UPGRADING OF LARGE TRANSFORMERS 3D-DESIGN: ENERGY-SAVING AND RELIABILITY OF ELECTRIC POWER SYSTEM

Transcription

1 UPGRADING OF LARGE TRANSFORMERS 3D-DESIGN: ENERGY-SAVING AND RELIABILITY OF ELECTRIC POWER SYSTEM Xose M. Lopez-Fernandez*, J. Turowski ** D. Souto Revenga*** and A. Soto Rodriguez*** * Dep. of Electrical Engineering, University of Vigo- Spain, xmlopez@uvigo.es ** Dept. of Intelligent Information Systems, WSHE Lodz Poland, Consultant for University of Vigo Spain, jt@p.lodz.pl *** EFACEC Energia SA Transformer Works, Porto, Portugal The global demand for power transformers is being more than doubled in the last five years, which have impacted on the power transformer industry with its suppliers, in how design to build with higher demand on quality, reliability and efficiency [1]. The power transformers are keys and costly components in network of the electric power systems, where their design have an essential consequence on safety, reliability and cost of electrical supply [2,3,4]. Methods of design and calculation of main active parts (core, windings) of transformers are based on rather easy 2-D field models and are sufficiently accurate. However, only 3-D design must be applied to inactive parts outside core windows [3,4,5,6]. In this work is pointed the meaning of reliability and efficiency of the large power transformers in the current market on the electric power systems from design point of view. For such purposes hybrid analytically-reluctance Network Method threedimensional RNM 3D [3,4,5] has proved here as one of the best for rapid design into the complex structure of modern transformers with extreme electromagnetic filed concentration, its crushing forces, eddy current loss and overheating hazard. Keywords: Power transformers, 3-D design, Energy-Saving, Reliability 1. Introduction The global demand for power transformers has more than doubled in the last five years. From about MVA during end of the 1990s the market has raised to roughly MVA this year. Some trends are pointing on a continuous increase. The capacity of transformer production is running at % which also includes the whole supply chain of transformer materials and components. A similar situation was found after the Second World War, 60 years ago. Today the main reasons are [1]: A huge growing demand in China, India and the rest of Asia and the Middle East. Needed reinforcement and replacement in the western world. The ongoing shift to increased use of electricity in the whole world followed by the need for energy efficiency and carbon dioxide reduction. Those big market changes have impacted the power transformer industry with its suppliers, in how design to build with higher demand on quality and reliability and efficiency. Such increasing demand of electrical energy and the growth of grid electric power systems the transformer technology is faced to new problems to be solved. So, today s designs with high material prices and low loss evaluations will see more intensive material utilization with increased stresses. Such material overload compare with the past is shown how recent catastrophic black-outs have exposed major vulnerabilities in existing generation, transmission and distribution systems [4,6]. Improper design and technology have created a situation where reliability of the entire system is often under question. It also has a negative environmental impact by significantly increasing losses. The power transformer technology platform can be classified into mainly two levels [1]: Transformer Physics o Basic Dielectrics o Core Performance and Acoustics o Load Losses and Thermal Performance o Short Circuit Strength o 2D and 3 D Simulations The Engineering Tools and Manufacturing Technology with accurate Testing Transformer Physics has the goal to maximize the reliability and efficiency without increasing the cost of the transformer. Here 3-D is where plays an important role, and more precisely in the evaluation and control of stray field since in large power transformers stray losses and overheating hazard became as decisive as load losses. Today 3D simulation programs are available for calculating electromagnetic losses in structural parts. However still, they require high time and computer capacity to be used by designer in the day to day [6] November 2007, ISEC-Coimbra Portugal

