Research on performance improvement and industrial application of static electromagnetic equipment using iron-based amorphous alloys-Doctoral Dissertation (Thesis(doctor)----Tohoku University-2019-JAPAN
ABSTRACT：Iron-based amorphous alloy has the advantages of lower iron loss and higher permeability compared with conventional grain-oriented silicon steel due to disappearance of magnetocrystalline anisotropy derived from randomly aligned magnetic atoms. One can expect for considerable improvementinpower efficiency of presently commercialized staticelectromagnetic machinessuch as distribution transformer and filtering reactor for inverter system by replacing the core material from silicon steel to iron-based amorphous alloy. Moreover, the material’s satisfactory performance at higher frequency enables application to the fieldof next generationin power distribution system. However, the commercially available iron-based amorphous alloy produced with meltquenching method has a shape of 20 to 30 m-thick thin foil; the coremade from the stacked and wound several hundred to thousand foils isfragile. This form of amorphous core causes difficulty in mass-production of the low cost and larger power capacity electromagnetic machines. Consequently, the range of industryapplication of amorphous alloy has been limited at present. Hence, this study aims at contribution to the savings of energy and CO2emission by theproposalsof low cost structures and design methodologies of three typesof amorphous electromagnetic machinethoseenable extensions of power capacity and excitation frequency. Additionally,the effect of improvement inpower efficiencies of them isdemonstrated by way of prototype tests. This thesis consists of six chapters;the research results are shown in accordance with the following organization. The chapter 1 describes the background and the objective of this study. The chapter 2 describes conventional technology and problems of amorphous electromagnetic machines. First, the advantages and disadvantages of presently mass-produced iron-based amorphous alloy were organizedcomparing the magnetic and physical properties of it with those of conventional grain-oriented silicon steel. Then, this chapter reviewed production methods of presently commercialized amorphous electromagnetic machinesandclarified the problems to be solved for extension of range of industryapplication. The chapter 3 describes the development of larger capacity distribution transformer with amorphous wound cores as the first case. This study considered a 30 MVA classed three phase amorphous core transformer that was not implementedup to nowbecause of structural problems, and proposed a structure that supports the fragile core while suppressing increase of iron loss. In advance of design, an estimation method of iron loss taking thecompressive stressin the core into account was established, which enabled the quantification of relationship between support method of larger wound core and its iron loss. The cores were divided into inner and outer components suspended independently; thedesign buffered the compressive stress affected in the cores and reduced the iron loss by 32% compared with that with a conventional support structure. In addition, providing the shielding components where the leakage field is concentrated resulted
in 66% decrease in strayloss by utilizing the electromagnetic analysis. Then, a proposed support structure including 10 MVA single phase three legs amorphous core and windings were manufactured separately, and loss performancesof them were evaluated. This study determined totalloss at a load factor of 50% using measured values of iron loss and copper loss and analytical values of stray loss assuming an averaged load operating condition of transformer; as a result, the totalloss in a 30 MVA three phase amorphous core transformer could be reduced by 35% against a conventional silicon steel core transformer of same power capacity. The chapter 4 describes the development of amorphous core reactor for filtering component in uninterruptible power system (UPS) as the second case. This study invented two types of core structure for low cost and larger capacity three phase AC reactors formed from the toroidally wound amorphous yoke cores and the magnetic leg cores cut from solidified toroidal amorphous cores. Calculation models of iron loss in the cores and gap loss induced from the fringing flux between core components were established on the basis of a technique for extracting the coordinate components of the magnetic flux density in accordance with the anisotropic magnetization curvesin plane and laminated directionsof the amorphous foils. It was confirmed that the calculated iron losses at utility and carrier frequencies agree with measured losses within a 10% error. The prototyped amorphous reactors had approximately half the total losses of that of a conventional silicon steel core reactor and increased the power efficiency of the 400 kVA UPS by up to 0.55%. Furthermore, this study demonstrated the practicalityof aminiaturized amorphous reactor designed with a magnetic flux density of 1.2 T increased from standard oneof 0.8 T. It was verified that total loss and unit volume of the prototyped miniaturized reactor could be reduced by 35% and 43% respectivelycompared with those of a silicon steel core reactor. The chapter 5 describes the development of high frequency amorphous transformer for isolated DC-DC converter in DC-interconnected offshore wind farm system as the third case. This study designed and prototyped a core-type 3 kHz-excited 500 kVA transformer consisting of a single phase lap-joint amorphous wound core and windings with primary and secondary copper (Cu)sheets wound alternately in turns. The alternately wound winding structure suppressed the proximity effect between Cu sheets and the in-plane eddy current due to the fringing flux crossing the edge of sheets and fixtures, and the copper loss at 3 kHz was 61% lower than that of conventionally designed winding with primary and secondary sheets wound continuously. The rated total loss of the transformer with alternately wound windings was 21% lower than that of a conventional one. Furthermore, this study proposed a guideline for iron loss-reducing design of the lap-joint part in the amorphous core based onthe measured iron loss at high frequencies and the results of electromagnetic analysis. The chapter 6 concludes the loss reduction effect of three types of amorphous electromagnetic machinedeveloped in this study and describes the remaining issues.
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