A2-00. GENERAL REPORT FOR SC A2 (Transformers)

CIGRE 2012 A2-00 GENERAL REPORT FOR SC A2 (Transformers) Chairman: Secretary: C. RAJOTTE (CA) P. PICHER (CA) Special Reporters: C.J. DUPONT (BR) (PS1) R. MERTENS (BE) (PS2) S. VOSS (DE) (PS3) Discussion summary prepared by: P. PICHER (CA) SESSION OPENING Claude Rajotte, SC A2 Chairman, opened the session at 09:00 with approximately 450 delegates attending. He welcomed attendees and explained the CIGRE Paris session organization with special reports, questions and contributions. He announced that two new WGs have been created recently. In total, twelve WG's are active. Two new brochures will be published end 2012 and a revision of two existing brochures will be made. He ended his remarks by presenting the preferential subjects of the 2012 session and those that were selected for the next SC A2 colloquium (jointly with SC C4) that will be held in Zurich (8-14 September 2013). * dupont@cepel.br; ronny.mertens@cgglobal.com; stephan.voss@siemens.com 1

PREFERENTIAL SUBJECT 1: TRANSFORMERS IN THE NETWORK OF THE FUTURE A total of 15 papers have been submitted, according the following sub-topics: intelligent monitoring, algorithms, access to new data and information; optimal utilization taking into account monitoring information, dynamic rating and overloading; transformer applications resulting from new technologies; impact of harmonics. PS1 - Keynote presentation by Dr. Jos Wetzer (NL) Dr. Wetzer presented the global and network trends that will lead to future network characteristics. He discussed global trends like increased electricity demand, environmental responsibility, declining fossil fuel supplies, aging assets and workforce, advanced technology and maintain of reliability. Network trends cover large scale renewable sources and storage, long distance & cross border transport, off-shore grids, electrification of transportation, distributed generation, etc. Asset management is also progressing from traditional time-based maintenance to risk-based and smart maintenance. At the same time, society becomes more vulnerable to power and communication (data) interruptions. These trends have several implications for HV & MV power transformers including new design of EHV and converter transformers, load flexibility, impact of harmonics, EMI robustness, smart diagnostics and maintenance, low-loss, green transformers, etc. The Special Reporter Mr. Carlos Dupont formulated the following questions to structure the contributions from delegates to the SC A2 Paris 2012 meeting. Question 1.1: What are the most important aspects related to the on-line continuous data? Are simple assessments of the measured signals deviation from fixed limits enough for a good condition diagnostic? What are the users currently accepted limits? Question 1.2: What is the most sensitive monitoring method to derive bushings capacitance and power dissipation factor values? A user's experience show that bushing degradation can evolve very quickly, is there enough experience to consider the possibility of using bushing monitoring as a tool to trip the transformer? Question 1.3: Is there enough experience to consider MeOH (methanol) as a new marker for early stage paper degradation in transformers? What are the known drawbacks and advantages related to the MeOH utilization when comparing to 2FAL? Question 1.4: What are the users most significant problems (technical / organizational or of human nature) associated with the implementation, operation and maintenance of continuous on-line monitoring systems? To what extent do these problems prevent the on-line continuous monitoring technology consolidation? Question 1.5: What are the most frequent commissioning problems encountered when implementing on-line condition monitoring systems? What kind of commissioning tests are 2