2 2. Today s big market change In the past the transformer prices in the western world consisted roughly of 50% material costs and the rest was related to personal and financial costs. The new emerging markets had prices where about 60 % was material and 40% for the rest. This has now dramatically changed with the increase of the raw materials so materials are now dominating with % of the prices. This quick quantum leap is not fully understood by the market [1]. This means of course that transformer manufacturers today are forced to reduce materials by better utilization of core steel and copper. The big question from the market is how this will affect the quality, reliability and efficiency of transformers now delivered and to be delivered. What is added to the material prices is that the loss evaluations have in relation to the material prices been decreased by a factor 3 or 4. Large utilities and power plants are calculating with much higher interest rates and low electricity costs in the planning of the projects. All those big changes drive the utilization of the material much higher today than in the past. Therefore the question of future reliability is one of the most relevant issues in this industry and the answers have to be found within the departments of design in the power transformer industry [1]. 3. Rating Today large power transformers are built for the highest transmission voltages presently in service, viz. 800 kv and with three-phase ratings of 3000 MVA and above for system intertie transformers and up to 2000 MVA for generator step-up transformers. The net extension plans in China and India and possible in other huge continents go now for 1100 kv and 1200 kv AC grids [1]. 4. Reliability Reliability of power transformers, is one of most important concern of present times, which affect reliability of transformers and whole power system, except overvoltages, are excessive local heating due to uncontrollable local concentration of eddy currents, induced in solid parts of transformers by strong stray fields, especially at overloading. If not examined accurately, they can easily cause the unplanned automatic shutdowns of a power plant. Such shutdown cost more than US$ 0.5 million a day each 1. As per Rickley (1997) the cost for replacement of power transformer at unplanned automatic shutdowns of a power plant, costs even from US$ 0.5 to 0.8 million or more per day A decade of nuclear progress, - Power Engineering Review. No 5, 1989, p.25. Alarming warning were the last power disasters: the biggest power failure in history called the Great North-East Blackout of There was a complete shutdown of electric service throughout New York, 8 north-eastern States and major portion of Canada. In the Province of Ontario, some 30 million people were without power for as long as 13 hours 2, which confirmed importance of transformers reliability. It happened again in New York ( ), France (1978), USA Pacific Region (1996) 4 in Canada, New York 5 ( ), and recently in France and North Italy ( ), and in Barcelona this current year ( ). This cascade tripping was mostly due to the overloading of transformers and is the sufficient reason not to ignore the stray field phenomena, especially at overloading 6 [4, 6]. 5. Energy-Saving With increasing size in power rating and transmission voltage the effects of eddy currents and electromagnetic forces in terms of local heating and mechanical withstand on through faults or short circuits are most critical [2, 4, 6]. At the same time, capitalised load-loss, depending on the time and place of operation, costs from 3000 to US$/kW [4] and no load loss even more. The cost of capitalised load losses for generator transformers is little above 6000 US$/kW and it will be about 3000 US$/kW for network transformers. Now the power loss cost more and life time of transformers is longer [4]. At last but not least, each 1 kwh of electrical energy needs to burn up to 0.4 kg of coal, which again is of no little ecological account. 6. Stray losses. Calculation and control Load losses must be now calculated with the highest accuracy than before, in order to guarantee temperatures in all service conditions and not only for windings as the standards are stipulating but also for vital structural parts. Stray power losses are those, which are induced in the passive, made of solid metal, parts of a transformer. They are of significant importance, especially in large transformers, where a large magnetic field concentration may appear. It causes not only a power loss comparable to that in the windings, but also gives rise to local overheating 2 IEEE PES, January Author (JT) participated in discussion of CIGRE experts with N. York Chief Power Engineer in this question, in Bruges, Belgium. 4 Spectrum IEEE, April Time, August 25, 2003, pp Only thanks to the author s (JT) expertise it was not rejected at the acceptance test the biggest transformer in Canada, November 2007, ISEC-Coimbra Portugal 581

