New Method Based On Waveform Identification For Avoiding Differential Protection Of Transformer To Mal-Operate At Out-Area Faults

Current transformer(CT) is an important current measure element in powersystems, especially in relay protection which needs accurate current measurement infault state. However, because of the saturation of current transformer, relay alwayscannot act correctly. Due to capacity increase or structure change of the power system,an extra-high current possibly 100 times of the rated current may appear in somecircuits of the power system when a short-circuit happens nearby, which will causesaturation of the current transformer iron core. In the network of large capacity andover-voltage, direct current component reduces slowly, which may deteriorateseverely the flux transmissibility of the current transformer winding.Serving as main transformer protection for internal faults,differential protectioncan function easily based on the principle of comparing the angle and value betweenthe current transformer secondary sides of transformer windings. But at out-area faultsand current transformer saturation,the current of primary side can't be transformedcorrectly, which leads to the distortion of secondary current and false protectingactions. The CT saturation is studied in this paper by ATP and PSCAD/EMTDCsimulation. Based on this, some conclusions of CT saturation features are drawn.Moreover, major factors that affect current transformer saturation are analyzed, andmethods of detecting transformer saturation are discussed. The mathematical modelsof transient current transformer simulation are reviewed and analyzed. The modelingmethod for these three models is described in detail. Through the comparison ofsimulation results of practical examples, some suggestions for the application fields ofthe different CT models are proposed.In this paper, through analyzing the sampling values of differential current, a newmethod based on waveform identification of differential current sampling values ispresented to prevent differential protection of transformer from mal-operation, whichcan really lock the protection at out-area faults and current transformer saturation.Using the method of phase comparison the system can be monitored effectivelyduring locked time. When the out-area faults turn into internal faults, the protectioncan be set free from locking. The simulation results show that the method is efficientand correct.