performed in the monitoring systems to evaluate the algorithm applicability? Is there any specific national standard or company procedure specific to the monitoring system testing? Question 1.6: Does the availability of better telecommunications infrastructure, fast processors, cost reduction of sensors and continuous on-line data acquisition changes the way diagnostic is performed? To what extent an adequate condition evaluation analysis and a reduction of failures consequences can be achieved only by using on-line continuous monitoring data and sensors? Which off-line techniques are now evolving for continuous online use? Question 1.7: What are the most challenging design, manufacturing and testing aspects related to UHV AC transformers? Is there enough technology available today to ensure high reliability in such UHV projects? Question 1.8: What special features are necessary in a transformer design to cope with the high level of harmonics created by firing thyristors and diode rectifiers in industrial applications? What is the user's experience with failure rates and the reliability of such transformers? Question 1.1 A contribution from UK reports that diagnostic data provided by an on-line monitor must meet two simultaneous requirements: valid data which is an indication of present condition and levels of deterioration; the present condition must be a valid indicator of future condition. The final element of condition monitoring is to have data which provides the basis for conclusions and thus allow actions and interventions. Currently accepted limits and rates of change must be seen as consensus and a guide for action, rather than a constraint. A contribution from France reports two examples of transformers that where operated far beyond any standards (IEC and others) just to emphasise the complexity of decision making when a transformer is diagnosed in bad condition. Conclusions are that on-line DGA comes with changes mainly related to operating procedures. On-line DGA helps to intercept abnormal operating condition with respect to a reference signature that is generally only applicable to single transformer or a family of identical transformers. DGA interpretation is not a science mainly because different faults can lead to the same signature and also because the location of the default is essential to estimate the overall risk of failure. A contribution from Germany reports on DGA analysis on on-load-tap-changer (OLTC). Depending on the OLTC type and the operational data of the application, the absolute gas amounts and gas compositions may vary extremely. For vacuum switching OLTC technology, the total amounts of gases generated are in the same range as the gases in transformers and can be analysed in the same way. If the DGA database contains 50 data sets at least, statistical methods can be applied to generate pre-failure- and alarm values for one OLTC class. The Duval triangle for OLTCs represents relations of key gases and so works without fixed limit values. It has been designed for arc-breaking-in-oil tap-changers with high acetylene values during normal service, but astonishingly in many cases it is also applicable to OLTCs with vacuum switching technology. Ongoing work tries to improve the applicability of this triangle by defining variable fault zones which are based on a statistical analysis of online data. 3

Question 1.2 A contribution from Poland reports that there are not sufficient grounds to activate the trip off function for the transformer based on a bushing monitoring system. During the last stormy summer the rapid increases of tgδ and C were observed many times. The phenomenon had lasted no longer than several minutes or an hour, then the measurements returned to the previous level. The situation might have been caused by heavy rain and rapid temperature decrease. In such environment, the emergency switch off for the transformer could be very dangerous even for the whole power system. A contribution from UK reports bushing monitoring experience with bushing monitoring based on the comparison of three bushings connected in the three phases. The approach uses a combination of expert system analysis of data collected to provide a baseline of normal operation, in addition to providing a nameplate comparison. This approach allows identifying normal variation and seeking out the cases where abnormal variation is present. Based on the return of experience over 10000 bushings monitored, it has been found that when the monitoring is applied with the correct procedures, it provides adequate warning of both graceful and sudden failure modes. Sudden failure modes are uncommon and are associated preferentially with particular bushing types. For those bushings, depending on the monitoring system type and reliability, it could be envisaged to trip the transformer when a rapid deterioration is detected. A contribution from Germany reports that the best bushing monitoring approach is to use voltage transformer as a reference for bushing monitoring, and as a fall back solution comparing bushings connected on the same bus. Comparing bushings in a three-phase system using the sum current approach is less sensitive because it is influenced by network unbalance. Question 1.3 A joint contribution from France and Canada reports that many papers described different laboratories ageing studies which validate the reliability of methanol (MeOH) as a chemical marker for cellulose paper degradation. MeOH can be produce from both standard and thermally upgraded Kraft papers under different ageing conditions and MeOH is a stable compound over the transformer temperature operation range. Two field case studies from Hydro-Québec in Canada and EDF in France demonstrated the correlation between paper ageing and the MeOH concentration in oil. The advantages of using MeOH over 2-FAL are the direct relationship with the cellulose chain scission no matter the insulation paper is a standard or a thermally upgraded Kraft paper. Like moisture in oil, MeOH is highly influenced by the oil temperature and it is very important to correct the concentration at a given temperature for interpretation. Hydro-Québec found some oil that may abnormally generate a small amount of MeOH that could interfere with the diagnosis. A contribution from Belgium reports that the MeOH is not a degradation product of hemicellulose as 2FAL. Since the MeOH is detected at higher DP values than 2FAL (<800) it allows to plan corrective actions at early stage of degradation. The MeOH equilibrium between paper and oil is influenced by temperature and acidity of the oil (polarity). Best applicability of MeOH is for closed type transformer units because free breathing open units seems to lose MeOH. 4

