Enhanced transformer differential protection – design, test and field experience

Transformer differential protection normally use dual slope or multiple slope characteristics to provide stability during through fault and load conditions. It also uses harmonic blocking criteria to provide stability during various transformer inrush conditions. The advancement in the materials used in transformers can cause a reduction in harmonics during a transformer inrush which can lead to reduced 2nd harmonic settings down to 10% or lower in some applications. This paper discusses the impact of a reduced 2nd harmonic threshold on the differential protection and highlights the benefits of waveform recognition algorithms such as the gap detection and the CT saturation techniques. These techniques are used to unblock the differential protection during internal faults, where CT saturation may lead to current waveforms that contain high levels of 2nd harmonic. This paper also highlights the benefits of an adaptive transient bias technique to provide high stability during through fault scenarios and includes results of extensive RTDS testing and some field performance examples.