Residual flux of core can be generated by DC resistance measurement and circuit breaker opening of transformer

which is mainly derived from the hysteresis characteristic of ferromagnetic material of core. Firstly

the flux computation method based on integral algorithm with baseline correction is studied. The residual flux measurement method based on real-time flux tracking is then proposed. Test results show that the deviation of flux waveform from the normal value will be generated by the trend term due to sinusoidal voltage integration. The integral algorithm with baseline correction can effectively eliminate the trend term

which provides the accurate flux measurement results. Afterwards

a transformer model with single-phase four-limb core is designed according to the core structure of main transformer of UHV transformer

and the simulation test circuits are built in laboratory to investigate the residual flux characteristics of the transformer model after DC excitation and circuit breaker opening. The experimental results of DC excitation show that the residual flux and DC current regularly vary as follows: first increasing and then decreasing when the initial residual flux of core is equal to zero. At the knee point of magnetization curve

the residual flux becomes the largest

accounting for about 52.63% of the saturation value. By contrast

the residual flux has hardly changes with variety of DC current excitation when the non-zero initial residual flux exists in the core

which is mainly caused by the impediment of initial residual flux. The experimental results of circuit breaker opening show that the relationship between residual flux and opening angles presents an approximately statistical cosine distribution. The residual fluxes of 0° and 180° are the largest

accounting for about 58.2% of the saturation value

while the smallest values of 90° and 270° are obtained

only accounting for about 4.14% of the saturation value.