https://doi.org/10.3311/PPee.13000 Creative Commons Attribution b |37 Periodica Polytechnica Electrical Engineering and Computer Science, 63(1), pp. 37–50, 2019 Evolution and Modern Approaches of the Power Transformer Cost Optimization Methods Tamás Orosz1* 1 Department of Theory of Electrical Engineering, Faculty of Electrical Engineering, University of West Bohemia, Universitni 26, 306 14 Pilsen, Czech Republic * Corresponding author, e-mail: [email protected] Received: 14 August 2018, Accepted: 04 October 2018, Published online: 21 January 2019 Abstract Transformer design is a challenging engineering task, where the different physical fields have to be harmonized together to fulfill the implied specifications. Due to the difficulty of this task, it can be separated into several subproblems. The first subproblem, in the pre-concept phase, during the transformer design is the calculation of the cost optimal key-design parameters, where not only the technical but also the economical parameters have to be considered, as well. This subproblem belongs to the most general branch of the non-linear mathematical optimization problems. This paper presents the main directions of the evolution and trends in the power transformer design. Main directions of the considered research and the future trends in the field of preliminary design transformer optimization methods are summarized. Keywords power transformer, design optimization, mathematical optimization 1 Introduction The design process of a power transformer is a com- plex optimization task, where not only the technological requirements but also the influence of the economic feasi- bility have to be considered. Moreover, the design method- ology may vary significantly according to the transformer type (e.g.: core or shell form, instrument transformer) and the applied manufacturing technology [1]. To create a competitive design, many non-linear con- straints have to be considered simultaneously to fulfill the imposed specifications. The searched design parameters can interact with each other and belong to different phys- ical domains – such as electrical, thermal, mechanical, etc. Therefore, the design of power transformers generally leads to a coupled, multidisciplinary optimization prob- lem, where the problem is not only to find a feasible solu- tion but also the cost-optimal solution [2]. Due to the complexity of the problem, classically, the design process of a power transformer is divided into the Fig. 1 The classical transformer design partitions [2, 6]. following three subproblems [3-5] (Fig. 1): • Pre-concept stage: design, which is usually sufficient for the proposi- The cost optimization of a transformer design is tion and a state of requisition. The design variables made in the preliminary (pre-concept, tender) design or parameters of this concept study is called the stage. The result is a design study, a simplified key-design parameters. Cite this article as: Orosz, T. "Evolution and Modern Approaches of the Power Transformer Cost Optimization Methods", Periodica Polytechnica Electrical Engineering and Computer Science, 63(1), pp. 37–50, 2019. https://doi.org/10.3311/PPee.13000 Orosz 38|Period. Polytech. Elec. Eng. Comp. Sci., 63(1), pp. 37–50, 2019 • Final design stage: The final design is essentially the refinement of the preliminary design to the most minute and complete details. Its output is the final design engineering record and the drafting and manufacturing instructions. • Checking stage: Checking of the final design, usually by another designer. This paper focuses on the optimization methods and approaches used in the pre-concept stage. The mathematical representation of this transformer design optimization subproblem belongs to the most gen- Fig. 2 Categorization of different transformer types by their nominal power and possible categorization of power transformers. eral branch of the non-linear mathematical optimization methods [7-9]. The first analytical methods were intro- duced at the beginning of the twentieth century [10] – only according to individual factors such as voltage, power, cli- some years later than the first power transformers were mate, system topology, sound level and others. Therefore produced [11] – showing the importance of the topic. One these machines are usually not interchangeable, thus the of the first application of the digital computers in the trans- reliability is a key factor [15]. former industry is an iterative solution for this design opti- The power rating level of this transformer category mization subproblem [2, 3, 8]. has been increased a lot during the last century due to the During the last century not only the applied methods advancements of applied materials and manufacturing have been evolved, but also the transformer industry, the technology [7, 9, 16-20]. The first power transformer was manufacturing methods, the construction of the trans- shipped on September 16, 1884 by Ganz & Company. This formers and the applied materials have been evolved a lot. first unit was a one-phase, shell-form transformer with