A contribution from Israel reports on a technology to determine the hot spot on selected components inside transformers. Question 1.4 A contribution from Australia reports the problems with on-line monitoring experienced from various users. Problems were related to suitability for ambient temperature, communication interface with maintenance system, issues with software licenses, false alarms or trip due to faults in the monitoring system, major damage of new bushings due to bushing-tap sensor problem and lack of training of operators to react after a monitoring system alarm. A contribution from Spain reports the problems when using multi-gas on line-monitoring as a mobile instrument. Problems were found in the configuration and commissioning tools, maintenance, communications and pipeline network. The use of multi-gas on-line monitoring equipments has proved useful, and when provided with portability the investment becomes justifiable for a large fleet of transformers. However manufacturers need to improve some aspects that make it easier the mobility of these devices, such as: friendly commissioning software and low maintenance equipments allowing them to attract more customers and consolidated into new markets. A contribution from Japan reports actual examples of measured data by on-line DGA monitoring. Judgement criteria, maintenance costs and detection sensitivity are the main parameters to examine before implementing such technology. Diagnosis of transformer with combination between on-line and off-line data contributes to a more reliable maintenance. A contribution from Poland reports that they installed and have operated more than 50 transformer monitoring system since the last four years. Different sensor reliability problems are reported for different types of sensors as hydrogen, moisture, carbon dioxide or on computer accessories. Other organisational and human issues are also discussed. Concerning the expansion of such a system, it is proposed to include power quality assessment as overvoltages and shortages registrations in order to assess the impact on the transformer lifetime. A contribution from UK reports with installation issued (wiring) and lack of understanding of variations with on-line data especially for on-line dissipation factor and capacitance measurements on bushings. The bushing tap adapter failure rate has been less than 0,26 %. A contribution from USA reports that the most significant problems are: common understanding of what transformer monitoring is; multiple types of sensors providing different outputs for the same parameter (i.e. gas sensor); multiple types of communication and parameter definition; difficulty to implement solution at a centralized level; need to have expert knowledge incorporated into enterprise; how to convert data/algorithms into actionable information; need for sophisticate IT platform to interact with multiple types of databases, online and offline data, multiple vendors, sensor agnostic type of solution. Question 1.5 A contribution from UK reports that the most frequent commissioning problem faced is poor wiring by contractors who have not followed installation guidelines: mislabelling of wires, reversing of connections and high impedance connections are among the most frequent problems. A solution developed is the use of a set of installation cables which are prefabricated and which require only suite connection with no wiring installation. 5

Question 1.6 A contribution from Sweden reports that the following off-line diagnostic measurements may be replaced by monitoring usually with no extra sensors needed: ratio, winding resistance at different tap positions, impedance, insulation resistance, dielectric loss, excitation current, load & stray losses as well as no load losses, used tap positions. Monitoring of those parameters provides information at low cost on core, windings, connections and OLTC. A contribution from UK reports that the development of communications infrastructure combined with the need for increased utilization of transformers is a dual driver for increased deployment of on-line diagnostic devices. Increased monitoring of thermal effects on transformers is seen as a growth area. This would allow better utilization of power transformers while simultaneously reducing likelihood of failure. Such an approach requires a combination of thermal sensors, on line DGA and PD sensors to give a more comprehensive view of the asset. Question 1.7 A contribution from Japan reports the experience with the development of UHV transformers in the following steps: development of the main components, testing of the prototype, lab testing of the full-scale transformer (1000 MVA), field test program since 1996 with energizing at 1,05 pu for about 2500 days. A contribution from Switzerland reports the challenges related to insulation in UHV transformers. For UHV transformers with extreme voltages, high field strength, large volumes and high financial capital bound, the following main requirements have to be considered: spacing elements as radial spacers and strips with excellent characteristics regarding the mechanical compression for compact windings and clean pressboard, free from extraneous material, particularly from any metallic particles. A contribution from India reports critical technical aspects during design, manufacturing and testing of India s first 1200 kv UHVAC transformer. Multiple challenges were found and resolved. The transformer was successfully commissioned on 27 th January, 2012 and is operating satisfactorily. Question 1.8 No contribution was provided in advance for this question. Oral contributions were, however, given during the discussion period. The reported necessary features in a transformer design to cope with harmonics, according contributions from India, are related basically to magnetic circuit, winding design, screen shield on core and proper selection of non-magnetic material. Influence of harmonics should be seen in each of the design criteria s (Dielectric, Thermal, Electromagnetic etc). Harmonics losses are calculated based on IEC 61378-1 and OFAF extra cooling to harmonic losses should be considered. Discussion Several spontaneous contributions were raised from the floor including several on the applications of bushing monitoring. Contributors were invited to provide a written version of their comments for inclusion in the session proceeding. 6

