False Trips on Transformer Inrush - Actions to Avoid the Unavoidable

摘要

The nature of the magnetizing circuit of a transformer requires that under some initial closing conditions there will be a significant current from the closing source; appearing to a differential relay as a difference current - a fault. Because, as the wave position at the instant of closing will vary, protection is complicated. Considering that a fault is possible at the moment of energization also adds complications. Over the years there have been a number of different schemes to avoid false tripping during transformer inrush. These have included increased pickups, time delays, harmonic blocking and harmonic restraint. Harmonic elements have been useful, as one characteristic of inrush has been a harmonic component. Changes in transformer designs, such as high efficiency core steel, have caused changes in transformer inrush characteristics. One change that has been observed is a reduction of the harmonic component of the inrush current, which is going along with a longer time for the inrush to decay. This paper examines a misoperation of a transformer differential relay that applied a harmonic restraint system. The inrush waveform was captured and played back into different relays; applying different restraint characteristics. Results are evaluated for different restraints, including a new waveform analysis method. The impact on restraint method and settings, at least for the case leading to the false trip, are evaluated and recommendations given to avoid future misoperations.