the The first analytical methods just considered oversimpli- following specifications: 1400 W rated power, 42 Hz net- fied models consisting of only the active materials, such work frequency, 120/72 Volts, 11.6/19.4 Amperes, and 1.67:1 as the core and the windings. These models neglected the turn ratio [11, 21-24]. The first three-phase unit developed impact of neglecting the cooling, the insulation system soon at the Allgemeine Elektricitäts- Gesellschaft ('General or other non-linear component at this pre-concept design Electricity Company') in 1899 [25]. The rated power of the phase [7, 12-14]. above mentioned first core-form power transformer units This paper provides an overview from the evolution of was lesser than 10 kVA (Fig. 3) [11, 26]. Interestingly, this these cost optimization methods, based on extensive num- power rate would not be considered as a power transformer ber of publications. according to the nowadays classification. The need for increasing grid power and larger distances 2 Trends in Transformer Technology induced the usage of larger power transformer units and A wide range of transformer types exists from the small electronic devices till the largest components of the power transmission network. Although these devices are based on the same physical principle, viz. the electromagnetic induction. They can be categorized in several different ways: by their rated power (Fig. 2), the type of the applica- tion, manufacturing technology etc. This paper focuses on the evolution of the preliminary design methods, which relates with large power trans- Fig. 3 The left side of the picture shows one of the first core type formers (LPT). LPT is a large, custom-built equipment power transformers, manufactured at the GANZ factory in 1886, rated in the bulk transmission grid. LPTs are very expensive, power was 7.5 kW [27]. On the right-side the size of this transformer is unique machines, tailored to the customers' requirements compared with a modern large power transformer. Orosz Period. Polytech. Elec. Eng. Comp. Sci., 63(1), pp. 37–50, 2019|39 the usage of larger voltage levels due to economic rea- are used: paraffinic-based and the naphthenic-based oils sons [18]. Nowadays, the nominal power of the largest [48]. During the aging processes insoluble sludge has been transformers reaches the 1500 MVA and the applied high produced in the case of paraffinic-based oils. This sludge voltage level is more than 1000 kV [20, 28, 29]. The trans- increases the pour point, the viscosity of the oil and causes former business and the load of a transformer design has overheating and reduced service life [48]. Naphthenic- doubled in every 11 years till 1945 when standardized, based oils has been developed to overcome these limita- repetitive transformer designs were introduced from 500 tions, which with aromatic compounds can remain fluid to 10000 kVA to reduce the engineering work and the at -40 °C But nowadays, the isoparaffinic based oils can shipment times [2]. have better performance [49]. Other disadvantage of the Nowadays, the term LPT refers to those power trans- mineral based oils is their inflammability and they are not formers which have more than 2,500 kVA rated power and biodegradable, viz. not environment friendly materials. do not include a winding intended for connection to the Not only mineral oil based insulation liquids are used low-voltage distribution network (Fig. 3) [16, 17, 30]. The in power transformers. Polychlorinated biphenyl (PCB) common in the large power transformers from the differ- based insulation materials has been widely applied in the ent decades of the last century is that they are too large electrical apparatus where the fire safety was the most or too complex for bulk production. Thus, these machines intrinsic design criteria [9, 45]. Till the end of the 60's are designed customly for the specific requirements of the when PCB's environmental toxicity was proved and the application, for example [15] states that approximately PCB's was classified as persistent organic compounds [50]. 1.3 LPTs are produced for each transformer design [16]. Since then the silicon oil and the synthetic ester based Due to the growth of the rated power the load and the insulation liquids, which flashing point are above than no-load losses are increased as well. In the case of large 300 °C are successfully applied in fire hazardous environ- power transformers the sum of these losses can be hun- ments. There are many other attempts with the application dreds of kilowatts. Accordingly, the diversion of heat is a of the SF6 gas in power transformers [51]. In the last twenty challenging engineering task. Different winding and cool- years, the most promising trend is the replacement of the ing systems with direct and natural oil flow has been intro- mineral oils with biodegradable, natural ester based liq- duced to solve this problem [17, 18, 31, 32]. uids [48, 52-55]. Both the thermal and the dielectric prop- One of the main challenges in insulation system design erties of these materials are very promising.