PREFERENTIAL SUBJECT 2: Transformer Eco design / Eco use A total of 11 papers have been submitted, according the following sub-topics: improved material, better efficiency; environmental considerations - low noise, oil containment, oil recycling, fire protection, and life cycle costs, life extension, repair/refurbish/replace decision. PS2 - Keynote presentation by Paul Van Tichelen (BE) Mr. Tichelen presented the outcomes of the European Sustainable Product Policy Study on Power and Distribution Transformers. A whole series of technologies and design options are available: decrease current density, conductor material selection; decreasing the transformer flux; step limb core cross-section; high permeability Grain Oriented Electrical Steel; amorphous Steel (distribution transformers); new insulation; new liquids; new steel coatings; core construction techniques; superconducting technology. From a system point of view, higher line voltage and switching off transformers when possible are possible avenues. Power and distribution transformers losses are responsible for about 2,6 % from the final energy consumption (figures EU27-2005). New EU Eco-design related legislation and supporting standardization is under preparation. Such legislation and/or standardization can hopefully support the market uptake of technologies to improve eco-design including energy efficiency. The Special Reporter Mr. Ronny Mertens, formulated the following questions to structure the contributions from delegates to the SC A2 Paris 2012 meeting. Question 2.1: How can the whole T&D community contribute to more environmentally friendly transformers in terms of life cycle assessment i.e. production, operation of transformers and disposal? Question 2.2: What roles do biodegradability and low losses play during the writing of the specifications and the following procurement process? Question 2.3: What other eco aspects should be considered e.g. low noise, integration, etc.? What other design and/or construction techniques should be considered at the same time? Question 2.4: What operational experiences are available with eco friendly transformers? What know how e.g. in terms of diagnostics, long term behaviour, etc., is missing to take a leap forward? Question 2.5: When significant actions are taken to extend the lifetime of a transformer, which second lifetime is envisaged? What are the criteria to take such a decision and is eco friendliness part of this decision? Question 2.1 A contribution from Spain reports the economical specification at Iberdrola Distribución Eléctrica S.A.U that takes into consideration all the cost during life of power transformers like initial purchase costs, capitalized value of load and no-load losses, installation costs, end of life disposal costs and operational costs. All the factors that take part in the transformer loss 7

