By identifying the characteristic equation of the specific no-load losses of each iron sheet based on the core configuration, it is possible to reach a high level of accuracy in detecting no-load losses over a wide range of flux density. The method is applicable and frequently used for reference as well as for new sheet types that appear on the market. The results presented in this paper are based on one year experience, showing that the total weight and volume of transformers have clearly been reduced

Ahmed Gamil

Franz Schatzl

Hrčak - Portal of scientific journals of Croatia

ABSTRACT By identifying the characteristic equation of the specific no­load los­ses of each iron sheet based on the core configuration, it is possible to reach a high level of accuracy in de­tecting no­load losses over a wide range of flux density. The method is applicable and frequently used for reference as well as for new sheet types that appear on the market. The results presented in this paper are based on one year experience, show­ing that the total weight and volume of transformers have clearly been re­duced. KEYWORDStransformers, core parameter, core corners, Reference Sheet Type (RST), New Sheet Type (NST), building factorDetermining transformer core losses based on investigation of core material behaviour during test and operation – resultsEVENTSTRANSFORMERS  MAGAZINE  |  Volume 3,  Issue 1CORE MATERIALS50    4. ResultsThe results of this study are divided into two sections. The optimisation results define the characteristic equation of each iron sheet material, combined with its histogram density function, which shows the probability of reaching a certain to­lerance. Second section concerns practi­cal results, which provide an overview of the real deviation between the measured and calculated no­load losses using the new approach over the last year. Besides, some transformer design examples will be considered in a comparison between the first designs built many years ago and the new designs depending on the new ap­proach. On the one hand, the core weight and consequently the whole transformer volume will be reduced. On the other, an accurate estimation of no­load losses will assist in avoiding exceeding guarantee li­mits and penalty consequences.Part 1 of this paper, published in Transformers Magazine Vol 2 Issue 4, described a mathematical model of a new approach for accurate deter-mination of no-load losses. In Part 2 of the paper, the results and practical experience with the application of the new approach are presented.4.1 Optimisation resultsDefining the core parameters means that the characteristic equation of the core ma­terial can be obtained separately for the corners and for the rest of the core (limb & yoke). Figure 6a illustrates how Pspc changes with the change of B for the core corners and the rest of the core. These cur­ves result from material handling and core manufacturing for four different materi­als. These materials have specific no­load losses in the range of 0.75 to 0.91 W/kg and the grade range from 23 to 30, which means that the material manufacturer is not able to detect this behaviour as long as there is no information from the core manufacture. Besides, these curves could only be used concerning a certain known path of the core material, namely: certain core manufacturing plus certain material handling plus core­form transformer.As depicted in Figure 6b, PRspc for ma­  t erials 1 and 2 is similar with a very slight difference. However, there is a huge dif­ference between the same mat erials in PEspc. Materials with a very slight difference in no-load losses in the rest of the core (PRspc) can display a huge difference in no-load losses in the corners (PEspc)Figure 6b. Enlargement of Figure 6a for the flux density region between 1.4 and 1.7 Tesla (T)Figure 6a. Specific no­load losses for four iron sheet materials different in thickness, and spe­cific losses for core corners and limb­yoke regions (plain lines are used for core corners, and circle markers for the rest of the core)www.transformers-magaz ine .com   51 Ahmed GAMIL, Franz SCHATZL Even with high accuracy for determining the core material characteristic equation, there still must be some deviation bet­ween the measured and calculated values. This deviation could be estimated using the probability density function. This function, based on a certain scattering of values, describes the frequency of the event that a certain value of this scattering occurs with respect to the total scattering. This could be presented as relative fre­quency or percentage frequency. In Figure 7, concerning quality a, the probability of having no deviation (equals zero) reaches 20 %. This means that if this material is used for 100 transformers, 20 of them are expected to show no deviation between the measured and calculated no­load los­ses. Using the probability density func­tion, the resulting relative frequency will define a deviation interval for each core material and in turn a practical indication of the quality, as shown in Figure 7.As there is zero deviation between the measured and calculated value, one can see that the material with quality b has the highest probability. This means that in situations where exceeding the gua­rantee limits will lead to refusal or high penalty, quality b is preferable. In other cases where tolerance is allowed from the customer side, the choice among qualities is taken from a cost point of view. If the tolerance is about +2 %, and from the graph one can see that the area under the three curves of a, b and c up to 2 % is nearly similar, the choice will depend on the material price and deli­very time.4.2 Practical  resultsSince the beginning of 2012, the new approach for determining no­load los­ses has been used in our own algorithm systems (built for SGB Regensburg, Ger­many). So far, a clear improvement in the design of new and rebuilt transformers has been recorded. The high accuracy of this approach results in cost saving in terms of the core weight, which means savings in terms of entire transformer materials and the total volume, as shown in Table 1. On the other hand, its accura­cy helps to avoid exceeding the customer guarantee value. The histogram in Figure 8 covers the transformer manufacturing by SGB from the beginning of 2012, which shows a probability of 83 % in a tolerance range of ± 3 % from the calcu­lated value.Figure 8 shows an average deviation of ­1.7 % with the standard deviation of 1.7 %. This means that the measured no­load losses, on average, lie under the estimated value by 1.7 %, where the lowest recorded deviation from the calculated value