Study on Residual Flux Evaluation Method Based on Variable-Regional Integral during the Voltage Attenuation Process

Because of the hysteresis characteristic of the transformer core, the residual magnetism is often left in the core during the removal of the transformer, causing inrush current when the transformer no-load switch on, and endangering the safety of equipments and systems. Then, to quantify the main transformer residual flux, a residual flux evaluation method based on variable-regional integral during the voltage attenuation process is proposed. This method determines interrupting time of the transformer interval breaker by detecting the mutation time of the transformer port voltage, and calculates integral through waveform reconstruction from secondary side voltage to primary side voltage, considering spectrum characteristics of potential transformer, and then converts transformer core magnetic field detection, which is theoretically impossible, into voltage detection through the relationship between flux and voltage, and gets the residual flux through the detected voltage. Finally, the method in this paper is applied to the commissioning test of Sichuan-Tibet Interconnection Project. Results tell that the calculated values of residual flux and inrush current with the main transformer unloaded are completely consistent with measured values, proving the validity and great practical value of this method.