capitalization formula are directly or indirectly fix by factors like economical market and operational procedures. Other technical specifications are also aimed to environmental aspects like low noise emission level, definition of accessories (vacuum OLTC, RIP bushings) and use of biodegradable dielectric fluid. A contribution from Sweden reports that 50% (110 TWh) of the T&D losses in EU-27 countries are attributed to power and distribution transformers. Reduction of losses in transformers is the first priority to reduce T&D losses. Life cycle analysis shows that 95 % of environmental impact of a transformer is caused by its losses. European commission Eco Design directives for transformer discusses that the total cost of ownership concept by specifying loss capitalization factors but also minimum energy efficiency standards could be used. By selecting appropriate capitalization factors, it could be possible to reduce transformer losses by 40-50 %. A contribution from Germany reports on vacuum switching technology as a part of an environmentally friendly transformer. In terms of life cycle assessment, on-load tap-changer monitoring is an appropriate tool to fulfill the requirements of the user (i.e., to use the operating equipment more efficiently). Accordingly, the tap-changer monitoring system provides the basis for the following features: diagnostics, life cycle assessment, early detection of faults, reliability of operating equipment, flexible use of monitoring equipment and reduction in maintenance costs. Question 2.2 A contribution from France reports the impacts on the transformer design of the change in the capital cost of the losses that was raised by RTE by a factor 4,5 in two steps. The first step was an increase by a factor 2,5 which led to a reduction of the no-load losses by a factor 1,6 for large 400 kv (over 300 MVA) transformers, but the load losses remained unchanged. In order to have opportunity for further reduction of the losses, RTE made an extensive study on the transportation limits and increased the gauge. The maximum transport weight was raised by 16 %. At the same time the capitalisation cost was raised again by a factor 1,8 making 4,5 times its original value. As a result the no load losses reduced by another 1,25 factor making them half of their original value and the load losses decreased by 30 %. This experience has proven that capitalisation of the losses is the most effective way of defining green and loss effective transformers covering all the aspect of the loss reduction. A contribution from Japan reports on hydrolysis in soil of silicone liquid for transformer fluid. Investigation for soil pollution of silicone liquid using soil simulating actual field conditions proved that silicone oil is less polluting than mineral oil and biodegradable in typical substation oil. Currently, several laws, regulations, and guidelines are available to reduce the effect of pollution on environment, but not in view of biodegradation. Specific criteria for biodegradability for substation and transformer will be discussed in the standard development process covering transformer design, manufacturing and application. A contribution from Australia reports that all their utilities specify a loss formula when buying transformers. As Australia now has a Carbon Tax, the cost of electricity, and therefore losses will increase slightly, so ideally utilities should adjust their loss formula and buy lower loss transformers. However, under the current market model, utilities do not normally pay for the losses. Furthermore, utilities are currently under pressure to reduce capital costs, and this may adversely affect the premium that utilities are prepared to pay for a low loss transformer. There is no standard loss formula used but most utilities are using 8

similar figures for similar types of transformers. On the other aspect of the question, a number of utilities are now specifying transformers filled with biodegradable fluids. One utility has specified that transformers be designed so they can be used with either mineral oil or biodegradable oil. Australian Environmental Agencies treat leaks and spills of biodegradable oils the same as leaks and spills of mineral oil. Another major utility wants to determine the effect of different biodegradable oils being mixed and how to do a midlife refurbishment on a transformer filled with biodegradable fluid. A contribution from Switzerland reports that there are two types of classifications of biodegradability, primary and ultimate. Both types are measured as a percentage change in relation to a period of time under standard conditions. The key point when specifying biodegradability is to ensure that a required percentage is set in reference to a specific test method. Prior agreement to these parameters will serve to prevent confusion or misinterpretation of the reported test results. Question 2.3 A contribution from Japan reports the experience with UHV transformer on-site assembly that contributed to reducing impact on the environment. After completing factory tests, the transformers are taken apart into smaller parts suitable for road transport by trailers. This technology was successfully applied to deliver one of the world s largest substation transformer (1500 MVA, three-phase, 500 kv). By application of the on-site assembly technology (one tank instead of three), the amount of necessary materials decreased: the quantity of copper wire decreased by 50%, transformer losses decreased by 20%, total weight decreased by 35% and installation area decreased by 40%. A contribution from Switzerland reports that applying the new resin impregnated synthetics bushings (RIS), like conventional resin impregnated paper bushing technology (RIP), is a good way to minimize the application of oil and reduce the risk of fire. The RIS bushings contain neither cellulose nor any other material with high moisture absorption ability. This fact, together with the application of the inorganic filler, impermeable to humidity, frees the RIS core material from any sensitivity to a humid storage environment. RIS technology used lower amount of materials of significant impact, such as aluminum and epoxy resin, which contributes to reduce even more the environmental impact. A contribution from Austria reports means to calculate the maximum stresses on the tank due to an arc pressure in order to design a tank with sufficient strength to avoid its rupture. The weakest part is near the top so in the event of leakage, the amount of insulation fluid loss is minor. Improved tank design together with high flash point liquids like esters will increase the safety aspect and so the eco aspect as well. For noise reduction, calculation models relating to magnetic forces, magnetostriction and dynamic mechanical models are used to deliver low noise transformer designs. Question 2.4 A contribution from Brazil, Sweden and USA reports information extracted from installed ester-filled transformers and experiments of laboratory induced failure modes. It was found that the acidity and power factor of ester oil increases significantly with ageing. But, these increases in acidity and power factor alone have not been followed by equivalent degradation of other properties of the ester oil impacting in the performance of the fluid, such as dielectric breakdown or viscosity. DGA: studies of natural ester liquids have shown that the gases 9

