Harmonic Analysis of the Currents in the Power Transformer Differential Protection Circuits in the Cases of External and Internal Faults

The usage of the high-harmonic restraint is one of the most common ways to ensure the non-operation of the power transformer differential protection when a magnetizing current inrush occurs. The application of this method allows not only to lower the minimum operating current of the protection, which increases its sensitivity to the developing inter-winding faults, but also provides inaction of the protection when unbalance currents increase during a transient caused by an external fault. However, there is no accurate information in the literature on possible ranges of variation of content of higher harmonics in the differential current in the cases when external or internal faults occur. This complicates the calculation of settings of protective relays, ensuring their most effective functioning. In this paper authors analyze the content of the higher harmonics in the differential current in the cases of external and internal faults, taking into account the saturation of the measuring current transformers in order to determine the range of variation of the higher harmonics under these conditions. It is shown that in all cases the ranges of variation of higher harmonics overlap which makes it difficult to create an effective differential protection that recognizes emergency and normal operation conditions operation only by harmonic analysis of the operating signal.