Rate of Rise of Differential Current Based Protection of Power Transformer

This paper proposes a new algorithm for the protection of power transformer depending on the rate of rise of differential currents. In which, all the winding currents of all the phases of power transformer are sensed through current transformer and based on the phasor difference between currents, differential currents of all phases are computed. Based on the value of differential current, abnormal condition is detected and rate of rise of differential currents (RRDC) is calculated. Further peak of the sum of derivative of RRDC function (SDRRDC) of all three phases is calculated and which is used to distinguish between external disturbances (magnetic inrush) and internal faults of transformer. For the validation of proposed protection algorithm, internal faults (symmetrical and asymmetrical) like line-to-ground (LG), line-to-line (LL), line-to-line-to-ground (LLG), line-to-line-to-line (LLL) and line-to-line-to-line-ground (LLLG), magnetic inrush as external disturbance and energisation of faulted transformer events are considered. Several test cases are generated, simulated and verified using PSCAD/EMTDC and MATLAB software packages. The proposed scheme precisely discriminates between magnetic inrush and internal faults condition with less response time than the conventional biased differential current protection scheme.