3 hazards. Behind, stray fields works with their usually complicated three-dimensional behaviour, which can lead to a failure of the whole unit as well as their evaluation and control being potentially a reserve for significant cost savings. One of the main tools of stray loss control in transformers, except winding shaping, is screening and shunting in leakage field region properly [5]. Stray losses in large transformers with not screened tank grow rapidly with increase of their rated power [2-3] and at values over MVA can dramatically exceed other losses. This is why tank walls should be carefully screened considering the spatial three directions together [6]. 7. Large transformer design An optimum design can be worked out with due considerations of design and manufacturing constraints. With the ever increasing competition in the global marketplace, there are continuous efforts to optimize the material cost of transformers. In most of the contracts, the transformers have to be delivered in a short period of time, and hence the speed of design and manufacture of the transformers is the key issue [1]. Unfortunately such 2 D formulae can not be properly justified and are often very controversial [2]. They are not general and limited to specific group of transformers. Nonetheless, with introduction of semi empirical corrections, 2 D formulae can deliver, in some range, results near to real ones. However, such approach can lead designers to incorrect results [6]. for highly qualified specialist. At the same time the accuracy of FEM 3D is not better than much simpler programs like for instance the RNM 3D recommended by authors [2]. Investigations carried out by author of [2] and his numerous collaborators have shown that his packages of the RNM-3D class, has proved as the most convenient, rapid and low-cost tool of reduction power losses and hot-spot hazard. It was confirmed industrially by broad application of these packages in more than 40 transformer works all over the world [3, 5]. 9. RNM-3D The Reluctance Network Method RNM belongs to one of the simplest and fastest methods of modelling and computation [3,5]. Its three dimensional version RNM 3D was introduced to calculate the leakage field on three phase transformers by Professor J. Turowski in It is extremely competitive when compared with popular FEM 3D and permits the analysis of complex three dimensional, three phase, non linear electromagnetic structures. The main secret of success is application of extended and deep theoretically base Knowledge Base [6], which lightens the programming process itself. In order to illustrate the computation of network parameter reluctances a application on shell-type large power transformers was considered according to Fig.1. Despite, that leakage field beyond core windows has complicated 3-D structure, many transformer works still use for this region a primitive, not adequate 2-D statistic formulae. Such approach is wrong, because stray field in this region has not only axial component, but also a strong circumferential one [2]. 8. Large transformer 3-D design There are sensitive and delicate regions needs more and more precise 3-D analysis for removal unnecessary power loss as well as monitoring and predict possible local overheating hazard. They can be classified into there many hazardous places as: tank walls with different screening and shunting, yoke-beams, clamping plates and bolts, bushing turrets, etc. Application to these regions of 3-D commercial programs like FEM-3D, FDM-3D, BEM-3D, also failed [6]. They are too costly, too labour-consuming and heavy still. In spite of serious increase (since 1985) of computational capability, our recent experience shows that FEM 3D needs several dozens, if not some hundreds, hours of work Fig.1 Single phase shell-type transformer modelled. The methods of reluctance network is based on simple, easy understandable Ohm's and Kirchhoff's Laws. FMMk =(NI)k = Rk Φk (1) lk Rk = (2) µ k sk Φ k = 0 FMM k = 0 (3) November 2007, ISEC-Coimbra Portugal

4 PRE-PROCESSOR MODEL DATA RELUCTANCES SOER RESULT Fig. 4. Developed interactive design structure of RNM-3D. POST-PROCESSOR Φ [Weber] H [A/m] B [T] P [W] Where in any axis direction xyz, according to Fig.3b, once the study domain was discretized there will be to identify: FMMk is the magnetic voltage or magnetomotive force of excitation coil with (NI)k amper-turns of k-th branch; Rk is the reluctance of k-th element; Φk=Bk sk is the magnetic flux in k- branch; Hk and Bk are the magnetic field strength and flux density respectively; lk and sk are the length and cross-section area of the reluctance element respectively. The number of k-branches of network is determined exactly by its discrete coordinate xyz of the Cartesian coordinate system. For every reluctance in Fig.3 there are corresponding magnetic flux Φx, Φy and Φz passing through this element. All quantities FMMk, Rk and Φk are uniquely localized in the network with aid of indices xyz corresponding to discrete coordinates of the system. HV CORE HV TANK TANK There are two main kinds of reluctances: - Magnetic resistance. The reluctances are calculated from various geometrical dimensions and electrical parameters of the transformer, according with equation (2). - Magnetic impedance. Complex reluctances Rµ, for conductive area of the metal element of the transformer such as tank wall and cover, made of solid steel with real Rr and imaginary Ri part (equation 5). These reluctances take into account analytically the skin effect, eddy current reaction with phase shift, nonlinear permeability inside solid metals, equivalent depth (equation 6) and screening effect due to alternating field. R µ = R r + jr i (4) δ 2 ωµσ = (6) Fig.2 3D-Model of a shell type Transformer. Study domain after applying symmetries. node-j (NI)k Rk node-i k-branch a) b) Fig.3 a) 3D-Reluctance Network Model of the study b) Representative k-branch of the network. The scheme of iterative process design is shown in Fig. 4. It is essentially a trial-and-error approach, where the designer making all important decision with the programmed tool, RNM-3D software, running into a computer. Solving by means a nodal equation method magnetic flux is obtained in each of k-branch of reluctance network as result. From computed magnetic flux en then easy postprocessing the magnetic field strength, flux density, losses, hot spots, electromagnetic forces, etc, into all 3-D domain of transformer November 2007, ISEC-Coimbra Portugal 583

