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

SUMMARY Whenever a power transformer in a no‐load condition is manually tripped, a residual flux appears in the transformer core, which causes an inrush current when the transformer is later re‐energized. However, the true nature of residual fluxes has not yet been experimentally elucidated. The authors interpreted the residual flux as representing the ending states of transient phenomena after tripping, and tested this interpretation experimentally. In the authors' interpretation, a three‐phase balanced transient phenomenon of the voltage, current, and core flux occurs immediately after the transformer is tripped at the time t op0 , and it continues until time t op1 . The true nature of the residual flux is the core fluxesφ a ( t op 1 ) ,φ b ( t op 1 ) ,φ c ( t op 1 )at t op1 . Furthermore, these residual fluxes as well as the voltages and currents during the transient interval are practically three‐phase balanced, so that they can be expressed as three‐phase balanced equilateral triangular phasors. The core flux values and waveforms cannot be directly measured but they can be digitally generated as the integrals of the voltage waveform. Thus a test of the residual flux under the above interpretation can be performed indirectly by preparing (1) measured voltage waveforms just after transformer tripping, (2) flux waveforms mathematically generated by voltage integration just after tripping, and (3) measured transient inrush current i rusha , i rushb , i rushc , occurring immediately after the transformer is re‐energized at time θcl, and then comparing these three data as characteristics in the 3‐D coordinates of [ θ op 0 , θ cl , i rush ] and of [ θ op 1 , θ cl , i rush ] . Verification tests were performed utilizing a simulation test circuit in which large numbers of on–off switching tests of a transformer were conducted. The test results clearly indicated that the inrush current reaches its maximum whenever θ cl is in antiphase with θ op1 (instead of θ op0 ), and reaches its minimum whenever θ cl is in phase with θ op1 . These test results confirmed the authors' interpretation of the true nature of the transient phenomena and the residual flux after tripping. The test results suggest essential algorithms for inrush current restraining control in order to appropriately restrain inrush current phenomena. Field test results at a 66‐kV wind power station where commercial equipment based on the above described theory and method were in service are also presented.