A Novel Differential Protective Relay Of Transformers

Power transformers are essential and important elements of power systems, andthe reliablility of power transformers directly determines whether the power systemworks safety or not. At present, the main protection of internal faults of the powertransformer is the differential protection. When the unloaded power transformer isconnected to the power system or the power transformer is reconnected to the powersystem after fault cleared, it will usually produce magnetizing inrush current, whichoften causes the differential protection relay acting wrongly. Therefore, how toidentifying inrush current and internal fault current is the major problem of powertransformer differential protection. The study on magnetizing inrush current is furthered more and more since boththe importance of power transformer differential protection and the complexity ofinrush current. At present, there are some methods of identifying inrush current. Inthis paper, second harmonic brake, dead angle, waveform symmetry, waveformmatching and virtual third harmonic brake mothod are introduced and analysied,which are all based on different current. This paper analyzed the waveform and minimal dead angle of inrush currents,and then presents a new mothod of identifying inrush current based on waveformlongitudinal symmetry. Internal fault current is longitudinal symmentical, so itslongitudinal symmetry value is very little and not more than 0.10. Meanwhile,one-way inrush current is not longitudinal symmentical, so its longitudinal symmetryvalue is large and large than 0.15 in most conditions. But symmetric inrush islongitudinal, so we should use OR brake when protecting three phase transformer.This method can suppress disturbance of high harmonics and can resist CT saturation. In the end, a practical two-sourcepower system is simulated using MATLAB, anda lot of simulations verify the correctness of the mothd which is mentioned above.