paraffinic oils have similar heat transfer properties. Calculations were also performed for a high end synthetic ester, and in comparison it has limitations for use as a transformer fluid in power transformers.



Recently, nanodielectric fluids have been proposed to be used as transformer liquid insulation. These materials are obtained by dispersing nanoparticles in a matrix fluid. The addition of particles seeks to improve the dielectric and thermal properties of the base liquids, giving rise to a new generation of insulating fluids. Several authors claim that the performance of nanodielectric fluids as insulators and coolers is superior to that of the base liquids.
The interest on these type of fluids has risen exponentially over the last years, as can be derived from the number of scientific works published in international journals and conferences.
Nanodielectric fluids are not a feasible technical solution nowadays. Some issues related to their stability, the interaction with the magnetic fields present in the transformer, the effect of the nanoparticles on the transformer solid insulation performance, and the production costs should be studied and addressed before they could be applied in real transformers.
The aim of this lecture is to present a state of the art on Nanodielectric fluids for electrotechnical use. The experimental results obtained by different authors are reviewed and compared. Additionally, practical issues, such as the manufacturing process of the liquids, and basic theoretical aspects are reviewed.




In this lecture the oil speed in horizontal ducts of an OD cooled winding is investigated by experimental and numerical methods. The presented winding model offers insight into the horizontal cooling channels perpendicular to the main oil flow direction. To visualize the oil flow, tracing particles were added to the cooling oil and illuminated by high power LEDs. The particle velocities were then determined by taking photographs with a defined exposure time. The design of the sophisticated winding model is described in the contribution. In addition to the experimental results, this contribution presents a comparison with respective numerical results from 3D CFD calculations. Finally, numerical results from the 3D winding model concerning the oil flow distribution inside the winding at various operating conditions are presented. The investigation indicated a strong inhomogeneous oil flow distribution on the horizontal channels. The presented results give a deep insight into the oil flow and temperature behaviour of windings enabling the designer to optimize the cooling of power transformer windings.



Transformers are critical components of HV transmission networks. Any outage, planned or accidental, means a long period till service recovery due to repair times and transportation difficulties (given their dimension and weight).
As an efficiency measure to guarantee the power supply through the whole HV grid, REE (Red Eléctrica de España), the system operator in Spain, has three 400 kV (250 MVA) mobile (“fast-deployable”) transformers with hybrid insulation as strategic reserve units, which allow to reduce both the downtime in case of failure and the associated costs.
This lecture presents the definition process of these strategic reserve units as well as the advantages of this solution describing two real cases.




The maintenance and operation recommendations based on the results obtained from continuous monitoring on two identical old transmission transformers are presented. The 130 MVA 230/115 kV transformers have been in service in similar load conditions for more than 40 years. A monitoring system for bushings and active part insulation was installed on these transformers. Additionally, oil sampling for lab DGA was performed. The monitoring data was processed to obtain an Insulation Health Index value, which is a number indicating in this case the poor condition of the insulation systems. This information triggered a detailed monitoring data evaluation in order to find the potential defect type and its location – information not provided directly by the Insulation Health Index algorithm.



Protection to fast front transients of power transformers connected to Gas Insulated Substations (GISs) requires not only detailed model of the GIS but also of the power system components connected to it including the power transformers themselves. Transformer models with different degree of modeling detail have been proposed in the literature. However, there is no single model able to represent at the same time the transient performance both outside and inside transformer when connected to the power system. This paper reports an investigation that shows the valuable information obtained when a white+black box representation is used to model the transformer in fast front transient simulations. Precisely, transient overvoltages will be simulated and the time domain severity factor (TDSF) will be monitored in an actual GIS model in the ATP/EMTP program.



The high voltage equipment engineers face the challenge of providing increased reliability and availability of power transformers in the scenario of new applications of electricity, such as electric cars as well as in distributed generation and renewable energy sources. In this scenario, the Equipment Condition Monitoring Systems (ECM) for on-line and continuous monitoring and diagnostic of high voltage equipment condition, during normal operation, are essential tools for more effective and intelligent management of those assets, by enabling predictive maintenance, based on actual condition, to replace preventive maintenance, based on time.
This paper presents the experience of Treetech with the design, implementation, operation and maintenance of on-line monitoring systems composed of smart sensors, communications and data processing software for diagnostics and prognostics of high voltage equipment condition, mainly for power transformers.
The centralized and decentralized architectures for sensors data acquisition are analyzed and their characteristics, advantages and disadvantages are presented, as well as case studies resulting from a large amount of monitoring systems deployed in field.




Between F.A.T (factory acceptance test) and commissioning test take place several processes in which dielectric oil is involved. An incorrect oil handling and store could alter its characteristics and reduce the dielectric strength of the insulation system or result in acceleration of the rate of ageing of the insulation system.
In this lecture some cases are described in order to demonstrate how easy it is to make a mistake and how complicate is to solve it.




This paper describes the “State Of the Art” for inventory, control, management, decontamination of electrical equipment and insulating liquids containing DBDS/Corrosion & PCBs. The Best Available Techniques (BAT) and Best Environmental Practices (BEP) for Life Cycle Management (LCM) of electrical equipment impregnated with insulating liquids, according to the prescriptions of the Stockholm Convention on Persistent Organic Pollutants (POPs) entered into force on May 17th 2004, are presented, according to art. 5 Annex C and IEC and CENELEC standards. The quantitative determination of total corrosive sulfur (TCS) in insulating liquids permits an objective ranking of sulfur compounds according to their corrosiveness towards copper. Case Histories of Diagnostics & Treatments are presented in according to IEC & CENELEC include On -Load treatments in power transformers and shunt reactors, in Brazil, South America, Europe and worldwide. The integrated treatment normally runs at 80-100 °C and has the capability to decontaminate equipment on-site through continuous circulation of the oil a closed system (without draining the oil or using auxiliary tanks) using the solvent capability of the oil for continuous extraction of PCBs and DBDS from solid materials inside the equipment (according IEC 60422 art. 11.4.4).



The occurrence of incidents in the interconnecting elements, like insulated cable links of power transformers to their respective bays in a GIS system of the High Voltage Grid, may lead into a prolonged unavailability of equipment and produce important solicitations due the short circuits to the transformer and also to the enclosure that contain it.
Union Fenosa Distribucion (UFD), in order to comply with the regulatory requirements in terms of electricity supply quality in urban areas, improved the technology developed with Polytechnic University Madrid (UPM), which is generally applied in the field of High Voltage insulated power lines for detecting partial discharge (PD) by advanced methods, to use it not only in cables but also in the transformer. So that, through an online monitoring it was achieved the necessary quality levels for the repairs and upgradings carried out.
Moreover it was essential to confirm the perfect state and condition of the transformer for a safe reconnection through the necessary discrete electrical measurements, tests and analysis, which are adapted to the characteristics of the system and counting with the restricted accessibility to active parts. Also, the infrastructure of the enclosure where the transformer is content it was revised and strengthened in order to enhance the resistence against the overpressures generated by internal explosion of any element, retaining the early detection of the fire protection systems, ensuring the electrical disconnection of the switchgear and activating the automatic fire extinguishing system at the right time.
Therefore, it is important to highlight the efforts on the adaptations and the evolutions of the early detection methods to diagnose and locate possible failures in power transformers and its associated elements, which will also lead to carry out a proper follow up or monitoring of operating conditions through an optimum combination of sensors and equipment.





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