5 B[T] 0.21 X Y Axis position 1 An example of results of the flux density on the tank wall along three spatial directions (Fig.5) is shown in Fig.6. In a simple way one can quickly find the hot spots on tank wall (place 2,3 and 5), and at the bottom (place 1) [3]. 10. Conclusions Z Fig. 5. 3D-Model of shell-type transformer. 1) Popular assumption that outside the core window of large three-phase transformer exists only 2 D axial H mx field component is misleading. There exists also a strong, 30 to 50% or bigger circumferential component H my, which create there complicated three dimensional (3-D) electromagnetic and thermal field. 2) Magnetic screens (shunts) are most popular now, because they seems more safety. However shunts should be laminated so, that the leakage flux lines pass always along a sheets plane. In opposite case, when shunt lamination is situated perpendicularly to field lines, the losses will be not eliminated, but even can be up to %. Each construction variant needs therefore careful interactive 3-D analysis of field lines distribution and shunt lay out. Upgrading a quality costs, but improvement of shunting can easily reduce e.g kw losses 3000 to US$/kW capitalised costs. 3) Accurate complex design and acceptance test of modern energy saving and reliable transformers, based on science, needs extraordinarily careful calculation of 3-D stray field, loss, overheating risk and screening with the help of complex 3-D program, based on rapid expert, interactive approach with considering thermal phenomena, eddy current reaction and iron nonlinearity ) Magnetic shunts, if are well designed are effective and safety. However they are heavy. If the correct and rapid 3-D program exists, one can try to return to much lighter Al electromagnetic screens, which have been used extensively before.. But here especial careful calculation of 3-D eddy currents and temperature analysis is necessary, especially on the edges of screens. RNM-3D approach is good for that job, because it is easy to understand clearly all necessary phenomena. Reward of such analysis could be much lighter transformer. Acknowledgments Authors wish to express their sincere gratitude to EFACEC Energía transformers factory in Porto- Portugal, which partly supports this work under a collaboration Project with University of Vigo-Spain ( ). References 4 Fig. 6. Flux density results on tank obtained with model of Fig. 5. Tank bottom (place 1) and tank wall (place 2, 3, and 4). [1] Thomas Fogelber. Reliability of Power Transformers - Important Aspects to Drive the Industry to Focus on Quality Proceedings of Advanced Research Workshop on Transformers (ARWtr2007) Baiona-Spain, October [2] J. Turowski Xose M. Lopez-Fernandez, A. Souto Rodriguez and D. Souto Revenga. Stray Losses Control in Core-and Shell Type Transformers. Upgrading of Energy-Saving and Reliability of Large Transformer Proceedings of Advanced Research Workshop on Transformers (ARWtr2007) Baiona-Spain, October [3] J. Turowski Xose M. Lopez-Fernandez, A. Souto Rodriguez and D. Souto Revenga. Stray Losses Control in Shell Type November 2007, ISEC-Coimbra Portugal

6 Transformers. Method of Three-Dimensional Reluctance Network Solution (RNM-3D Shell) Proceedings of Advanced Research Workshop on Transformers (ARWtr2007) Baiona- Spain, October [4] J. Turowski. Rapid Upgrading of Energy-Saving and Reliability of Large Transformers. Proceedings of TRANSFORMATOR 2007 Conference, Torun-Poland. pp , May [5] J. Turowski, M. Turowski, M. Kopec: Method of threedimensional network solution of leakage field of three-phase transformers. IEEE Transactions on Magnetics. Vol.26, No 5, September 1990, pp [6] Xose M. Lopez-Fernandez, J. Turowski, E. Lesniewska, B. Ertan (Co-authors and Co-editors): TRANSFORMERS in Practice. on CD ISBN , Vigo, Spain, November 2007, ISEC-Coimbra Portugal 585