A novel inductance-based technique for discrimination of internal faults from magnetizing inrush currents in power transformers

This paper presents a new methodology for discrimination between inrush currents and internal faults for a three-phase power transformer. This algorithm is based on instantaneous inductance. First, this method calculates differential inductance of transformer phases from primary side view of transformer by using voltage and current signals. Then, the algorithm compares differential inductance with a threshold value. If the calculated criterion is over than the threshold, disturbance will be inrush current. Otherwise, if the calculated criterion is lower than the threshold, disturbance will be internal fault. The operating time is less than 5 ms, less than 1/4 rd of power frequency cycle. The scheme uses voltage and current signal; hence it can work reliably in the presence of transformer tap variation, fault resistance and as shown later, the criterion works properly in CT saturation or over-flux condition. Simulation studies with respect to different faults and inrush conditions have been conducted and the results proved that the proposed technique is able to offer fast responses in protection and accurately discriminate between inrush currents and internal faults.