lies at 3.4 % (1.7 % + 1.7 %). The histogram also includes the NST amount, whose Pspc esti­mation has improved with time, as more information is gained. A very important advantage of the new approach is that the quality of RST is always under monito­ring. If the deviation between the measu­red and calculated P0 is out of the expected range, the history path of the core material is investigated from the sheet manufacture to the transformer test.ConclusionThe new approach is very promising in the present and the future, providing an opportunity for quality control not just for the core material types but also for the en­tire core manufacturing process. Of course, as for any method or approach, there are benefits and risks. One of the main advan­tages is implementing the building factor in the difference between PEspc and PRspc, which seems to be a function of flux den­sity as shown in Figure 6. The main risk lies in detecting the core parameter for NST. Some precautions must be taken when using NST to avoid any undesirable results or deviations. However, until today only one type of NST has shown unexpected results, which has been proved to have re­sulted from the change of quality of the iron sheet manufacturer. During the application of this NST, the sheet category has changed from NST to RST based on the acquired measurement history.A very important ad-vantage of the new approach is that the qual ity of RST is always under monitoringCORE MATERIALSFigure 7. Relative frequency of the expected deviation for different core material qualitiesTable 1. Amount of iron weight saving considering the new approach to improving old trans­former designs according to their same requirements  Transformer data    Iron weight saving [%]           110 MVA (50 Hz) 110/30 kV  4.0            80 MVA (60 Hz) 115/69 kV  18.3          35 MVA (50 Hz)  69.2/9 kV  3.4Since the beginning of the application of the new approach for determining no-load losses in 2012, a clear improvement in the design of new and rebuilt transformers has been recorded52    TRANSFORMERS  MAGAZINE  |  Volume 3,  Issue 1own sheet types. In all cases, the principle of the new approach is the same – one char­acteristic equation for the corner and one for the limb­yoke regions.The new approach is a continuous pro­cess of quality improvement. A cyclic in­vestigation has been performed over one year to increase its accuracy and avoid any undesirable deviations caused by any segment in the material chain: from iron manufacturing to transformer test. A high accuracy of this method has resulted in cost savings in terms of the core size and consequently, the entire transformer ma­terial needs.Bibliography[1] J.C. Olivares­Galván and R.Escarela­Pérez, Separation of No­Load Losses for Distribution Transformers Using Experi­mental Methods, 7th Mediterranean Con­ference on Power Generation, Nov. 2010.[2] Y. Chen and P. Pillay, An Improved Formula for Lamination Core Losses Calculations, 2002 IEEE IAS Annual Meeting, pp 759­766.[3] F. Zhalefar and M. Sanaye­Pasand, In­ternational Colloquium Transformer Re­search and Asset Management, Nov. 2009. [4] S. V. Kulkarni, Transformer Enginee­ring Design and Practice, 2004.[5] M. Robert, Transformer Design Prin­ciple with Application to Core­Form Po­wer Transformers, 2nd Edition, 2010.[6] J. Martin, The J&P Transformer Book, 12th Edition, 1998.[7] R. Feinberg, Modern Power Transfor­mer Practice, 1979.[8] J. John, Power Transformers Princip­les and Applications, 2002.[9] T. McLyman, Magnetic Core Selection for Transformers and Inductors, May 1997.Further, there are some cases of inad­equate material handling by the manufac­turer, so that the deviation span is wide of the estimated characteristic equation (hig­her standard deviation). In this case, the sheet type is used in limited amount and very cautiously to be sure that the final de­viation is in the negative region, as shown in Figure 8, in the range between ­4 and ­6 % (less than 10 transformers in 2 years). This method has already been applied to over 1,000 units.One of the main principles of this method is that changing of the core corners weight depends only on the changing of the core cross sectional area, which is inversely proportional to flux density if the volts per winding turn are kept constant. If changing the limb and/or the yoke lengths is causing changes in the corners volume/weight, then another consideration must be taken into account to estimate Pspc in an accurate way. In this case a 3D­illustration is requi­red based on the changes in the core weight and flux density. However, as mentioned before, each transformer manufacturer will have its own characteristic equation for its The main risk of the new approach may occur while detecting the core parameter for NST, and some precautions must be taken to avoid any undesirable results or deviationsFigure 8. Histogram of the deviation of no­load losses from 2012 until today, including over 200 transformer designs (different core designs with different material types)AuthorsAhmed Gamil completed a M.Sc. in electrical power engin­eering in 2004 and has worked in transformer protection and monitoring systems as well as software development through different positions at AREVA, Siemens and German Research Association (DFG). His work involved simulation of network short circuit and transformer analysis under lightning impulse voltage as well as programming of offline/online monitoring systems. Today he works for SGB Regensburg, Germany, in R&D sector and leads different development projects in power transformer design. He is also an active par­ticipant in international conferences such as CIGRE and CWIEME.Franz Schatzl graduated from the University of Vienna in the field of electrical power engineering in 1998.  In 1999 he joined Siemens Transformers Austria (STA) AG as an electrical design engineer. He was responsible for special applications such as low noise transformers and the use of alternative liquids. From 2007 to 2009 he oversaw the electrical design department. Since 2010 he has been Technical and R&D Manager at SGB Regensburg in Germany in the power transformer division. Franz is also member of IEC and CIGRE working groups and is author of several papers on different power transform­er subjects.  Correction of the equation (1) from Part 1 of this article published in  Volume 2, Issue 4 on page 23:P0 = GE . PEspc + ( GT – GE)  .  PRspc            www.transformers-magaz ine .com   53 

Determining transformer core losses based on investigation of core material behaviour during test and operation – Results

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