generated in esters are similar to the gases generated in mineral oil, but there is a need for specific diagnostic interpretation rules for ester liquids which in some cases needs to include the natural ester liquid chemistry also. Early oxidation: in a transformer filled with dry air (during shipping) can lead to the formation of a thin layer of polymerized liquid on surfaces (cellulose and tank walls) inside the transformer. A contribution from Australia reports that two utilities has installed monitoring systems on transformers filled with natural ester fluid and that no problem has been found. Before these transformers could be installed, it was necessary for procedures to be developed for handling the ester fluid. This was necessary to ensure that no residual fluid was left in the processing plant, as it would otherwise oxidise, or potentially contaminate mineral oil transformers which would later be processed using the same plan. Bleeding of the conservator bags was difficult on both of these transformers. Correct bleeding was considered essential since the ester fluid should not be exposed to oxygen. Question 2.5 A contribution from Brazil, Sweden and USA reports that because of the high moisture solubility of natural ester fluids, more water is drawn from the cellulose into the fluid under even quasi-equilibrium conditions. In addition to that, the reverse migration of the moisture from the natural ester to the cellulose is slowed by a phenomenon that is not completely understood yet, but that could be attributed to a chemical reaction between the water molecules and the natural ester fluid. The dryer paper then, ages at a considerable slower rate than if immersed in mineral oil. The practical result is that cellulose insulation has a 20 C thermal advantage when aged in ester fluids over when aged in mineral oil, i.e. for the same life expectancy, cellulose impregnated with ester fluids can be maintained at 20 C higher temperature than with mineral oil. Therefore the cellulose material applied in transformers filled with natural esters fluids can be expected to last more than twice as long as mineral oilinsulated paper when submitted to similar thermal stresses. A contribution from Sweden and Switzerland reports the eco friendliness and beneficial effects of reclaiming instead of changing the oil after ageing. For oil-filled equipment, a properly carried out on-line reclaiming will restore the oil to practically as-new condition. Any damage done to the solid insulation is irreversible, but the future rate of its degradation will be lower, since all the components that speed up paper ageing (low molecular weight acids, sludge and water) are reduced to a great extent by the treatment. The cleaned oil circulating through the transformer will extract acids and other degradation products from the cellulosic materials which allows to keep good oil properties during many years while we usually see oil acidity increasing after a few years after oil exchange. It is not possible to state very precisely the effect of reclaiming on ageing rate, but based on what research that is available, it seems reasonable to expect at least a reduction of the paper ageing rate by half. Discussion Several spontaneous contributions were raised from the floor. Contributors were invited to provide a written version of their comments for inclusion in the session proceeding. 10

