The Effect Of DC Magnetic Bias To Transformer Differential Protection And Its Suppression

DC magnetic bias is an abnormal operation state of transformer, which is DCcomponent appearing in the transformer excitation current. When DC current entersthe transformer, it causes the transformer core to saturate in a half-cycle, then a largenumber of harmonic current is generated in excitation current, and it causes thetransformer loss increases, temperature increases, overheats locally, noise increases,vibration increases, system voltage drops, relay protection malfunctions, and etc., andit threatens the safe operation of power seriously.In the research of DC magnetic bias, domestic and foreign scholars primarilyfocus on DC component calculation and restraining measures and the research onthe transformer protection lacks. Therefore, in this paper, theoretical analysis of DCmagnetic bias mechanism is presented firstly, then the characteristics and situationof the transformer flux, excitation current,and harmonic currents in different DCmagnetic bias conditions is analyzed, and the effect harmonic wave changes may givetransformer differential protection which is analyzed too.In view of the above theoretical analysis, a specific transformer simulationmodel is built in PSCAD/EMTDC in this paper, then in different DC magnetic biasconditions, the characteristics and change situation of transformer normal operationand inrush current,excitation current and harmonic current when transformer internalfault is simulated, then the data exported from PSCAD is programmed by Matlab, andtransformer protection operation situation is obtained. Comparative analysis showsthat when a large straight flow into the transformer and internal fault occurs DCmagnetic bias can cause the refusal of transformer differential protection operation.Currently, there are two main ways to solve this refused operation problem: first,to reduce the transformer neutral point DC component; and second, to improveprotection algorithm on the basis of the original protection. Domestic and international research shows that the main three measures to reduce the transformerneutral point DC component have their own advantages and disadvantages in theeconomy and security. So it rarely operated in the grid. Therefore, in this paper, thesecond way is used to solve the transformer protection failure operation caused byDC magnetic bias. Through a lot of simulation studies of transformer internal faultand differential protections in the of refused operation in DC magnetic bias, theunique harmonic characteristics is found, thus in this paper, a new protectionalgorithms is proposed accordingly. Through analysis, this method is simple, reliable,easy to put into practice and does not cause additional harm to the grid.