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

Abstract

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 top0, and it continues until time top1. The true nature of the residual flux is the core fluxes φa(t op 1), φb(t op 1), φc(t op 1) at top1. 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 rush 0.16ema, i rush 0.16emb, i rush 0.16emc, 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 θcl is in antiphase with θop1 (instead of θop0), and reaches its minimum whenever θcl is in phase with θop1. 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.

A Method for Estimating the Current–Flux Curve of a Single‐Phase Transformer for Electromagnetic Transient Simulations of Inrush Currents

“…Besides, we measured inrush current in case that residual flux Φ 0 was eliminated by AC demagnetization, and in case that the transformer was energized with residual flux as it was after opening. In order to obtain residual flux, voltage waveform was time‐integrated, and the difference was found with respect to center value of magnetic flux at the moment when transients converged after disconnection of source voltage . Saturation flux Φ s of the test transformer was calculated assuming that iron flux saturates at the moment when measured waveform of inrush current shows a sharp rise.…”

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

“…Thus whenever breaker Br1 is tripped, the tripping of all phase poles will be completed simultaneously at instant t op0 by chopping‐mode tripping of the breaker at the instant when the three‐phase moving contacts move apart from the fixed contacts almost simultaneously. This is in contrast to current‐zero mode tripping, which is the ordinary tripping mode in case of fault current tripping and/or load current tripping, where the tripping of the second and third poles is completed about 10 ms after the first pole trips, because each phase breaker pole can cut off a large current only at the time of current zero .…”

Investigations into the transformer inrush current problem

“…The absolute value of the residual flux, which remains in the ferromagnetic core, can be quite different from one transformer to another. Its true nature has not been experimentally clarified because the flux values in the transformer core cannot be measured directly and field tests cannot be easily conducted [2,3]. It is generally believed that the residual flux, following deenergising of the transformer, will decay slowly over time in a matter of minutes or hours [4] depending on the temperature; at normal temperature we should expect a slower process but it can also remain for a long time.…”