PREFERENTIAL SUBJECT 3: TRANSFORMER MAGNETIC CIRCUIT A total of 6 papers have been submitted, according the following sub-topics: core and core structure design, modelling, temperature evaluation; material characteristics, manufacturing, assembling; saturation - inrush, direct current, geomagnetically-induced current. PS3 - Keynote presentation by Michael Hastenrath (DE) Mr. Hastenrath presented the present status and expected developments on grain oriented electrical steel for power and distribution transformers. He showed the past development of grain oriented steel and the improvements from classical grain oriented (CGO) to the highly oriented grades (HGO). The hot and cold manufacturing processing and the processing from GO electrical steel to transformer properties were exposed. Potentials for further loss improvements are smoother surfaces, improved coating, sharper texture, optimized domain refining and thinner gauges. Mr. Hastenrath concluded that the grain oriented electrical steel significantly contributed to the fact that transformers are among the electrical machines with the highest efficiency. The Special Reporter Mr. Stephan Voss formulated the following questions to structure the contributions from delegates to the SC A2 Paris 2012 meeting. Question 3.1: Is more power quality related information needed in transformer and reactor specification? Question 3.2: How to handle direct current and its effects in transformer operation? What are typical and critical limits and performance requirements? Which measures are preferable for utilities? Question 3.3: How to design (and review) transformers immunity to GIC impacts? How to prove the long term reliability of a transformer at the design stage? Which measures can be taken to handle GIC impacts? Question 3.4: How to reduce the risk related to inrush currents when energizing transformers? Question 3.5: How will active control change transformer and reactor technology in the future? How to cope with the risk of control malfunction? What are the experiences and the acceptance criteria regarding active control of transformers and reactors (e.g. increased sound level when regulating reactor core magnetization)? 11

Question 3.6: How will modern simulation tools change transformer and reactor design/design review process and technology in the future? Major design criteria still depend on empirics including safety margins. How to align and validate numerical model complexity and simulation results in case of missing reference designs? Question 3.7: What may be the decision criteria for selecting a triangular core design? Does less volume but different shape limit the applicability for replacement of old units? For triangular transformer design, magnetic fields decay faster with distance from the outer windings circumference. Does the closer vicinity of windings and yokes cause higher losses and thermal stresses inside the windings in converter applications (especially due to higher harmonics and eddy currents)? Question 3.1 A contribution from UK reports the IEC standard harmonic limits. In National Grid the harmonic current limit is 3%, the power quality is checked only for specific connections and nothing is said about power quality in transformer specification. Further questions on this topic would be: is this enough for the future? What about earthing transformers? Question 3.2 A contribution from Austria reports that the best way to mitigate DC current influence on power transformers and the grid is the active DC compensation. By measuring DC magnetization of the core, the same amount of DC ampere turns with opposite sign can be introduced. Thus, there is no resulting direct magnetization of the core and all negative effects are cancelled. As a consequence, power quality is improved, losses are reduced and noise is reduced to the original value. A contribution from South Africa reports on the effects of GICs on transformers. Even short, rapid and relatively low magnitude heating can cause hot spots that start the deterioration process. Rapid heating generates gas bubbles. Case of PD initiated by very small direct currents has been previously reported. Even small GICs can affect transformer and system operation and eventually cause transformer failure. A contribution from France reports experiments carried out in order to evaluate the effect of DC current on the magnetic behaviour of transformer cores. The transformer operation under DC bias excitation can be both measured and modelled using a combination of different techniques. The effect is reasonably understood and can be taken into account in the transformer design stage when specified by the user. DC bias excitation causes the saturation of the core during half cycle excitation and as a result a larger amount of leakage flux should be expected unless appropriate measures are taken. In addition, it causes an increase in the no load losses of the core however this is not thought to be significant enough to cause increased hot spots inside the core and thus problems during the transformer operation. A spontaneous contribution from Finland reports experiments on a three phase transformer with five limb core design withstanding a DC current level up to 200 A at full load. Question 3.3 A contribution from South Africa reports simulation studies which demonstrated that the three limb core design is less susceptible to saturation due to DC current than to the five limb design. For the five-limb core, for 40 A DC the temperature rise of the tie plates goes up to 12

