Experimental Study of Transformer Residual Flux and the Method of Restraining Inrush Current

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  • 変圧器残留磁束の実態解明と励磁突入電流抑制法
  • ヘンアツキ ザンリュウジソク ノ ジッタイ カイメイ ト レイジトツニュウ デンリュウ ヨクセイホウ

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Whenever a power transformer under no load condition is manually tripped, residual flux appears in the transformer core which vitally causes inrush current when the transformer is later reenergized. However real aspects of residual fluxes in relation with the tripping time have not yet been experimentally cleared. The authors established their interpretation of residual flux as of the ending states of transient phenomena after tripping, and they executed experimental proof test. By the authors' interpretation, three phase balanced transient phenomenon of voltage, current and core-flux is caused immediately after the transformer is tripped at top0, and it continues until time top1. The real aspect of the residual flux should be the core flux φa(top1), φb(top1), φc(top1)</i> at top1. Further such residual flux as well as voltages and currents during the transient interval are practically three phase balanced so that they can be expressed as three phase balanced equilateral triangle phasors. By the way, the core flux values and the waveforms can not be directly measured while they can be digitally generated as integration of voltage waveform. So, the proof test of the residual flux by the above interpretation can be executed indirectly by preparing (a) measured voltage waveforms just after transformer tripping, (b) mathematically generated flux waveforms by voltage integration just after tripping and (c) measured transient inrush current irush a, irush b, irush c caused immediately after the transformer is reenergized at timing θcl, and then by comparing all these three data as the characteristics on the 3-D coordinates of [θop0, θcl, irush] and of [θop1, θcl, irush].<br>The authors executed experimental proof test using a mimic-test-circuit where large number of on-off-switching tests of a transformer were conducted. The test result clearly indicated that inrush current becomes the largest whenever θcl is reverse angular timing toward θop1 (in stead of θop0), and it becomes the smallest whenever θcl is in-phase with θop1. All these test results satisfied the authors' interpretation of real aspect of the transient phenomena and the residual flux after tripping. The test results suggest essential algorisms of inrush current restraining control in order to appropriately restrain inrush current phenomena.

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