almost 100 K which can only tolerated for a few minutes. For a 3-limb core, the simulations showed that the effect of GIC is almost negligible up to 400 A DC because the core is not driven into saturation. ESKOM will revise the transformer specification to specify only 3- limb designs where transport heights can still be achieved. A contribution from Japan reports experimental tests on large scale transformer model to determine the effect of DC excitation. Core plates in core type transformer and core support in shell type design were the parts showing the highest temperature rises. Temperature rise of about 110 C at excitation current of 8x10 4 A/m were measured and this temperature rise was reduced to approximately 1/10 by using non-magnetic steel. The lifetime reduction of this temperature under a fairly large GIC level is low and the duration of the effect is short. A contribution from France reports the experience and specifications relating to GIC and contribution to design choices and review. Users in areas where stray DC or GIC may occur often specify that transformers and shunt reactors shall withstand superimposed DC currents without damage or excessive temperature rise. The DC currents are usually specified with levels as high as 100 A and durations from seconds to several minutes. Capability of transformers and shunt reactors to withstand the DC currents needs to be demonstrated in the Design Review. Use of numerical modelling (like electric analysis tool and finite element analysis), comforted by experimental background, contributes to support the design choices and the design evaluation process. A contribution from Spain reports a methodology to design a transformer immune to GIC. The avenues are to reduce the transformer susceptibility to core saturation, reduce the transformer susceptibility to overheating and use higher temperature insulation in parts of the windings & structural parts expected to have high temperature levels for the GIC duration. A contribution from Austria reports the experience with the calculation, design/manufacture and tests of GIC-safe power transformers. Specific use of non-magnetic steel inserts in tank and clamping are used to reduce high saturation and overheating levels at extreme GIC. Question 3.4 A contribution from Brazil reports that by reducing the magnitudes of the inrush current using controlled switching, the impacts in power systems, such as temporary harmonic overvoltage, momentary voltage dips, stresses related with internal mechanical efforts on the transformer windings, undesired protection operation by high currents in capacitor banks and in transformers neutrals will be mitigated. The circuit breaker plays a crucial role in controlled switching as it must work in a stable way with a relatively small (close/open) time span. A contribution from France reports that the reduction of inrush currents at the energization of transformers is a major issue in existing and aged networks, as they may reach high values and lead to repetitive stresses for certain operating conditions. The use of adapted palliatives devices, like synchronous controllers, lead to very low values for the inrush currents, and also the overvoltages, due to the limitation of the magnetic flux in the iron core at the energization. Question 3.5 No contribution was presented for this question. 13

Question 3.6 A contribution from France reports modelling and experimental results on various cores to calculate the no-load losses. The numerical modelling is based on 2D FEM and a coupled circuit model to the mesh domain. The anisotropy of the core material was not taken into consideration, and the eddy currents which circulate between laminations were not modeled in this 2D study. By measuring a large number of different size cores and obtaining accurate numerical data for the tested cores, a high confidence level of prediction of the core no-load loss can be achieved. Statistical analysis carried out in conjunction with the numerical calculations, enables the accurate estimation of the air-gaps present inside the cores and allows the numerical calculations to be carried out while taking the air-gaps into account. A contribution from France reports on the resonance effect on no-load noise of a 100 kva magnetic core. FEM simulations and experiments using acceleration sensors were carried out and the measurements confirmed that the magnetostriction forces vary with a double frequency of that of the power source. The exciting frequency can only excite the natural modes which have the frequencies of even times of exciting frequency. A contribution from Finland describes a study of a power transformer submitted to severe over-excitation and overload condition when equipped with temperature sensors in different locations of the core and core structure. The study reveals the hottest spot location and temperature does not only depends on transformer design, but also depends on loading conditions as shifts of location was observed depends on loading situation. The author concludes temperature tests on the core are not meaningful for regular thermal testing, whereas such tests are very valuable for aligning and validating the calculation methods. Question 3.7 A contribution from Switzerland, Spain and USA reports that the reasons to select a triangular core design are to get: a more compact footprint; a more efficient transformer, a safer transformer, reduced sound level and reduced inrush current and harmonics. Discussion Several spontaneous contributions were raised from the floor. Contributors were invited to provide a written version of their comments for inclusion in the session proceeding. 14

CLOSING REMARKS In his closing address to A2, Chairman Claude Rajotte summarized the highlights of the 50 contributions that were presented during the day. Mr. Rajotte drew this A2 2012 Paris Session to a close by confirming the next SC A2 event is the Colloquium being held in Zurich Switzerland, on 8-14 September 2013. This is a joint meeting with SC C4 and the topics are: Interaction between transformer and the Power System Experience with the use of Phase-Shifting transformers Network planning in the context of an ageing transformer fleet The session was closed at 17.45. 15